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CHAPTER 1.
DOS TECHNICAL INFORMATION
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
SOME HISTORY
Development of MSDOS/PCDOS began in October 1980, when IBM began searching
the market for an operating system for the yet-to-be-introduced IBM PC.
Microsoft had no real operating system to sell, but after some research licensed
Seattle Computer Products' 86-DOS, which had been written by a man named Tim
Paterson for use on the company's line of 8086, S100 bus micros. This was
hurriedly polished up and presented to IBM for evaluation. IBM had originally
intended to use Digital Research's CP/M operating system, which was the industry
standard at the time. Folklore reports everything from obscure legal
entanglements to outright snubbing of the IBM representatives by Digital,
irregardless, IBM found itself left with Microsoft's offering of "Microsoft Disk
Operating System 1.0". An agreement was reached between the two, and "IBM PC-DOS
1.0" was ready for the introduction of the IBM PC in October 1981. IBM subjected
the operating system to an extensive quality-assurance program, found well over
300 bugs, and decided to rewrite the programs. This is why PC-DOS is copyrighted
by both IBM and Microsoft.
It is sometimes amusing to reflect on the fact that the IBM PC was not
originally intended to run MSDOS. The target operating system at the end of the
development was for a (not yet in existence) 8086 version of CP/M. On the other
hand, when DOS was originally written the IBM PC did not yet exist! Although
PC-DOS was bundled with the computer, Digital Research's CP/M-86 would probably
have been the main operating system for the PC except for two things - Digital
Research wanted $495 for CP/M-86 (considering PC-DOS was essentially free) and
many software developers found it easier to port existing CP/M software to DOS
than to the new version of CP/M.
MSDOS and PC-DOS have been run on more than just the IBM-PC and clones. There
was an expansion board for the Apple ][ that allowed one to run (some) well -
behaved DOS programs. There are expansion boards for the Commodore Amiga 2000,
the Apple MacIntosh II, and the IBM RT PC allowing them to run DOS, and the IBM
3270 PC, which ran DOS on a 68000 microprocessor. The Atari STs can run an
emulator program and boot MSDOS.
Specific Versions of MS/PC-DOS:
DOS version nomenclature: major.minor.minor. The digit to the left of the
decimal point indicates a major DOS version change. 1.0 was the first version.
2.0 added subdirectories, etc. 3.0 added file handles and network support.
The first minor version indicates customization for a major application. For
example, 2.1 for the PCjr, 3.3 for the PS/2s. The second minor version does not
seem to have any particular meaning.
The main versions of DOS are:
PC-DOS 1.0 October 1981 original release
PC-DOS 1.1 June 1982 bugfix, double sided drive support
MS-DOS 1.25 June 1982 for early compatibles
PC-DOS 2.0 March 1983 for PC/XT, many UNIX-like functions
PC-DOS 2.1 October 1983 for PCjr, bugfixes for 2.0
MS-DOS 2.11 October 1983 compatible equivalent to 2.1
PC-DOS 3.0 August 1984 for PC/AT, network support
PC-DOS 3.1 November 1984 bugfix for 3.0
MS-DOS 2.25 October 1985 compatible; extended foreign language support
PC-DOS 3.2 July 1986 3.5 inch drive support for Convertible
PC-DOS 3.3 April 1987 for PS/2 series
Some versions of MS-DOS varied from PC-DOS in the availible external commands.
Some OEMs only licensed the basic operating system code (the xxxDOS and xxxBIO
programs, and COMMAND.COM) from Microsoft, and either wrote the rest themselves
or contracted them from outside software houses like Phoenix. Most of the
external programs for DOS 3.x are written in "C" while the 1.x and 2.x utilities
were written in assembly language. Other OEMs required customized versions of
DOS for their specific hardware configurations, such as Sanyo 55x and early
Tandy computers, which were unable to exchange their DOS with the IBM version.
At least two versions of DOS have been modified to be run entirely out of ROM.
The Sharp PC5000 had MSDOS 1.25 in ROM, and the Toshiba 1100 and some Tandy
models have MSDOS 2.11 in ROM.
THE OPERATING SYSTEM HIERARCHY
The Disk Operating System (DOS) and the ROM BIOS serve as an insulating layer
between the application program and the machine, and as a source of services
to the application program.
The system heirarchy may be thought of as a tree, with the lowest level being
the actual hardware. The 8088 or V20 processor sees the computer's address
space as a ladder two bytes wide and one million bytes long. Parts of this
ladder are in ROM, parts in RAM, and parts are not assigned. There are also
256 "ports" that the processor can use to control devices.
The hardware is normally addressed by the ROM BIOS, which will always know
where everything is in its particular system. The chips may usually also be
written to directly, by telling the processor to write to a specific address or
port. This sometimes does not work as the chips may not always be at the same
addresses or have the same functions from machine to machine.
DOS STRUCTURE
DOS consists of four components:
* The boot record
* The ROM BIOS interface (IBMBIO.COM or IO.SYS)
* The DOS program file (IBMDOS.COM or MSDOS.SYS)
* The command processor (COMMAND.COM or aftermarket replacement)
The boot record begins on track 0, sector 1, side 0 of every diskette formatted
by the DOS FORMAT command. The boot record is placed on diskettes to produce an
error message if you try to start up the system with a nonsystem diskette in
drive A. For hard disks, the boot record resides on the first sector of the DOS
partition. All media supported by DOS use one sector for the boot record.
- Read Only Memory (ROM) BIOS Interface
The file IBMBIO.COM or IO.SYS is the interface module to the ROM BIOS.
This file provides a low-level interface to the ROM BIOS device routines and
may contain extensions or changes to the system board ROMs. Some compatibles do
not have a ROM BIOS to extend, and load the entire BIOS from disk. (Sanyo 55x,
Viasyn)
The actual DOS program is file IBMDOS.COM or MSDOS.SYS. It provides a high-
level interface for user (application) programs. This program consists of file
management routines, data blocking/deblocking for the disk routines, and a
variety of built-in functions easily accessible by user programs.
When a user program calls these function routines, they accept high-level
information by way of register and control block contents. For device
operations, the functions translate the requirement into one or more calls to
IBMBIO.COM or MSDOS.SYS to complete the request.
The Command interpreter, COMMAND.COM, consists of these parts:
Resident Portion:
The resident portion resides in memory immediately following IBMDOS.COM and its
data area. This portion contains routines to process interrupts 22h (Terminate
Address), 23h (Ctrl-Break Handler), and 24h (Critical Error Handler), as well as
a routine to reload the transient portion if needed. For DOS 3.x, this portion
also contains a routine to load and execute external commands, such as files
with exensions of COM or EXE.
When a program terminates, a checksum is used to determine if the application
program overlaid the transient portion of COMMAND.COM. If so, the resident
portion will reload the transient portion from the area designated by COMSPEC=
in the DOS environment. If COMMAND.COM cannot be found, the system will halt.
NOTE: DOS 3.3 checks for the presence of a hard disk, and will default to
COMSPEC=C:\. Previous versions default to COMSPEC=A:\. Under some DOS
versions, if COMMAND.COM is not immediately availible for reloading
(i.e., swapping to a floppy with COMMAND.COM on it) DOS may crash.
All standard DOS error handling is done within the resident portion of
COMMAND.COM. This includes displaying error messages and interpreting the
replies of Abort, Retry, Ignore, Fail.
An initialization routine is included in the resident portion and assumes
control during startup. This routine contains the AUTOEXEC.BAT file handler and
determines the segment address where user application programs may be loaded.
The initialization routine is then no longer needed and is overlaid by the first
program COMMAND.COM loads.
NOTE: AUTOEXEC.BAT may be a hidden file.
A transient portion is loaded at the high end of memory. This is the command
processor itself, containing all of the internal command processors and the
batch file processor. For DOS 2.x, this portion also contains a routine to load
and execute external commands, such as files with extensions of COM or EXE.
This portion of COMMAND.COM also produces the DOS prompt (such as "A>"), reads
the command from the standard input device (usually the keyboard or a batch
file), and executes the command. For external commands, it builds a command line
and issues an EXEC function call to load and transfer control to the program.
NOTE: COMMAND.COM may be a hidden file.
NOTE: For Dos 2.x, the transient portion of the command processor contains the
EXEC routine that loads and executes external commands. For DOS 3.x, the
resident portion of the command processor contains the EXEC routine.
DOS Initialization
The system is initialized by a software reset (Ctrl-Alt-Del), a hardware reset
(reset button), or by turning the computer on. The Intel 80x8x series processors
always look for their first instruction at the end of their address space
(0FFFF0h) when powered up or reset. This address contains a jump to the first
instruction for the ROM BIOS.
Built-in ROM programs (Power-On Self-Test, or POST, in the IBM) check machine
status and run inspection programs of various sorts. Some machines set up a
reserved RAM area with bytes indicating installed equipment (AT and PCjr).
The ROM routine looks for a disk drive at A: or an option ROM (usually a hard
disk) at absolute address C:800h. If no floppy drive or option ROM is found, the
BIOS calls int 19h (ROM BASIC if it is an IBM) or displays error message.
If a bootable disk is found, the ROM BIOS loads the first sector of information
from the disk and then jumps into the RAM location holding that code. This code
normally is a routine to load the rest of the code off the disk, or to "boot"
the system.
The following actions occur after a system initialization:
1. The boot record is read into memory and given control.
2. The boot record then checks the root directory to assure that the first
two files are IBMBIO.COM and IBMDOS.COM. These two files must be the
first two files, and they must be in that order (IBMBIO.COM first, with
its sectors in contiguous order).
NOTE: IBMDOS.COM need not be contiguous in version 3.x.
3. The boot record loads IBMBIO.COM into memory.
4. The initialization code in IBMBIO.COM loads IBMDOS.COM, determines
equipment status, resets the disk system, initializes the attached
devices, sets the system parameters and loads any installable device
drivers according to the CONFIG.SYS file in the root directory (if
present), sets the low-numbered interrupt vectors, relocates IBMDOS.COM
downward, and calls the first byte of DOS.
NOTE: CONFIG.SYS may be a hidden file.
5. DOS initializes its internal working tables, initializes the interrupt
vectors for interrupts 20h through 27h, and builds a Program Segment
Prefix for COMMAND.COM at the lowest available segment. For DOS versions
3.10 up, DOS initializes interrupt vectors for interrupts 0Fh through 3Fh.
6. IBMBIO.COM uses the EXEC function call to load and start the top-level
command processor. The default command processor is COMMAND.COM.
CHAPTER 2
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
CPU Port Assignments, System Memory Map, BIOS Data Area, Interrupts 00h to 09h
SYSTEM MEMORY MAP - OVERALL
The IBM PC handles its address space in 64k segments, divided into 16k
fractions and then further as nescessary.
start start end
addr. addr. addr. usage
(dec) (hex)
00000 **** 640k *************** system data, drivers....
0000:0000 hardware interrupt vectors
0000:0040 BIOS interrupt vectors
0k start of RAM | 0000:0080 DOS interrupt vector table
16k 00000-03FFF | 0000:0300 Stack area during POST and bootstrap routine
32k 04000-07FFF | 0000:0400 BIOS Data Area
48k 08000-0BFFF | 0000:04F0 Intra-Application Communications Area
|
64k 10000-13FFF | 0000:0500 DOS reserved communication area
80k 14000-17FFF | xxxx:0000 IO.SYS - DOS interface to ROM I/O routines
96k 18000-1BFFF | xxxx:0000 MSDOS.SYS - DOS interrupt handlers, service
112k 1C000-1FFFF | routines (int 21 functions)
|
128k 20000-23FFF | xxxx:xxxx DOS buffers, control areas, and installed
144k 24000-27FFF | device drivers.
160k 28000-2BFFF | xxxx:0000 resident portion of COMMAND.COM, interrupt
176k 2C000-2FFFF | handlers for int 22h, 23h,24h, and code to
| reload the transient portion
192k 30000-33FFF | xxxx:0000 master environment block, default 64 bytes
208k 34000-37FFF | xxxx:0000 environment for next program
224k 38000-3BFFF | xxxx:0000 external commands or utilities (COM or EXE
240k 3C000-3FFFF | files)
|
256k 40000-43FFF | ----:---- application programs
272k 44000-47FFF | xxxx:0000 user stack for COM files (256 bytes)
288k 48000-4BFFF | xxxx:0000 transient portion of COMMAND.COM
304k 4C000-4FFFF |
|
320k 50000-53FFF |
336k 54000-57FFF |
352k 58000-5BFFF |
368k 5C000-5FFFF |
|
384k 60000-63FFF |
400k 64000-67FFF |
416k 68000-6BFFF |
432k 6C000-6FFFF |
|
448k 70000-73FFF |
464k 74000-77FFF |
480k 78000-7BFFF |
496k 7C000-7FFFF |
|
512k 80000-83FFF |
528k 84000-87FFF |
544k 88000-8BFFF | original IBM PC-1 BIOS limited memory to 544k
560k 8C000-8FFFF |
|
576k 90000-93FFF |
592k 94000-97FFF |
609k 98000-9BFFF |
624k 9C000-9FFFF | to 640k (top of RAM address space)
A0000 ***** 64k *************** EGA address
640k A0000-A95B0 MCGA 320x200 256 color video buffer
AF8C0 MCGA 640x480 2 color video buffer
-A3FFF
656k A4000-A7FFF
672k A8000-ABFFF
688k AC000-AFFFF
B0000 ***** 64k *************** mono and CGA address
704k B0000-B3FFF mono uses only 4k | The PCjr and early Tandy 1000
720k B4000-B7FFF | BIOSs revector direct writes to
736k B8000-BBFFF CGA uses entire 16k | the B8 area to the Video Gate
756k BC000-BFFFF | Array and reserved system RAM
C0000 ***** 64k *************** expansion ROM
768k C0000-C3FFF 16k EGA BIOS C000:001E EGA BIOS signature (the letters IBM)
784k C4000-C5FFF
C6000-C63FF 256 bytes Professional Graphics Display communication area
C6400-C7FFF
800k C8000-CBFFF 16k hard disk controller BIOS, drive 0 default
816k CC000-CDFFF 8k IBM PC Network NETBIOS
CE000-CFFFF
D0000 ***** 64k *************** expansion ROM | PCjr first ROM cartridge
832k D0000-D7FFF 32k IBM Cluster Adapter | address area.
DA000 voice communications |
848k D4000-D7FFF | Common expanded memory board
864k D8000-DBFFF | paging area.
880k DC000-DFFFF |
E0000 ***** 64k *************** expansion ROM | PCjr second ROM
896k E0000-E3FFF | cartridge address
912k E4000-E7FFF | area
928k E8000-EBFFF |
944k EC000-EFFFF |
F0000 ***** 64k *************** system | PCjr optional ROM
960k F0000-F3FFF reserved by IBM | cartridge address
976k F4000- | area (cartridge
F6000 ROM BASIC Begins | BASIC)
992k F8000-FB000 |
1008k FC000-FFFFF ROM BASIC and original |
BIOS (Compatibility BIOS |
in PS/2) |
1024k FFFFF end of memory (1024k) for 8088 machines
F000:FFF5 BIOS release date
F000:FFFE PC model identification
384k 100000-15FFFF 80286/AT extended memory area, 1Mb motherboard
15Mb 100000-FFFFFF 80286/AT extended memory address space
15Mb 160000-FDFFFF Micro Channel RAM expansion (15Mb extended memory)
128k FE0000-FFFFFF system board ROM (PS/2 Advanced BIOS)
PC Port Assignment, Intel 8088, 80C88, 8086, 80286, 80386 CPUs
hex addr. Function
0000-000F 8237 DMA controller
0010-001F 8237 DMA controller (AT, PS/2)
0020-0027 8259A interrupt controller
0020-003F 8259A interrupt controller (AT)
0040-005F 8253-5 programmable timers
(note: 0041 was memory refresh in PCs. Not used in PS/2)
0060-0067 8255 peripheral interface
0060-006F 8042 keyboard controller (AT)
0200-020F game-control adapter
0210-0217 expansion box (PC, XT)
0278-027F LPT3
02F8-02FF COM2
0300-031F prototype card
0320-032F hard disk controller
0378-037F LPT2
03BC-03BF LPT1
03D0-03DF CGA, MCGA, VGA adapter control
03F0-03F7 floppy disk controller
03F8-03FF COM1
note: These are functions common across the IBM range. The PCjr, PC
Convertible and PS/2 (both buses) have enhancements. In some cases, the
AT and PS/2 series ignore, duplicate, or reassign ports arbitrarily. If
your code incorporates specific port addresses for video or system board
control it would be wise to have your application determine the machine
type and video adapter and address the ports as required.
Reserved Memory Locations in the IBM PC
addr. size description
000h-3FFh DOS interrupt vector table
30:00h- used as a stack area during POST and bootstrap routines. This
3F:FFh stack area may be revectored by an application program.
The BIOS Data Area addr. from 400h to 4FFh
40:00 word COM1 port address | These addresses are zeroed out in the
40:02 word COM2 port address | OS/2 DOS Compatibility Box if any of
40:04 word COM3 port address | the OS/2 COM??.SYS drivers are loaded.
40:06 word COM4 port address |
40:08 word LPT1 port address
40:0A word LPT2 port address
40:0C word LPT3 port address
40:0E word LPT4 port address (not valid in PS/2 machines)
40:0E word PS/2 pointer to 1k extended BIOS Data Area at top of RAM
40:10 word equipment flag (see int 11h)
bits:
0 1 if floppy drive present (see bits 6&7) 0 if not
1 1 if 80x87 installed (not valid in PCjr)
2,3 system board RAM (not used on AT or PS/2)
00 16k
01 32k
10 48k
11 64k
4,5 initial video mode
00 no video adapter
01 40 column color (PCjr)
10 80 column color
11 MDA
6,7 number of diskette drives
00 1 drive
01 2 drives
10 3 drives
11 4 drives
8 0 DMA present
1 DMA not present (PCjr)
9,A,B number of RS232 serial ports
C game adapter (joystick)
0 no game adapter
1 if game adapter
D serial printer (PCjr only)
0 no printer
1 serial printer present
E,F number of parallel printers installed
note 1) The IBM PC and AT store the settings of the system board
switches or CMOS RAM setup information (as obtained by the BIOS
in the Power-On Self Test (POST)) at addresses 40:10h and
40:13h. 00000001b indicates "on", 00000000b is "off".
2) CMOS RAM map, PC/AT:
offset contents
00h Seconds
01h Second Alarm
02h Minutes
03h Minute Alarm
04h Hours
05h Hour Alarm
06h Day of the Week
07h Day of the Month
08h Month
09h Year
0Ah Status Register A
0Bh Status Register B
0Ch Status Register C
0Dh Status Register D
0Eh Diagnostic Status Byte
0Fh Shutdown Status Byte
10h Disk Drive Type for Drives A: and B:
The drive-type bytes use bits 0:3 for the first
drive and 4:7 for the other
Disk drive types:
00h no drive present
01h double sided 360k
02h high capacity (1.2 meg)
03h-0Fh reserved
11h (AT):Reserved (PS/2):drive type for hard disk C:
12h (PS/2):drive type for hard disk D:
(AT, XT/286):hard disk type for drives C: and D:
Format of drive-type entry for AT, XT/286:
0 number of cyls in drive (0-1023 allowed)
2 number of heads per drive (0-15 allowed)
3 starting reduced write compensation (not
used on AT)
5 starting cylinder for write compensation
7 max. ECC data burst length, XT only
8 control byte
Bit
7 disable disk-access retries
6 disable ECC retries
5-4 reserved, set to zero
3 more than 8 heads
2-0 drive option on XT (not used by AT)
9 timeout value for XT (not used by AT)
12 landing zone cylinder number
14 number of sectors per track (default 17,
0-17 allowed)
13h Reserved
14h Equipment Byte (corresponds to sw. 1 on PC and XT)
15h-16h Base Memory Size (low,high)
17h-18h Expansion Memory Size (low,high)
19h-20h Reserved
(PS/2) POS information Model 50 (60 and 80 use a 2k
CMOS RAM that is not accessible through software)
21h-2Dh Reserved (not checksumed)
2Eh-2Fh Checksum of Bytes 10 Through 20 (low,high)
30h-31h Exp. Memory Size as Det. by POST (low,high)
32h Date Century Byte
33h Information Flags (set during power-on)
34h-3Fh Reserved
3) The alarm function is used to drive the BIOS wait function (int
15h function 90h).
4) To access the configuration RAM write the byte address (00-3Fh)
you need to access to I/O port 70h, then access the data via I/O
port 71h.
5) CMOS RAM chip is a Motorola 146818
6) The equipment byte is used to determine the configuration for the
power-on diagnostics.
7) Bytes 00-0Dh are defined by the chip for timing functions, bytes
0Eh-3Fh are defined by IBM.
40:12 byte number of errors detected by infrared keyboard link (PCjr only)
40:13 word availible memory size in Kbytes (less display RAM in PCjr)
this is the value returned by int 12h
40:17 byte keyboard flag byte 0 (see int 9h)
bit 7 insert mode on 3 alt pressed
6 capslock on 2 ctrl pressed
5 numlock on 1 left shift pressed
4 scrollock on 0 right shift pressed
40:18 byte keyboard flag byte 1 (see int 9h)
bit 7 insert pressed 3 ctrl-numlock (pause) toggled
6 capslock pressed 2 PCjr keyboard click active
5 numlock pressed 1 PCjr ctrl-alt-capslock held
4 scrollock pressed 0
40:19 byte storage for alternate keypad entry (not normally used)
40:1A word pointer to keyboard buffer head character
40:1C word pointer to keyboard buffer tail character
40:1E 32bytes 16 2-byte entries for keyboard circular buffer, read by int 16h
40:3E byte drive seek status - if bit=0, next seek will recalibrate by
repositioning to Track 0.
bit 3 drive D bit 2 drive C
1 drive B 0 drive A
40:3F byte diskette motor status
bit 7 1, write in progress 3 1, D: motor on (floppy 3)
6 2 1, C: motor on (floppy 2)
5 1 1, B: motor on
4 0 1, A: motor on
40:40 byte motor off counter
starts at 37 and is decremented 1 by each system clock tick.
motor is shut off when count = 0.
40:41 byte status of last diskette operation where:
bit 7 timeout failure bit 3 DMA overrun
6 seek failure 2 sector not found
5 controller failure 1 address not found
4 CRC failure 0 bad command
40:42 7 bytes NEC status
40:49 byte current CRT mode (hex value)
00h 40x25 BW (CGA) 01h 40x25 color (CGA)
02h 80x25 BW (CGA) 03h 80x25 color (CGA)
04h 320x200 color (CGA) 05h 320x200 BW (CGA)
06h 640x200 BW (CGA) 07h monochrome (MDA)
extended video modes (EGA/MCGA/VGA or other)
08h lores,16 color 09h med res,16 color
0Ah hires,4 color 0Bh n/a
0Ch med res,16 color 0Dh hires,16 color
0Eh hires,4 color 0Fh hires,64 color
40:4A word number of columns on screen, coded as hex number of columns
20 col = 14h (video mode 8, low resolution 160x200 CGA graphics)
40 col = 28h
80 col = 46h
40:4C word screen buffer length in bytes
(number of bytes used per screen page, varies with video mode)
40:4E word current screen buffer starting offset (active page)
40:50 8 words cursor position pages 1-8
the first byte of each word gives the column (0-19, 39, or 79)
the second byte gives the row (0-24)
40:60 byte end line for cursor (normally 1)
40:61 byte start line for cursor (normally 0)
40:62 byte current video page being displayed (0-7)
40:63 word base port address of 6845 CRT controller or equivalent
for active display 3B4h=mono, 3D4h=color
40:65 byte current setting of the CRT mode register
40:66 byte current palette mask setting (CGA)
40:67 5 bytes temporary storage for SS:SP during shutdown (cassette interface)
40:6C word timer counter low word
40:6E word timer counter high word
40:69 byte HD_INSTALL (Columbia PCs) (not valid on most clone computers)
bit 0 = 0 8 inch external floppy drives
1 5-1/4 external floppy drives
1,2 = highest drive address which int 13 will accept
(since the floppy drives are assigned 0-3, subtract
3 to obtain the number of hard disks installed)
4,5 = # of hard disks connected to expansion controller
6,7 = # of hard disks on motherboard controller
(if bit 6 or 7 = 1, no A: floppy is present and
the maximum number of floppies from int 11 is 3)
40:70 byte 24 hour timer overflow 1 if timer went past midnight
it is reset to 0 each time it is read by int 1Ah
40:71 byte BIOS break flag (bit 7 = 1 means break key hit)
40:72 word reset flag (1234 = soft reset, memory check will be bypassed)
PCjr keeps 1234h here for softboot when a cartridge is installed
40:74 byte status of last hard disk operation; PCjr special diskette control
40:75 byte # of hard disks attached (0-2) ; PCjr special diskette control
40:76 byte hd control byte; temporary holding area for 6th param table entry
40:77 byte port offset to current hd adapter ; PCjr special diskette control
40:78 4 bytes timeout value for LPT1,LPT2,LPT3,LPT4
40:7C 4 bytes timeout value for COM1,COM2,COM3,COM4 (0-FFh seconds, default 1)
40:80 word pointer to start of circular keyboard buffer, default 03:1E
40:82 word pointer to end of circular keyboard buffer, default 03:3E
40:84 byte rows on the screen (EGA only)
40:84 byte PCjr interrupt flag; timer channel 0 (used by POST)
40:85 word bytes per character (EGA only)
40:85 2 bytes (PCjr only) typamatic char to repeat
40:86 2 bytes (PCjr only) typamatic initial delay
40:87 byte mode options (EGA only)
Bit 1 0 = EGA is connected to a color display
1 = EGA is monochrome.
Bit 3 0 = EGA is the active display,
1 = "other" display is active.
Mode combinations:
Bit3 Bit1 Meaning
0 0 EGA is active display and is color
0 1 EGA is active display and is monochrome
1 0 EGA is not active, a mono card is active
1 1 EGA is not active, a CGA is active
40:87 byte (PCjr only) current Fn key code
40:88 byte feature bit switches (EGA only) 0=on, 1=off
bit 3 = switch 4
bit 2 = switch 3
bit 1 = switch 2
bit 0 = switch 1
40:88 byte (PCjr only) special keyboard status byte
bit 7 function flag 3 typamatic (0=enable,1=disable)
6 Fn-B break 2 typamatic speed (0=slow,1=fast)
5 Fn pressed 1 extra delay bef.typamatic (0=enable)
4 Fn lock 0 write char, typamatic delay elapsed
40:89 byte PCjr, current value of 6845 reg 2 (horiz.synch) used by
ctrl-alt-cursor screen positioning routine in ROM
40:8A byte PCjrCRT/CPU Page Register Image, default 3Fh
40:8B byte last diskette data rate selected
40:8C byte hard disk status returned by controller
40:8D byte hard disk error returned by controller
40:8E byte hard disk interrupt (bit 7=working int)
40:90 4 bytes media state drive 0, 1, 2, 3
40:94 2 bytes track currently seeked to drive 0, 1
40:96 byte keyboard flag byte 3 (see int 9h)
40:97 byte keyboard flag byte 2 (see int 9h)
40:98 dword pointer to users wait flag
40:9C dword users timeout value in microseconds
40:A0 byte real time clock wait function in use
40:A1 byte LAN A DMA channel flags
40:A2 2 bytes status LAN A 0,1
40:A4 dword saved hard disk interrupt vector
40:A8 dword EGA pointer to parameter table
40:B4 byte keyboard NMI control flags (Convertible)
40:B5 dword keyboard break pending flags (Convertible)
40:B9 byte port 60 single byte queue (Convertible)
40:BA byte scan code of last key (Convertible)
40:BB byte pointer to NMI buffer head (Convertible)
40:BC byte pointer to NMI buffer tail (Convertible)
40:BD 16bytes NMI scan code buffer (Convertible)
40:CE word day counter (Convertible and after)
to -04:8F end of BIOS Data Area
40:90-40:EF reserved by IBM
04:F0 16 bytes Intra-Application Communications Area (for use by applications
04:FF to transfer data or parameters to each other)
05:00 byte DOS print screen status flag
00h not active or successful completion
01h print screen in progress
0FFh error during print screen operation
05:01 Used by BASIC
05:02-03 PCjr POST and diagnostics work area
05:04 byte Single drive mode status byte
00 logical drive A
01 logical drive B
05:05-0E PCjr POST and diagnostics work area
05:0F BASIC: SHELL flag (set to 02h if there is a current SHELL)
05:10 word BASIC: segment address storage (set with DEF SEG)
05:12 4 bytes BASIC: int 1Ch clock interrupt vector segment:offset storage
05:16 4 bytes BASIC: int 23h ctrl-break interrupt segment:offset storage
05:1A 4 bytes BASIC: int 24h disk error interrupt vector segment:offset storage
05:1B-1F Used by BASIC for dynamic storage
05:20-21 Used by DOS for dynamic storage
05:22-2C Used by DOS for diskette parameter table. See int 1Eh for values
05:30-33 Used by MODE command
05:34-FF Unknown - Reserved for DOS
At Absolute Addresses:
0008:0047 IO.SYS or IBMBIO.COM IRET instruction. This is the dummy routine that
interrupts 01h, 03h, and 0Fh are initialized to during POST.
C000:001E EGA BIOS signature (the letters IBM)
F000:FFF5 BIOS release date
F000:FFFE PC model identification
date model byte submodel byte revision
04/24/81 FF = PC-0 (16k) -- --
10/19/81 FF = PC-1 (64k) -- --
08/16/82 FF = PC, XT, XT/370 -- --
(256k motherboard)
10/27/82 FF = PC, XT, XT/370 -- --
(256k motherboard)
11/08/82 FE = XT, Portable PC -- --
XT/370, 3270PC
01/10/86 FB = XT 00 01
01/10/86 FB = XT-2 (early)
05/09/86 FB = XT-2 (640k) 00 02
06/01/83 FD = PCjr -- --
01/10/84 FC = AT -- --
06/10/85 FC = AT 00 01
11/15/85 FC = AT 01 00
04/21/86 FC = XT/286 02 00
09/13/85 F9 = Convertible 00 00
09/02/86 FA = PS/2 Model 30 00 00
11/15/86 FC = AT, Enhanced 8mHz
02/13/87 FC = PS/2 Model 50 04 00
02/13/87 FC = PS/2 Model 60 05 00
1987 F8 = PS/2 Model 80 00 00
2D = Compaq PC (4.77) -- --
9A = Compaq Plus (XT) -- --
00FC 7531/2 Industrial AT
06FC 7552 Gearbox
The IBM PC System Interrupts (Overview)
The interrupt table is stored in the very lowest location in memory, starting
at 0000:0000h. The locations are offset from segment 0, ie location 0000h has
the address for int 0, etc. Each address is four bytes long and its location in
memory can be found by multiplying the interrupt number by 4. For example, int
7 could be found by (7x4=28) or 1Bh (0000:001Bh).
These interrupt vectors normally point to ROM tables or are taken over by DOS
when an application is run. Some applications revector these interrupts to
their own code to change the way the system responds to the user.
Interrupt Address Function
Number (Hex)
0 00-03 CPU Divide by Zero
1 04-07 CPU Single Step
2 08-0B CPU Nonmaskable
3 0C-0F CPU Breakpoint
4 10-13 CPU Overflow
5 14-17 BIOS Print Screen
6 18-1B hdw Reserved
7 1C-1F hdw Reserved
8 20-23 hdw Time of Day
9 24-27 hdw Keyboard
A 28-2B hdw Reserved
B 2C-2F hdw Communications [8259]
C 30-33 hdw Communications
D 34-37 hdw Disk
E 38-3B hdw Diskette
F 3C-3F hdw Printer
10 40-43 BIOS Video
11 44-47 BIOS Equipment Check
12 48-4B BIOS Memory
13 4C-4F BIOS Diskette/Disk
14 50-53 BIOS Serial Communications
15 54-57 BIOS Cassette, System Services
16 58-5B BIOS Keyboard
17 5C-5F BIOS Parallel Printer
18 60-63 BIOS Resident BASIC
19 64-67 BIOS Bootstrap Loader
1A 68-6B BIOS Time of Day
1B 6C-6F BIOS Keyboard Break
1C 70-73 BIOS Timer Tick
1D 74-77 BIOS Video Initialization
1E 78-7B BIOS Diskette Parameters
1F 7C-7F BIOS Video Graphics Characters, second set
20 80-83 DOS General Program Termination
21 84-87 DOS DOS Services Function Request
22 88-8B DOS Called Program Termination Address
23 8C-8F DOS Control Break Termination Address
24 90-93 DOS Critical Error Handler
25 94-97 DOS Absolute Disk Read
26 98-9B DOS Absolute Disk Write
27 9C-9F DOS Terminate and Stay Resident
28-3F A0-FF DOS Reserved for DOS
40-43 100-115 BIOS Reserved for BIOS
44 116-119 BIOS First 128 Graphics Characters
45-47 120-131 BIOS Reserved for BIOS
48 132-135 BIOS PCjr Cordless Keyboard Translation
49 136-139 BIOS PCjr Non-Keyboard Scancode Translation Table
50-5F 140-17F BIOS Reserved for BIOS
60-67 180-19F Reserved for User Software Interrupts
68-7F 1A0-1FF Reserved by IBM
80-85 200-217 ROM BASIC
86-F0 218-3C3 Used by BASIC Interpreter When BASIC is Running
F1-FF 3C4-3FF Reserved by IBM
For consistency in this volume, all locations and offsets are in hexadecimal
unless otherwise specified. All hex numbers are prefaced with a leading zero
if they begin with an alphabetic character, and are terminated with a
lowercase H (h). The formats vary according to common usage.
The IBM-PC System Interrupts (in detail)
Interrupt 00h Divide by Zero (processor error). Automatically called at end
(0:0000h) of DIV or IDIV operation that results in error. Normally set by
DOS to display an error message and abort the program.
Interrupt 01h Single step - Taken after every instruction when CPU Trap Flag
(0:0004h) indicates single-step mode (bit 8 of FLAGS is 1). This is what
makes the T command of DEBUG work for single stepping. Is not
generated after MOV to segment register or POP of segment
register. (unless you have a very early 8088 with the microcode
bug).
Interrupt 02h Non-maskable interrupt - Vector not disabled via CLI. Used by
(0:0008h) parity check routine in POST, 8087 coprocessor, PCjr infrared
keyboard link.
Interrupt 03h Breakpoint - Taken when CPU executes the 1-byte int 3 (0CCh).
(0:000Ch) Generated by opcode 0CCh. Similar to 8080's RST instruction.
Generally used to set breakpoints for DEBUG.
Interrupt 04h Divide overflow - Generated by INTO instruction if OF flag is
(0:0010h) set. If flag is not set, INTO is effectively a NOP. Used to trap
any arithmetic errors when program is ready to handle them rather
than immediately when they occur.
Interrupt 05h Print Screen - service dumps the screen to the printer. Invoked
(0:0014h) by int 9 for shifted key 55 (PrtSc). Automatically called by
keyboard scan when PrtSc key is pressed. Normally executes
routine to print the screen, but may call any routine that can
safely be executed from inside the keyboard scanner. Status and
result byte are at address 0050:0000.
entry AH 05h
return absolute address 50:0
00h print screen has not been called, or upon return from a call
there were no errors.
01h print screen is already in progress.
0FFh error encountered during printing.
note 1) Uses BIOS services to read the screen
2) Output is directed to LPT1
3) Revectored into GRAPHICS.COM if GRAPHICS.COM is loaded
Interrupt 06h Reserved by IBM
(0:0018h)
Interrupt 07h Reserved by IBM
(0:00C0h)
Interrupt 08h Timer - 55ms timer "tick" taken 18.2 times per second. Updates
(0:0020h) (IRQ0) BIOS clock and turns off diskette drive motors after 2
seconds of inactivity.
entry AH 08h
return absolute addresses:
40:6C number of interrupts since power on (4 bytes)
40:70 number of days since power on (1 byte)
40:67 day counter on all products after AT
40:40 motor control count - gets decremented and shuts off diskette
motor if zero
note Int 1Ch invoked as a user interrupt.
Interrupt 09h Keyboard - taken whenever a key is pressed or released.
(0:0024h) (IRQ1) Stores characters/scan-codes in status at [0040:0017,18]
entry AH 09h
return at absolute memory addresses:
40:17 bit
0 right shift key depressed
1 left shift key depressed
2 control key depressed
3 alt key depressed
4 ScrollLock state has been toggled
5 NumLock state has been toggled
6 CapsLock state has been toggled
7 insert state is active
40:18 bit
0 left control key depressed
1 left alt key depressed
2 SysReq key depressed
3 Pause key has been toggled
4 ScrollLock key is depressed
5 NumLock key is depressed
6 CapsLock key is depressed
7 Insert key is depressed
40:96 bit
0 last code was the E1h hidden code
1 last code was the E0h hidden code
2 right control key down
3 right alt key down
4 101 key Enhanced keyboard installed
5 force NumLock if rd ID & kbx
6 last character was first ID character
7 doing a read ID (must be bit 0)
40:97 bit
0 ScrollLock indicator
1 NumLock indicator
2 CapsLock indicator
3 circus system indicator
4 ACK received
5 resend received flag
6 mode indicator update
7 keyboard transmit error flag
40:1E keyboard buffer (20h bytes)
40:1C buffer tail pointer
40:72 1234h if ctrl-alt-del pressed on keyboard
AL scan code
note 1) Int 05h invoked if PrtSc key pressed
2) Int 1Bh invoked if Ctrl-Break key sequence pressed
3) Int 15h, AH=85h invoked on AT and after if SysReq key is pressed
4) Int 15h, AH=4Fh invoked on machines after AT
Interrupt 0Ah EGA Vertical Retrace
(0:0028h) (IRQ2) used by EGA vertical retrace, hard disk
Interrupt 0Bh Communications Controller (serial port) hdw. entry
(0:002Ch) (IRQ3) Serial Port 2 (com2)
note IRQ 3 may be used by SDLC (synchronous data-link control) or
bisynchronous communications cards instead of a serial port.
Interrupt 0Ch Communications Controller (serial port) hdw. entry
(0:0030h) (IRQ4) Serial Port 1 (com1)
note IRQ 4 may be used by SDLC (synchronous data-link control) or
bisynchronous communications cards instead of a serial port.
Interrupt 0Dh Alternate Printer, PC/AT 80287
(0:0034h) (IRQ5) used by hard disk, 60 Hz RAM refresh, LPT2 on AT, XT/286,
and PS/2, dummy CRT vertical retrace on PCjr
Interrupt 0Eh Diskette - indicates that a seek is in progress
(0:0038h) (IRQ6) (sets bit 8 of 40:3E)
Interrupt 0Fh Reserved by IBM
(0:003Ch) (IRQ7) IRQ7 used by PPI interrupt (LPT1, LPT2)
CHAPTER 3 THE PC ROM BIOS
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
Interrupt 10h Video I/O - services to handle video output
(0:0040h) The ROM video routines in the original PC BIOS are designed for
use with the Color Graphics Adapter and incorporate code to test
for the horizontal retrace before writing. The check is
performed no matter what actual display adapter is installed.
The ROM character table for the first 128 characters is located
at 0FA6Eh in the PC. Int 01Fh can be used to point to a second
table of 128 characters.
CS, SS, DS, ES, BX, CX, DX are preserved during call. All
others are destroyed.
Function 00h Determine or Set Video State
entry AH 00h set video mode
AL display mode: CGA|PCjr|MDA|MCGA|EGA|VGA|8514
00h 40x25 B/W text CGA|PCjr| | |EGA| |
16 color 01h 40x25 color text CGA|PCjr| | |EGA| |
02h 80x25 B/W text CGA|PCjr| | |EGA| |
16 color 03h 80x25 color text CGA|PCjr| | |EGA|VGA|
4 color 04h 320x200 color graphics CGA|PCjr| | |EGA| |
4 tone gray 05h 320x200 B/W graphics CGA|PCjr| | |EGA| |
2 color 06h 640x200 B/W graphics CGA|PCjr| | |EGA| |
monochrome 07h 80x25 monochrome text | |MDA| |EGA| |
16 color 08h 160x200 color graphics CGA|PCjr| | | | |
16 color 09h 320x200 color graphics |PCjr| | | |VGA|
4 color 0Ah 640x200 color graphics |PCjr| | | | |
N/A 0Bh BIOS font load | | | |EGA|VGA|
N/A 0Ch BIOS font load | | | |EGA|VGA|
16 color 0Dh 320x200 graphics | | | |EGA|VGA|
16 color 0Eh 640x200 graphics | | | |EGA|VGA|
monochrome 0Fh 640x350 graphics | | | |EGA|VGA|
16 & 64 color 10h 640x350 color hi-res | | | |EGA|VGA|
2 color 11h 640x480 graphics | | |MCGA| |VGA|
16 color 12h 640x480 graphics | | | | |VGA|
256 color 13h 320x200 graphics | | |MCGA| |VGA|8514
14h-20h used by EGA and VGA graphics modes
18h 132x44 8x8 char mono | Tseng Labs EVA
19h 132x25 8x14 char mono | Tseng Labs EVA
1Ah 132x28 8x13 char mono | Tseng Labs EVA
monochrome 21h Hercules Graphics, Graphics Page 1
monochrome 22h Hercules Graphics, Graphics Page 2
22h 132x44 | Tseng, Ahead
23h 132x25 | Tseng Labs EVA
132x25 | Ahead Systems EGA2001
132x25 | ATI EGA Wonder
24h 132x28 | Tseng Labs EVA
25h 80x60 640x480 | Tseng Labs EVA
16 color 640x480 | VEGA VGA
26h 80x60 | Tseng Labs EVA
80x60 640x480 | Ahead Systems EGA2001
16 color 27h 720x512 | VEGA VGA
monochrome 132x25 | ATI EGA Wonder
28h unknown | VEGA VGA
16 color 29h 800x600 | VEGA VGA
256 color 2Dh 640x350 | VEGA VGA
256 color 2Eh 640x480 | VEGA VGA
256 color 2Fh 720x512 | VEGA VGA
256 color 30h 800x600 | VEGA VGA
unknown | AT&T 6300
16 color 36h 960x720 | VEGA VGA
16 color 37h 1024x768 | VEGA VGA
monochrome 132x44 | ATI EGA Wonder
2 color 40h 640x400 | AT&T 6300
80x43 | VEGA VGA
16 color 41h 640x200 | AT&T 6300
132x25 | VEGA VGA
16 color 42h 640x400 | AT&T 6300
132x43 | VEGA VGA
43h unsupported 640x200 of 640x400 viewport AT&T 6300
80x60 | VEGA VGA
44h disable VDC and DEB output | AT&T 6300
100x60 | VEGA VGA
48h 80x50 640x400 | AT&T 6300
4Dh 120x25 | VEGA VGA
4Eh 120x43 | VEGA VGA
4Fh 132x25 | VEGA VGA
monochrome 50h 132x25 | Ahead Systems EGA2001
16 color 640x480 | Paradise EGA-480
monochrome 80x43 | VEGA VGA
640x480 mono? | Taxan 565 EGA
51h 80x30 | Paradise EGA-480
monochrome 132x25 | VEGA VGA
640x480 ? | ATI EGA Wonder
monochrome 52h 132x44 | Ahead Systems EGA2001
monochrome 132x43 | VEGA VGA
752x410 ? | ATI EGA Wonder
53h 800x560 ? | ATI EGA Wonder
54h 132x43 | Paradise EGA-480
16 color 132x43 | Paradise VGA
16 color 132x43 | Paradise VGA on multisync
132x43 | Taxan 565 EGA
55h 132x25 | Paradise EGA-480
16 color 132x25 | Paradise VGA
16 color 132x25 | Paradise VGA on multisync
132x25 | Taxan 565 EGA
56h 132x43 | NSI Smart EGA+
132x43 | Paradise VGA
132x43 | Paradise VGA on multisync
monochrome 132x43 | Taxan 565 EGA
57h 132x25 | NSI Smart EGA+
132x25 | Paradise VGA
132x25 | Paradise VGA on multisync
monochrome 132x25 | Taxan 565 EGA
58h 100x75 800x600 16/256k | Paradise VGA
59h 100x75 800x600 | Paradise VGA
5Eh 640x400 | Paradise VGA,VEGA VGA
5Fh 640x480 | Paradise VGA
60h 80x??? ???x400 | Corona/Cordata BIOS v4.10+
752x410 | VEGA VGA
60h 400 line graphics+80 col text |(Corona/Cordata)
61h 400 line graphics | Corona/Cordata BIOS v4.10+
720x540 | VEGA VGA
62h 800x600 | VEGA VGA
16 color 71h 100x35 800x600 | NSI Smart EGA+
74h 640x400 graphics | Toshiba 3100
82h 80x25 B&W | AT&T VDC overlay mode *
83h 80x25 | AT&T VDC overlay mode *
86h 640x200 B&W | AT&T VDC overlay mode *
C0h 640x400 2/prog pallet | AT&T VDC overlay mode *
C4h disable output | AT&T VDC overlay mode *
D0h 640x400 | DEC VAXmate AT&T mode
unknown 640x225 | Z-100
unknown 640x400 | Z-100
note 1) If the high bit in AL is set, the display buffer is not cleared when a
new mode is selected. This may be used to mix modes on the display;
for example, characters of two difference sizes might be displayed
2) Modes 8-10 are available on the PCjr, Tandy 1000, and PS/2
3) IBM claims 100% software and hardware emulation of the CGA with the
MCGA chipset. All registers may be read and written as CGA. All
characters are double-scanned to give 80x25 with 400 line resolution.
The attributes for setting border color may be set on MCGA, but the
borders will remain the default color (they cannot actually be set)
4) The IBM Color Graphics Adapter (CGA) is too slow for the screen to
be updated before the vertical retrace of the monitor is completed.
If the video RAM is addressed directly, the screen will have "snow"
or interference. IBM's default is to turn the adapter off when it is
being updated, ie "flickering" when the display is scrolled.
5) The vertical retrace signal may be ignored when using the MCGA adapter.
The MCGA will not generate snow when written to. There is no flicker
with the MCGA.
6) The PCjr Video Gate Array uses a user-defined block of main system RAM
from 4 to 32k in size instead of having dedicated memory for the
display. Vertical retrace may be ignored when writing to the PCjr.
There is no flicker with the PCjr display.
7) The Hercules Graphics Card has 750x348 resolution
8) The Hercules Graphics Card takes 32k beginning at B:000 (same as MDA)
9) The CGA, MCGA, and VGA adapters use hardware address B:800
10) The BIOS clears the screen when the mode is set or reset.
11) For AT&T VDC overlay modes, BL contains the DEB mode, which may be 06h,
40h, or 44h
Function 01h Set Cursor Type - set the size of the cursor or turn it off
entry AH 01h
CH bit values:
bits 0-4 top line for cursor in character cell
bits 5-6 blink attribute
00 normal
01 invisible (no cursor)
10 slow (not used on original IBM PC)
11 fast
CL bit values:
bits 0-4 bottom line for cursor in character cell
return none
note 1) The ROM BIOS default cursors are: start end
monochrome mode 07h: 11 12
text modes 00h-03h: 6 7
2) The blinking in text mode is caused by hardware and cannot be turned
off, though some kludges can temporarily fake a nonblinking cursor
3) The cursor is automatically turned off in graphics mode
4) Another method of turning off the cursor in text mode is to position it
to a nondisplayable address, such as (X,Y)=(0,25)
5) Buggy on EGA systems - BIOS remaps cursor shape in 43 line modes, but
returns unmapped cursor shape
Function 02h Set Cursor Position - reposition the cursor to (X,Y)
entry AH 02h
BH video page
00h graphics mode
03h modes 2 and 3
07h modes 0 and 1
DH row (Y=0-24)
DL column (X=0-79 or 0-39)
return none
note 1) (0,0) is upper left corner of the screen
Function 03h Read Cursor Position - return the position of the cursor
entry AH 03h
BH page number
00h in graphics modes
03h in modes 2 & 3
07h in modes 0 & 1
return CH top line for cursor (bits 4-0)
CL bottom line for cursor (bits 4-0)
DH row number (Y=0-24)
DL column number (X=0-79 or 0-39)
Function 04h Read Light Pen - fetch light pen information
entry AH 04h
return AH 00h light pen not triggered
AH 01h light pen is triggered, values in resgisters
BX pixel column (X=0-319 or 0-639) graphics mode
CH raster line (Y=0-199) old graphics modes
CX (EGA) raster line (0-nnn) new graphics modes
DH row of current position (Y=0-24) text mode
DL column of current position (X=0-79 or 0-39) text mode
note Not supported on PS/2
Function 05h Select Active Page - set page number for services 6 and 7
entry AH 05h
AL number of new active page
0-7 modes 00h and 01h (CGA)
0-3 modes 02h and 03h (CGA)
0-7 modes 02h and 03h (EGA)
0-7 mode 0Dh (EGA)
0-3 mode 0Eh (EGA)
0-1 mode 0Fh (EGA)
0-1 mode 10h (EGA)
for PCjr only:
AL 80h to read CRT/CPU page registers
81h to set CPU page register to value in BL
82h to set CRT page register to value in BH
83h to set both CPU and page registers
(and Corona/Cordata BIOS v4.10+)
BH CRT page number for subfunctions 82h and 83h
BL CPU page register for subfunctions 81h and 83h
return standard PC none
PCjr if called with AH bit 7=1 then
BH CRT page register
BL CPU page register
DX segment of graphics bitmap buffer (video modes 60h,61h; AL0Fh)
note 1) Mono adapter has only one display page
2) CGA has four 80x25 text pages or eight 40x25 text pages
3) A separate cursor is maintained for each display page
4) Switching between pages does not affect their contents
5) Higher page numbers indicate higher memory positions
Function 06h Scroll Page Up - scroll up or clear a display "window"
entry AH 06h
AL number of lines blanked at bottom of page
0 = blank entire window
BH attributes to be used on blank line
CH row (Y) of upper left corner or window
CL column (X) of upper left corner of window
DH row (Y) of lower right corner of window
DL column (X) of lower right corner of window
return none
note 1) Push BP before scrolling, pop after
2) If in CGA text mode, affects current page only
Function 07h Scroll Page Down - scroll down or clear a display "window"
entry AH 07h
AL number of lines to be blanked at top of page
0 = blank entire window
BH attributes to be used on blank line
CH row (Y) of upper left corner or window
CL column (X) of upper left corner of window
DH row (Y) of lower right corner of window
DL column (X) of lower right corner of window
return none
note 1) Push BP before scrolling, pop after
2) If in CGA text mode, affects current page only
Function 08h Read Character Attribute - of character at current cursor pos.
entry AH 08h
BH display page number - text mode
return AH character attribute - text mode
AL ASCII code of character at current cursor position
Function 09h Write Character and Attribute - at current cursor position
entry AH 09h
AL ASCII code of character to display
BH display page number - text mode
BL attribute/color of character
CX number of characters to write
return none
note 1) CX should not exceed actual rows availible, or results may be erratic
2) Setting CX to zero will cause runaway
3) All values of AL result in some sort of display; the various control
characters are not recognized as special and do not change the current
cursor position
4) Does not change cursor position when called - the cursor must be
advanced with int 10 function 0Ah.
5) If used to write characters in graphics mode with bit 7 of AH set to 1
the character will by XORed with the current display contents.
6) In graphics mode the bit patterns for ASCII character codes 80h-0FFh
are obtained from a table. On the standard PC and AT, the location is at
interrupt vector 01Fh (0000:007C). For ASCII characters 00h-07Fh, the
table is at an address in ROM. On the PCjr the table is at interrupt
vector 44h (0000:00110) and is in addressable RAM (may be replaced by
the user)
7) All characters are displayed, including CR, LF, and BS
Function 0Ah Write Character - display character(s) (use current attribute)
entry AH 0Ah
AL ASCII code of character to display
BH display page - text mode
BL color of character (graphics mode, PCjr only)
CX number of times to write character
return none
note 1) CX should not exceed actual rows availible, or results may be erratic
2) All values of AL result in some sort of display; the various control
characters are not recognized as special and do not change the current
cursor position
3) If used to write characters in graphics mode with bit 7 of AH set to 1
the character will by XORed with the current display contents.
4) In graphics mode the bit patterns for ASCII character codes 80h-0FFh
are obtained from a table. On the standard PC and AT, the location is at
interrupt vector 01Fh (0000:007C). For ASCII characters 00h-07Fh, the
table is at an address in ROM. On the PCjr the table is at interrupt
vector 44h (0000:00110) and is in addressable RAM (may be replaced by
the user)
5) In EGA in graphics modes, replication count in CX works correctly only
if all characters written are contained on the same row
6) All characters are displayed, including CR, LF, and BS
Function 0Bh Set Color Palette - set palette for graphics or text border
entry AH 0Bh
BH 00h select border (text mode)
BL color 0-15, 16-31 for high-intensity characters
BH 01h set graphics palette with value in BL
(CGA) BL 0 green/red/yellow
1 cyan/magenta/white
(EGA) (graphics modes)
BH 0
BL has border color (0-15) & high intensity bkgr'd color (16-31)
BH 1
BL contains palette being selected (0-1)
return none
note 1) Valid in CGA mode 04h, PCjr modes 06h, 08h-0Ah
2) Although the registers in the MCGA may be set as if to change the
border, the MCGA will not display a border no matter what register
settings are used.
Function 0Ch Write Dot - plot one graphics pixel
entry AH 0Ch
AL dot color code (0/1 in mode 6, 0-3 in modes 4 and 5)
(set bit 7 to XOR the dot with current color)
0-3 mode 04h, 05h
0-1 mode 06h
BH page number (ignored if adapter supports only one page)
CX column (X=0000h - 027Fh)
(0 - 319 in modes 4,5,13, 0 - 639 in modes 6,14,15,16)
DX row (Y=0000h - 00C7h) (0 - 199 CGA)
return none
note Video graphics modes 4-6 only
Function 0Dh Read Dot - determine the color of one graphics pixel
entry AH 0Dh
CX column (X=0000h - 027Fh) (0-319 or 639)
DX row (Y=0000h - 00C7h) (0-199)
return AL color of dot
note Only valid in graphics mode
Function 0Eh Write TTY - write one character and update cursor. Also handles
CR (0Dh), beep (07h), backspace (10h), and scrolling
entry AH 0Eh
AL ASCII code of character to be written
BH page number (text)
BL foreground color (video modes 6 & 7 only) (graphics)
return none
note 1) The ASCII codes for bell, backspace, carriage return, and linefeed are
recognized and appropriate action taken. All other characters are
written to the screen and the cursor is advanced to the next position
2) Text can be written to any CGA page regardless of current active page
3) Automatic linewrap and scrolling are provided through this function
4) This is the function used by the DOS CON console driver.
5) This function does not explicitly allow the use of attributes to the
characters written. Attributes may be provided by first writing an ASCII
27h (blank) with the desired attributes using function 09h, then
overwriting with the actual character using this function. While clumsy
this allows use of the linewrap and scrolling services provided by
this function
Function 0Fh Return Current Video State - mode and size of the screen
entry AH 0Fh
return AH number of character columns on screen
AL mode currently set (see AH=00h for display mode codes)
BH current active display page
note If mode was set with bit 7 set ("no blanking"), the returned mode will
also have bit 7 set
Function 10h Set Palette Registers
PCjr, Tandy 1000, EGA, MCGA, VGA
entry AH 10h
AL 00h set individual palette register
01h set border color palette register
02h set all palette registers and overscan
03h toggle blink/intensity bit (EGA, MCGA, VGA)
04h unknown
05h unknown
06h unknown
07h read individual palette register (VGA)
08h read overscan (order color) (VGA)
09h read all palette registers and overscan register (VGA)
10h set individual video DAC color register (MCGA, VGA)
11h unknown
12h set block of video DAC color registers (MCGA, VGA)
13h set video DAC color page (VGA)
14h unknown
15h read individual video DAC color register (MCGA, VGA)
16h unknown
17h read block of video DAC color registers (MCGA, VGA)
18h unknown
19h unknown
1Ah read video DAC color-page state (VGA)
1Bh perform gray-scale summing (VGA)
BH color value
BL if AL=00h palette register to set (00h-0Fh)
if AL=03h 0 to enable intensity
1 to enable blinking
ES:DX if AL=02h pointer to 16-byte table of register values
followed by the overscan value:
bytes 0-15 values for palette registers 0-15
byte 16 value for border register
return none
note DAC is Digital to Analog Convertor circuit in MCGA/VGA chips
Function 11h Character Generator Routine (EGA and after)
entry AH 11h
The following functions will cause a mode set, completely
resetting the video environment, but without clearing the video
buffer.
AL 00h, 10h load user-specified patterns
ES:BP pointer to user table
CX count of patterns to store
DX character offset into map 2 block
BL block to load in map 2
BH number of bytes per character pattern
AL 01h, 11h load ROM monochrome patterns (8 by 14)
BL block to load
AL 02h, 12h load ROM 8 by 8 double-dot patterns
BL block to load
AL 03h set block specifier
BL block specifier
AL 04h load 8x16 text characters (MCGA, VGA)
AL 14h set 8x16 text characters (MCGA, VGA)
The routines called with AL=1x are designed to be called only
immediately after a mode set and are similar to the routines
called with AL=0x, except that:
Page 0 must be active.
Bytes/character is recalculated.
Max character rows is recalculated.
CRT buffer length is recalculated.
CRTC registers are reprogrammed as follows:
reg09h bytes/char-1; max scan line (mode 7 only)
reg0Ah bytes/char-2; cursor start
reg0Bh 0 ; cursor end
reg12h ((rows+1)*(bytes/char))-1
; vertical display end
reg14h bytes/char ; underline loc
(*** BUG: should be 1 less ***)
The following functions are meant to be called only after a
mode set:
AL 20h user 8 by 8 graphics characters (INT 1FH)
ES:BP = pointer to user table
AL 21h user graphics characters
ES:BP pointer to user table
CX bytes per character
BL row specifier
0 user set - DL = number of rows
1 14 rows
2 25 rows
3 43 rows
AL 22h ROM 8 by 14 set
BL row specifier
AL 23h ROM 8 by 8 double dot
BL row specifier
AL 24h load 8x16 graphics characters (MCGA, VGA)
AL 30h return information
BH pointer specifier
0 int 1Fh pointer
1 int 44h pointer
2 ROM 8 by 14 character font pointer
3 ROM 8 by 8 double dot font pointer
4 ROM 8 by 8 DD font (top half)
5 ROM text alternate (9 by 14) pointer
return ES:BP specified pointer value
CX bytes/character
DL character rows on screen
Function 12h Alternate Select (EGA and after)
entry AH 12h
AL 00h unknown
01h unknown
02h select 400 line mode (VGA)
BL 10h return EGA information
20h select alternate print screen routine
30h select vertical resolution for text modes (VGA)
31h enable/disable default palette loading (MCGA, VGA)
32h enable/disable video addressing (MCGA, VGA)
33h enable/disable default gray scale summing (MCGA, VGA)
34h enable/diable text cursor emulation (VGA)
35h display-switch interface
return BH 00h if color mode is in effect
01h if mono mode is in effect
BL 00h if 64k EGA memory
01h if 128k EGA memory
02h if 192k EGA memory
03h if 256k EGA memory
CH feature bits
CL switch settings
Function 13h Write String, Don't Move Cursor (AT, XT/286, PS/2)
entry AH 13h
AL 00h
BH display page number
BL attribute
CX length of string
DX starting cursor position
ES:BP pointer to start of string
return none
Function 13h Write String, Move Cursor (AT, XT/286, PS/2)
entry AH 13h
AL 01h
BH display page number
BL attribute
DX starting cursor position
CX length of string
ES:BP pointer to start of string
return none
Function 13h Write String of Alternating Characters and Attributes;
Don't Move Cursor (AT, XT/286, PS/2)
entry AH 13h
AL 02h
bit 0: set in order to move cursor after write
bit 1: set if string contains alternating chars and attributes
BH display page number
BL attribute if AL bit 1 clear
CX length of string
DH row of starting cursor position
DL column of starting cursor position
ES:BP pointer to start of string
return none
Function 13h Write String of Alternating Characters and Attributes;
Move Cursor (AT, XT/286, PS/2)
entry AH 13h
AL 03h
bit 0: set in order to move cursor after write
bit 1: set if string contains alternating characters and attributes
BH display page number
BL attribute if AL bit 1 clear
CX length of string
DH,DL row,column of starting cursor position
ES:BP pointer to start of string
return none
note Recognizes CR, LF, BS, and bell
Function 14h Load LCD Character Font (Convertible)
entry AH 14h
AL 00h load user specified font
BH number of bytes per character
BL 00h load main font (block 0)
01h load alternate font (block 1)
AL 01h load system ROM default font
BL 00h load main font (block 0)
01h load alternate font (block 1)
AL 02h set mapping of LCD high intensity attribute
BL 00h ignore high intensity attribute
01h map high intensity to underscore
02h map high intensity to reverse video
03h map high intensity to seleected alternate font
ES:DI pointer to character font
CX number of characters to store
DX character offset into RAM font area
Function 15h Return Physical Display Parameters (Convertible)
return AX Alternate display adapter type
ES:DI pointer to parameter table:
word # information
01h monitor model number
02h vertical pixels per meter
03h horizontal pixels per meter
04h total number of vertical pixels
05h total number of horizontal pixels
06h horizontal pixel separation in micrometers
07h vertical pixel separation in micrometers
Function 1Ah Display Combination Code (PS/2)
Using the compatibility BIOS of the PS/2 Models 50, 60, 80 there
is a way to determine which video controller and attached
display are on the system. The Display Combination Code (DCC)
is a Video BIOS function that provides the capability.
entry AH 1Ah
AL 00h read display combination code
01h write display combination code
return AL 1Ah indicates Compatibility BIOS is supported,
any other value is invalid
BH alternate display device
where:
00h no display
01h IBM monochrome display and printer adapter
02h IBM color/graphics monitor adapter
03h reserved
04h IBM EGA (color display)
05h IBM EGA (monochrome)
06h IBM PGA
07h VGA (analog monochrome display)
08h VGA (analog color display)
09h reserved
0Ah reserved
0Bh IBM PS/2 Model 30 (analog monochrome display)
0Ch IBM PS/2 Model 30 (analog color display)
BL active display device
Function 1Bh Functionality/State Information (MCGA, VGA)
entry AH 1Bh
return unknown
Function 1Ch Save/Restore Video State (VGA)
entry AH 1Ch
AL 00h return state buffer size
01h save video state
02h restore video state
return unknown
note VGA only
Function 70h Get Video RAM Address (Tandy 1000)
entry AH 70h
return AX segment addresses of the following
BX offset address of green plane
CX segment address of green plane
DX segment address of red/blue plane
note (red offset = 0, blue offset = 4000)
Function 71h Get INCRAM Addresses (Tandy 1000)
entry AH 71h
return AX segment address of the following
BX = segment address of INCRAM
CX = offset address of INCRAM
Function 72h Scroll Screen Right (Tandy 1000)
entry AH 72h
AL number of columns blanked at left of page
00h blank window
BH attributes to be used on blank columns
CH,CL row, column address of upper left corner
DH,DL row, column address of lower right corner
Function 73h Scroll Screen Left (Tandy 1000)
entry AH 73h
AL number of columns blanked at right of page
00h blank window
BH attributes to be used on blank columns
CH,CL row, column address of upper left corner
DH,DL row, column address of lower right corner
Function 81h DESQview video - Get something?
entry AH 81h
DX 4456h ('DV')
return ES segment of DESQview data structure for video buffer
byte ES:[0] = current window number
note This function is probably meant for internal use only, due to the
magic value required in DX
Function 82h DESQview - Get Current Window Info
entry AH 82h
DX 4456h ('DV')
return AH unknown
AL current window number
BH unknown
BL direct screen writes
0 program does not do direct writes
1 program does direct writes, so shadow buffer not usable
CH unknown
CL current video mode
DS segment in DESQview for data structure
in DV 2.00,
byte DS:[0] = window number
word DS:[1] = segment of other data structure
word DS:[3] = segment of window's object handle
ES segment of DESQview data structure for video buffer
note This function is probably meant for internal use only, due to the magic
value required in DX
Function 0F0h Microsoft Mouse driver EGA support - Read One Register
entry AH 0F0h
BL register number
DX group index
Pointer/data chips
00h CRT Controller (25 reg) 3B4h mono modes, 3D4h color modes
08h Sequencer (5 registers) 3C4h
10h Graphics Controller (9 registers) 3CEh
18h Attribute Controller (20 registers) 3C0h
Single registers
20h Miscellaneous Output register 3C2h
28h Feature Control register (3BAh mono modes, 3DAh color modes)
30h Graphics 1 Position register 3CCh
38h Graphics 2 Position register 3CAh
return BL data
Function 0F1h Microsoft Mouse driver EGA support - Write One Register
entry AH 0F1h
DX group index (see function F0h)
BL register number
BH value to write
return BL data
Function 0F2h Microsoft Mouse driver EGA support - Read Register Range
entry AH 0F2h
CH starting register number
CL number of registers (>1)
DX group index
00h CRTC (3B4h mono modes, 3D4h color modes)
08h Sequencer 3C4h
10h Graphics Controller 3CEh
18h Attribute Controller 3C0h
ES:BX pointer to buffer, CL bytes
Function 0F3h Microsoft Mouse driver EGA support - Write Register Range
entry AH 0F3h
CH starting register
CL number of registers (>1)
DX group index
00h CRTC (3B4h mono modes, 3D4h color modes)
08h Sequencer 3C4h
10h Graphics Controller 3CEh
18h Attribute Controller 3C0h
ES:BX pointer to buffer, CL bytes
Function 0F4h Microsoft Mouse driver EGA support - Read Register Set
entry AH 0F4h
CX number of registers (>1)
ES:BX pointer to table of records in this format:
bytes 1-2 group index
Pointer/data chips
00h CRTC (3B4h mono modes, 3D4h color modes)
08h Sequencer 3C4h
10h Graphics Controller 3CEh
18h Attribute Controller 3C0h
Single registers
20h Miscellaneous Output register 3C2h
28h Feature Control register (3BAh mono modes, 3DAh color)
30h Graphics 1 Position register 3CCh
38h Graphics 2 Position register 3CAh
byte 3 register number (0 for single registers)
byte 4 register value
Function 0F5h Microsoft Mouse driver EGA support - Read Register Set
entry AH 0F5h
CX number of registers (>1)
ES:BX pointer to table of records in this format:
bytes 1-2 port number
Pointer/data chips
00h CRTC (3B4h mono modes, 3D4h color modes)
08h Sequencer 3C4h
10h Graphics Controller 3CEh
18h Attribute Controller 3C0h
Single registers
20h Miscellaneous Output register 3C2h
28h Feature Control register (3BAh mono modes, 3DAh color)
30h Graphics 1 Position register 3CCh
38h Graphics 2 Position register 3CAh
byte 3 register number (0 for single registers)
byte 4 register value
Function 0F6h Microsoft Mouse driver EGA support
Revert to Default Registers
entry AH 0F6h
return unknown
Function 0F7h Microsoft Mouse driver EGA support
Define Default Register Table
entry AH 0F7h
DX port number
Pointer/data chips
00h CRTC (3B4h mono modes, 3D4h color modes)
08h Sequencer 3C4h
10h Graphics Controller 3CEh
18h Attribute Controller 3C0h
Single registers
20h Miscellaneous Output register 3C2h
28h Feature Control register (3BAh mono modes, 3DAh color modes)
30h Graphics 1 Position register 3CCh
38h Graphics 2 Position register 3CAh
ES:BX address of table of one byte entries, one byte
to be written to each register
Function 0FAh Microsoft Mouse driver EGA support - Interrogate Driver
entry AH 0FAh
BX 00h
return BX 00h if mouse driver not present
ES:BX pointer to EGA Register Interface version number, if present:
byte 1 major release number
byte 2 minor release number
Function 0FEh Get Alternate Screen Buffer Address (text mode only)
(Topview/DesQview/Taskview)
entry AH 0FEh
ES:DI segment:offset of assumed video buffer
return: ES:DI segment:offset of actual video buffer
note 1) This alternate video buffer can be written to directly, in the same
manner as writing to B:000 or B:800. The MT program will manage the
actual display.
2) There is no need to synchronize vertical retrace when writing to the
alternate buffer; this is managed by the MT program
3) If TopView or DESQview is not running, ES:DI is returned unchanged.
4) TopView requires that function 0FFh be called every time you write into
the buffer to tell TopView that something changed
Function 0FFh Update Real Display (text mode only) (TopView)
Update Video Buffer (Topview/DesQview/Taskview)
entry AH 0FFh
CX number of sequential characters that have been modified
DI offset of first character that has been modified
ES segment of video buffer
return unknown
note 1) DesQview supports this call, but does not require it
2) Avoid CX=0
Interrupt 11h Equipment Check
(0:0044h) fetch a code describing active peripherals.
entry AH 11h
return AX Equipment listing word Bits are:
0 number of floppy drives
0 no drives
1 bootable diskette installed
1 math chip
0 no math coprocessor (80x87) present
1 math coprocessor (80x87) present
(PS/2) 2 0 mouse not installed
1 mouse installed
(PC) 2,3 system board RAM
0,0 16k (PC-0, PC-1)
1,1 64k (PC-2, XT)
note 1) not commonly used. Set both bits to 1
2) both bits always 1 in AT
4,5 initial video mode
0,0 no video installed (use with dumb terminal)
0,1 40x25 color (CGA)
1,0 80x25 color (CGA)
1,1 80x25 monochrome (MDA or Hercules)
6,7 number of diskette drives (only if bit 0 1)
0,0 1 drives
0,1 2 drives
1,0 3 drives
1,1 4 drives
8 0 DMA present
1 no DMA on system (PCjr, some Tandy 1000s)
9,A,B number of RS232 serial ports (0-3)
0,0,0 none
0,0,1 1
0,1,0 2
0,1,1 3
1,0,0 4
C 0 no game I/O attached
1 game I/O attached (default for PCjr)
D serial accessory installation
0 no serial accessories installed
1 Convertible - internal modem installed
1 PCjr - serial printer attached
E,F number of parallel printers
0,0 none
0,1 one (LPT1, PRN)
1,0 two (LPT2)
1,1 three (LPT3)
note Models before PS/2 would allow a fourth
parallel printer. Remapping of the BIOS in the
PS/2s does not allow the use of LPT4.
Interrupt 12h Memory Size
(0:0048h) get system memory
return AX number of contiguous 1K RAM blocks
note 1) This service does not depend on the setting of the motherboard switches
2) This is the same value stored in absolute address 04:13h
Interrupt 13h Disk I/O - access the disk drives (floppy and hard disk)
(0:004Ch) does not try rereading disk if an error is returned
Function 00h Reset - reset the disk controller chip
entry AH 00h
DL drive (if bit 7 is set both hard disks and floppy disks reset)
return AH status
note 1) Forces controller chip to recalibrate read/write heads
2) Some systems (Sanyo 55x) this resets all drives
Function 01h Get Status of disk system
entry AH 01h
DL drive (hard disk if bit 7 set)
return AL status of most recent operation
00h successful completion
01h bad command
02h address mark not found
03h tried to write on write-protected disk
04h sector not found
05h reset failed (hard disk)
06h diskette removed or changed
07h bad parameter table (hard disk)
08h DMA overrun
09h attempt to DMA across 64K boundary
0Ah bad sector detected (hard disk)
0Bh bad track detected (hard disk)
0Ch unsupported track
0Dh invalid number of sectors on format (hard disk)
0Eh control data address mark detected (hard disk)
0Fh DMA arbitration error (hard disk)
10h bad CRC/EEC on read
11h data ECC corrected
20h controller failure
40h seek failed
80h timeout
0AAh drive not ready (hard disk)
0BBh undefined error (hard disk)
0CCh write fault (hard disk)
0E0h status error (hard disk)
0FFh sense operation failed (hard disk)
Function 02h Read Sectors - read one or more sectors from diskette
entry AH 02h
AL number of sectors to read
BX address of buffer (ES=segment)
CH track number (0-39 or 0-79 for floppies)
(for hard disk, bits 8,9 in high bits of CL)
CL sector number (1 to 18, not value checked)
DH head number (0 or 1)
DL drive (0=A, 1=B, etc.) (bit 7=0) (drive 0-7)
ES:BX address to store/fetch data (buffer to fill)
[0000:0078] dword pointer to diskette parms
return CF clear (0) for successful
set (1) failure
AH status (00h, 02h, 03h, 04h, 08h, 09h, 10h, 0Ah, 20h,
40h, 80h)
AL number of sectors transferred
note 1) Number of sectors begins with 1, not 0
2) Trying to read zero sectors is considered a programming error; results
are not defined
Function 03h Write Sectors - write from memory to disk
entry AH 03h
AL number of sectors to write (1-8)
CH track number (for hard disk, bits 8,9 in high bits of CL)
CL beginning sector number
(if hard disk, high two bits are high bits of track #)
DH head number
DL drive number (0-7)
ES:BX address of buffer for data
return CF set if error
AH status (see above)
AL number of sectors written
note 1) Number of sectors begins with 1, not 0
2) Trying to write zero sectors is considered a programming error; results
are not defined
Function 04h Verify - verify that a write operation was successful
entry AH 04h
AL number of sectors to verify (1-8)
CH track number (for hard disk, bits 8,9 in high bits of CL)
CL beginning sector number
DH head number
DL drive number (0-7)
return CF set on error
AH status (see above)
AL number of sectors verified
Function 05h Format Track - write sector ID bytes for 1 track
entry AH 05h
AL number of sectors to create on this track
CH track (or cylinder) number
CL sector number
DH head number (0, 1)
DL drive number (0-3)
ES:BX pointer to 4-byte address field (C-H-R-N)
byte 1 = (C) cylinder or track
byte 2 = (H) head
byte 3 = (R) sector
byte 4 = (N) bytes/sector (0 = 128, 1 = 256, 2 = 512, 3 = 1024)
return CF set if error occurred
AH status code (see above)
note Not valid for ESDI hard disks on PS/2
Function 06h Hard Disk - format track and set bad sector flags
(PC2, PC-XT, and Portable)
entry AH 06h
AL interleave value (XT only)
CH cylinder number (bits 8,9 in high bits of CL)
CL sector number
DH head
DL drive
ES:BX 512 byte format buffer
the first 2*(sectors/track) bytes contain f,n for each sector
f 00Fh for good sector
80h for bad sector
n sector number
return AH status code
Function 07h Hard Disk - format the drive starting at the desired track
(PC2, PC-XT and Portable)
entry AH 07h
AL interleave value (XT only) (01h-10h)
CH cylinder number (bits 8,9 in high bits of CL) (00h-03FFh)
CL sector number
DH head number (0-7)
DL drive number (80h-87h, 80h=C, 81h=D,...)
ES:BX format buffer, size = 512 bytes
the first 2*(sectors/track) bytes contain f,n for each sector
f=00h for good sector
80h for bad sector
n=sector number
return AH status code (see above)
Function 08h Read Drive Parameters (XT, CONV, AT, XT/286, PS/2)
entry AH 08h
DL drive number (0-2)
return CF set on error
AH status code (see above)
BL drive type (see AH=17h below) (AT/PS2 floppies only)
CH maximum useable value for cylinder number
CL maximum useable value for sector number or cylinder number
DH maximum usable value for head number
DL number of consecutive acknowledging drives (0-2)
ES:DI drive parameter table
Function 09h Initialize Two Fixed Disk Base Tables (XT, AT, XT/286, PS/2)
(install nonstandard drive)
entry AH 09h
return CF set on error
AH status code (see above)
data block definitions:
+0 maximum number of cylinders (dw)
+2 maximum number of heads (db)
+3 starting reduced write current cylinder (dw - XT only)
+5 starting write precomp cylinder (dw)
+7 maximum ECC data burst length (db - XT only)
+8 control byte: Bits
0,1,2 ? drive option
3,4,5 - always zero
6 - disable ECC retries
7 - disable access retries
note 1) Int 41h points to table for drive 0
2) Int 46h points to table for drive 1
3) 41h used by XT, 41h and 46h used by AT
Function 0Ah Read Long (Hard disk) (XT, AT, XT/286, PS/2)
entry AH 0Ah
CH cylinder number (bits 8,9 in high bits of CL)
CL sector number
DL drive ID
DH head number
ES:BX pointer to buffer to fill
return CF set on error
AH status code (see above)
AL number of sectors actually transferred
note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code
2) Used for diagnostics only on PS/2 systems
Function 0Bh Write Long (XT, AT, XT/286, PS/2)
entry AH 0Bh
CH cylinder (bits 8,9 in high bits of CL)
CL sector number
DH head number
DL drive ID
ES:BX pointer to buffer containing data
return CF set on error
AH status code (see above)
AL number of sectors actually transferred
note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code
2) Used for diagnostics only on PS/2 systems
Function 0Ch Seek To Cylinder (except PC, PCjr)
entry AH 0Ch
CH cylinder number (bits 8,9 in high bits of CL)
DH head number
DL drive ID
return CF set on error
AH status code (see above)
note 1) Positions heads over a particular cylinder
Function 0Dh Alternate Disk Reset (except PC, PCjr)
entry AH 0Dh
DL drive ID
return CF set on error
AH status code (see above)
note Not for PS/2 ESDI hard disks
Function 0Eh Read Sector Buffer (XT, Portable PS/2)
entry AH 0Eh
AL number of sectors
CH cylinder (bits 8,9 in top two bits of CL)
CL sector number
DH head number
DL drive number
ES:BX pointer to buffer
return CF set on error
AH status code (see above)
AL number of sectors actually transferred
note 1) Transfers controller's sector buffer. No data is read from the drive
2) Used for diagnostics only on PS/2 systems
Function 0Fh Write sector buffer (XT, Portable)
entry AH 0Fh
AL number of sectors
CH cylinder (bits 8,9 in top two bits of CL)
CL sector number
DH head number
DL drive number
ES:BX pointer to buffer
return CF set if error
AH status code
AL number of sectors actually transferred
note 1) Should be called before formatting to initialize the controller's
sector buffer.
2) Used for diagnostics only on PS/2 systems
Function 10h Test For Drive Ready
entry AH 10h
DL drive ID
return CF set on error
AH status code (see above)
Function 11h Recalibrate Drive
entry AH 11h
DL drive ID
return CF set on error
AH status code (see above)
Function 12h Controller RAM Diagnostic (XT, Portable, PS/2)
entry AH 12h
return CF set on error
AH status code (see AH=1 above)
note Used for diagnostics only on PS/2 systems
Function 13h Drive Diagnostic (XT, Portable)
entry AH 13h
return CF set on error
AH status code (see above)
note Used for diagnostics only on PS/2 systems
Function 14h Controller Internal Diagnostic (AT, XT/286)
entry AH 14h
return CF set on error
AH status code (see above)
note 1) OEM is Western Digital 1003-WA2 hard/floppy combination controller
in AT and XT/286.
2) Used for diagnostics only in PS/2 systems
Function 15h Get Disk Type (except PC and XT)
entry AH 15h
DL drive ID
return AH disk type
00h no drive is present
01h diskette, no change detection present
02h diskette, change detection present
03h fixed disk
CX:DX number of 512-byte sectors when AH = 03h
Function 16h Change of Disk Status (diskette) (except PC and XT)
entry AH 16h
return AH disk change status
00h no disk change
01h disk changed
DL drive that had disk change
Function 17h Set Disk Type for Format (diskette) (except PC and XT)
entry AH 17h
AL 00h no disk
01h 360kb diskette in 360Kb drive
02h 360kb diskette in 1.2M drive
03h 1.2M diskette in 1.2M drive
04h 720kb diskette in 720Kb drive
DL drive number
return AH status of operation
note This function is probably enhanced for the PS/2 series to detect
1.44 in 1.44 and 720k in 1.44.
Function 18h Set Media Type For Format (diskette) (AT, XT/286, PS/2)
entry AH 18h
CH lower 8 bits of number of tracks
CL high 2 bits of number of tracks (6,7) sectors per track
(bits 0-5)
DL drive number
return AH 00h if requested combination supported
01h if function not available
0Ch if not suppported or drive type unknown
80h if there is no media in the drive
ES:DI pointer to 11-byte parm table
Function 19h Park Hard Disk Heads (XT/286, PS/2)
entry AH 19h
DL drive
return CF set on error
AH error code
Function 1Ah ESDI Hard Disk - Format (PS/2)
entry AH 1Ah
AL defect table count
CL format modifiers
bit 0: ignore primary defect map
bit 1: ignore secondary defect map
bit 2: update secondary defect map
bit 3: perform surface analysis
bit 4: generate periodic interrupt
DL drive
ES:BX pointer to defect table
return CF set on error
AH status (see AH=1 above)
note If periodic interrupt selected, int 15h/AH=0Fh is called after each
cylinder is formatted
Interrupt 14h Initialize and Access Serial Port For Int 14
(0:0050h) the following status is defined:
serial status byte:
bits 0 delta clear to send
1 delta data set ready
2 trailing edge ring detector
3 delta receive line signal det.
4 clear to send
5 data set ready
6 ring indicator
7 receive line signal detect
line status byte:
bits 0 data ready
1 overrun error
2 parity error
3 framing error
4 break detect
5 transmit holding reg. empty
6 transmit shift register empty
7 time out note: if bit 7 set then other bits are invalid
All routines have AH=function number and DX=RS232 card number (0 based).
AL=character to send or received character on exit, unless otherwise noted.
entry AH 00h Initialize And Access Serial Communications Port
bit pattern: BBBPPSLL
BBB = baud rate: 110,150,300,600,1200,2400,4800,9600
PP = parity: 01 = odd, 11 = even
S = stop bits: 0 = 1, 1 = 2
LL = word length: 10 = 7-bits, 11 = 8-bits
AL parms for initialization:
bit pattern:
0 word length
1 word length
2 stop bits
3 parity
4 parity
5 baud rate
6 baud rate
7 baud rate
word length 10 7 bits
11 8 bits
stop bits 0 1 stop bit
1 2 stop bits
parity 00 none
01 odd
11 even
baud rate 000 110 baud
001 150 baud
010 300 baud
011 600 baud
100 1200 baud
101 2400 baud
110 4800 baud
111 9600 baud (4800 on PCjr)
DX port number
return AH line status
AL modem status
Function 01h Send Character in AL to Comm Port DX (0 or 1)
entry AH 01h
AL character
DX port number (0 or 1)
return AH RS232 status code
bit 0 data ready
1 overrun error
2 parity error
3 framing error
4 break detected
5 transmission buffer register empty
6 transmission shift register empty
7 timeout
AL modem status
bit
0 delta clear-to-send
1 delta data-set-ready
2 trailing edge ring detected
3 change, receive line signal detected
4 clear-to-send
5 data-set-ready
6 ring received
7 receive line signal detected
Function 02h Wait For A Character From Comm Port DX
entry AH 02h
return AL character received
AH error code (see above)(00h for no error)
Function 03h Fetch the Status of Comm Port DX (0 or 1)
entry AH 03h
return AH set bits (01h) indicate comm-line status
bit 7 timeout
bit 6 empty transmit shift register
bit 5 empty transmit holding register
bit 4 break detected ("long-space")
bit 3 framing error
bit 2 parity error
bit 1 overrun error
bit 0 data ready
AL set bits indicate modem status
bit 7 received line signal detect
bit 6 ring indicator
bit 5 data set ready
bit 4 clear to send
bit 3 delta receive line signal detect
bit 2 trailing edge ring detector
bit 1 delta data set ready
bit 0 delta clear to send
Function 04h Extended Initialize (PC Convertible)
entry AH 04h
AL break status
01h if break
00h if no break
BH parity
00h no parity
01h odd parity
02h even parity
03h stick parity odd
04h stick parity even
BL number of stop bits
00h one stop bit
01h 2 stop bits (1? if 5 bit word length)
CH word length
00h 5 bits
01h 6 bits
02h 7 bits
03h 8 bits
CL baud rate
00h 110
01h 150
02h 300
03h 600
04h 1200
05h 2400
06h 4800
07h 9600
08h 19200
return AL modem status
AH line control status
Function 05h Extended Communication Port Control (PS/2)
entry AH 05h
AL 00h read modem control register
return BL modem control reg (see AL=1)
AL 01h write modem control register
BL modem control register: (for AL=00 and AL=01) bits
0 data terminal ready
1 request to send
2 out1
3 out2
4 loop
5,6,7 reserved
return AH status
Interrupt 15h Cassette I/O
(0:0054h) Renamed "System Services" on PS/2 line
Function 00h Turn Cassette Motor On (PC, PCjr only)
entry AH 00h
return AH 86h no cassette present
CF set on error
note NOP for systems where cassette not supported
Function 01h Turn Cassette Motor Off (PC, PCjr only)
entry AH 01h
return AH 86h no cassette present
CF set on error
note NOP for systems where cassette not supported
Function 02h Read Blocks From Cassette (PC, PCjr only)
entry AH 02h
CX count of bytes to read
ES:BX pointer to data buffer
return CF set on error
AH error code
01h CRC error
02h bad tape signals
03h no data found on tape
04h no data
80h invalid command
86h no cassette present
DX count of bytes actually read
ES:BX pointer past last byte written
note 1) NOP for systems where cassette not supported
2) Cassette operations normally read 256 byte blocks
Function 03h Write Data Blocks to Cassette (PC, PCjr only)
entry AH 03h
CX count of bytes to write
ES:BX pointer to data buffer
return CF set on error
AH error code (see 02h)
CX 0
ES:BX pointer to last byte written+1
note 1) NOP for systems where cassette not supported
2) The last block is padded to 256 bytes with zeroes if needed
3) No errors are returned by this service
Function 0Fh ESDI Format Unit Periodic Interrupt (PS/2 50, 60, 80)
entry AH 0Fh
AL phase code
00h reserved
01h surface analysis
02h formatting
return CF clear if formatting should continue
set if it should terminate
note Called during ESDI drive formatting after each cylinder is completed
Function 10h TopView API Function Calls (TopView)
entry AX 00h PAUSE Give Up CPU Time
return 00h after other processes run
01h GETMEM allocate "system" memory
BX number of bytes to allocate
return ES:DI pointer to block of memory
02h PUTMEM deallocate "system" memory
ES:DI pointer to previously allocated block
return block freed
03h PRINTC display character/attribute on screen
BH attribute
BL character
DX segment of object handle for window
note BX=0 does not display anything, it only
positions the hardware cursor
10h unknown
AL 04h thru 12h
return TopView - unimplemented in DV 2.0x
pops up "Programming error" window in DV 2.0x
11h unknown
12h unknown
13h GETBIT define a 2nd-level interrupt handler
ES:DI pointer to FAR service routine
return BX bit mask indicating which bit was
allocated
0 if no more bits available
14h FREEBIT undefine a 2nd-level interrupt handler
BX bit mask from int 15/AH 13h
15h SETBIT schedule one or more 2nd-level interrupts
BX bit mask for interrupts to post
return indicated routines will be called at next ???
16h ISOBJ verify object handle
ES:DI possible object handle
return BX -1 if ES:DI is a valid object handle
0 if ES:DI is not
17h TopView - unimplemented in DV 2.00
return pops up "Programming Error" window in DV 2.00
18h LOCATE Find Window at a Given Screen Location
BH column
BL row
ES segment of object handle for ???
(0 = use default)
return ES segment of object handle for window
which is visible at the indicated
position
19h SOUND Make Tone
BX frequency in Hertz
CX duration in clock ticks (18.2 ticks/sec)
return immediately, tone continues to completion
note If another tone is already playing, the new tone
does not start until completion of the previous
one. In DV 2.00, it is possible to enqueue
about 32 tones before the process is blocked
until a note completes.
In DV 2.00, the lowest tone allowed is 20 Hz
1Ah OSTACK Switch to Task's Internal Stack
return stack switched
1Bh BEGINC Begin Critical Region
return task-switching temporarily disabled
note Will not task-switch until END CRITICAL REGION
(AH=1Ch) is called
1Ch ENDC End Critical Region
return task-switching enabled
1Dh STOP STOP TASK
ES segment of object handle for task to be stopped
(== handle of main window for that task)
return indicated task will no longer get CPU time
note At least in DV 2.00, this function is ignored
unless the indicated task is the current task.
1Eh START Start Task
ES segment of object handle for task to be started
(== handle of main window for that task)
return Indicated task is started up again
1Fh DISPEROR Pop-Up Error Window
BX bit fields:
0-12 number of characters to display
13,14 which mouse button may be pressed to
remove window
00 either
01 left
10 right
11 either
15 beep if 1
DS:DI pointer to text of message
CH width of error window (0 = default)
CL height of error window (0 = default)
DX segment of object handle
return BX status:
1 left button pressed
2 right button pressed
27 ESC key pressed
note Window remains on-screen until ESC or indicated
mouse button is pressed
20h TopView - unimplemented in DV 2.0x
return pops up "Programming Error" window in DV 2.0x
21h PGMINT Interrupt Another Task
BX segment of object handle for task to interrupt
DX:CX address of FAR routine to jump to next time
task is run
return nothing?
note The current ES, DS, SI, DI, and BP are passed
to the FAR routine
22h GETVER Get Version
BX 00h
return BX nonzero, TopView or compatible loaded
BH minor version
BL major version
notes TaskView returns BX = 0001h
DESQview 2.0 returns BX = 0A01h
23h POSWIN Position Window
BX segment of object handle for parent window
within which to position the window (0 = full
screen)
CH # columns to offset from position in DL
CL # rows to offset from position in DL
DL bit flags
0,1 horizontal position
00 current
01 center
10 left
11 right
2,3 vertical position
00 current
01 center
10 top
11 bottom
4 don't redraw screen if set
5-7 not used
ES segment of object handle for window to be
positioned
return nothing
24h GETBUF Get Virtual Screen Information
BX segment of object handle for window (0=default)
return CX size of virtual screen in bytes
DL 0 or 1, unknown
ES:DI address of virtual screen
25h USTACK Switch Back to User's Stack
return stack switched back
note Call only after int 15h,fn1Ah
26h
thru 2Ah DesQview (TopView?) - unimplemented in DV 2.0x
return pops up "Programming Error" window in DV 2.0x
2Bh POSTTASK Awaken Task DesQview 2.0 (TopView?)
BX segment of object handle for task
return nothing
2Ch Start New Application in New Process
DesQview 2.0 (TopView?)
ES:DI pointer to contents of .PIF/.DVP file
BX size of .PIF/.DVP info
return BX segment of object handle for new task
2Dh Keyboard Mouse Control DesQview 2.0
BL subfunction
00h determine whether using keyboard mouse
01h turn keyboard mouse on
02h turn keyboard mouse off
return (calling BL was 00h)
BL 0 using real mouse
1 using keyboard mouse
Function 20h PRINT.COM (DOS internal) (AT, XT-286, PS/2 50+)
entry AH 20h
AL subfunction
00h unknown (PRINT)
01h unknown (PRINT)
10h sets up SysReq routine on AT, XT/286, PS/2
11h completion of SysReq routine (software only)
note 1) AL=0 or 1 sets or resets some flags which affect what PRINT does when
it tries to access the disk
Function 21h Power-On Self Test (POST) Error Log (PS/2 50+)
entry AH 21h
AL 00h read POST log
01h write POST log
BH device ID
BL error code
return CF set on error
AH status
00h OK
01h list full
80h invalid cmd
86h unsupported
if function 00h:
BX number of error codes stored
ES:DI pointer to error log
note: The log is a series of words, the first byte of which identifies the
error code and the second the device.
Function 40h Read/Modify Profiles (Convertible)
entry AH 40h
AL 00h read system profile in CX,BX
01h write system profile from CX, BX
02h read internal modem profile in BX
03h write internal modem profile from BX
BX profile info
return BX internal modem profile (from 02h)
CX,BX system profile (from 00h)
Function 41h Wait On External Event (Convertible)
entry AH 41h
AL condition type
bits 0-2: condition to wait for
0 any external event
1 compare and return if equal
2 compare and return if not equal
3 test and return if not zero
4 test and return if zero
bit 3: reserved
bit 4: 1=port address, 0=user byte
bits 5-7: reserved
BH condition compare or mask value
condition codes:
0 any external event
1 compare and return if equal
2 compare and return if not equal
3 test and return if not zero
4 test and return if zero
BL timeout value times 55 milliseconds
0 if no time limit
DX I/O port address (if AL bit 4 = 1)
ES:DI pointer to user byte (if AL bit 4 = 0)
Function 42h Request System Power Off (Convertible)
entry AH 42h
AL 00h to use system profile
01h to force suspend regardless of profile
Function 43h Read System Status (Convertible)
entry AH 43h
return AL status bits:
0 LCD detached
1 reserved
2 RS232/parallel powered on
3 internal modem powered on
4 power activated by alarm
5 standby power lost
6 external power in use
7 battery low
Function 44h (De)activate Internal Modem Power (Convertible)
entry AH 44h
AL 00h to power off
01h to power on
Function 4Fh Keyboard Intercept (except PC, PCjr, and XT)
entry AH 4Fh
AL scan code, CF set
return AL scan code, CF set if processing desired
note Called by int 9 handler to translate scan codes
Function 80h Device Open (AT, XT/286, PS/2)
entry AH 80h
BX device ID
CX process ID
return CF set on error
AH status
Function 81h Device Close (AT, XT/286, PS/2)
entry AH 81h
BX device ID
CX process ID
return CF set on error
AH status
Function 82h Program Termination (AT, XT/286, PS/2)
AH 82h
BX device ID
return: CF set on error
AH status
note Closes all devices opened with function 80h
Function 83h Event Wait (AT, XT/286, Convertible, PS/2)
entry AH 83h
AL 00h to set interval
10h to cancel
CX,DX number of microseconds to wait (granularity is 976 microseconds)
ES:BX pointer to memory flag (bit 7 is set when interval expires)
(pointer is to caller's memory)
return CF set (1) if function already busy
Function 84h Read Joystick Input Settings (AT, XT/286, PS/2)
entry AH 84h
DX 00h to read the current switch settings (return in AL)
01h to read the resistive inputs
return AX A(X) value
BX A(Y) value
CX B(X) value
DX B(Y) value
AL switch settings (bits 7-4)
Function 85h System Request (SysReq) Key Pressed (except PC, PCjr, XT)
entry AH 85h
return AL 00h key pressed
01h key released
note Called by keyboard decode routine
Function 86h Elapsed Time Wait (except PC, PCjr, XT)
AH 86h
CX,DX number of microseconds to wait
return CF clear after wait elapses
CF set immediately due to error
note Only accurate to 977 microseconds
Function 87h Extended Memory Block Move (286/386 machines only)
AH 87h
CX number of words to move
ES:SI pointer to Global Descriptor Table (GDT)
offset 00h null descriptor
08h uninitialized, will be made into GDT descriptor
10h descriptor for source of move
18h descriptor for destination of move
20h uninitialized, used by BIOS
28h uninitialized, will be made into SS descriptor
return CF set on error
AH status
00h source copied into destination
01h parity error
02h interrupt error
03h address line 20 gating failed
Function 88h Extended Memory Size Determine (AT, XT/286, PS/2)
entry AH 88h
return AX # of contiguous 1K blocks of memory starting at address 1024k
Function 89h Switch Processor to Protected Mode (AT, XT/286, PS/2)
entry AH 89h
BH interrupt number of IRQ 8 (IRQ 9Fh use next 7 interrupts)
BL interrupt number of IRQ 0 (IRQ 17h use next 7 interrupts)
CX offset into protected mode CS to jump to
DS:SI pointer to Global Descriptor Table for protected mode
offset 00h null descriptor
08h GDT descriptor
10h IDT descriptor
18h DS
20h ES
28h SS
30h CS
38h uninitialized, used to build descriptor for
BIOS CS
return AH 0FFh error enabling address line 20
CF set on error
Function 90h Device Busy Loop (except PC, PCjr, XT)
entry AH 90h
AL type code:
00h disk
01h diskette
02h keyboard
03h PS/2 pointing device
80h network (ES:BX = ncb)
0FCh disk reset
0FDh diskette motor start
0FEh printer
ES:BX pointer to request block for type codes 80h through 0BFh
return CF 1 (set) if wait time satisfied
0 (clear) if driver must perform wait
note Used by NETBIOS
Type codes are allocated as follows:
00h-7Fh non-reentrant devices; OS must arbitrate access
80h-BFh reentrant devices; ES:BX points to a unique control block
C0h-FFh wait-only calls, no complementary int 15,fn91h call
Function 91h Set Flag and Complete Interrupt (except PC, PCjr, XT)
entry AH 91h
AL type code (see AH=90h above)
ES:BX pointer to request block for type codes 80h through 0BFh
return AH 0
note Used by NETBIOS
Function 0C0h Get System Configuration (XT after 1/10/86, PC Convertible,
XT/286, AT, PS/2)
entry AH 0C0h
return CF 1 if BIOS doesn't support call
ES:BX pointer to ROM system descriptor table
dword number of bytes following
byte ID byte: PC FF
XT FE or FB
PCjr FD
byte secondary ID distingushes between AT and XT/286, etc.
byte BIOS revision level, 0 for 1st release, 1 for 2nd, etc.
byte feature information
80h DMA channel 3 used by hard disk BIOS
40h 2nd 8259 installed
20h realtime clock installed
10h int 15h,fn 04h called upon int 09h
08h wait for external event supported
04h extended BIOS area allocated at 640k
03h reserved
02h bus is Micro Channel instead of PC
01h reserved
00h reserved
word unknown (set to 0)
word unknown (set to 0)
note Int 15h is also used for the Multitask Hook on PS/2 machines. No
register settings availible yet.
The 1/10/86 XT BIOS returns an incorrect value for the feature byte.
Function 0C1h System - Return Extended-BIOS Data-Area Segment Address (PS/2)
entry AH 0C1h
return CF set on error
ES segment of data area
Function 0C2h Pointing Device BIOS Interface (DesQview 2.x) (PS/2)
entry AH 0C2h
AL 00h enable/disable
BH 00h disable
01h reset
return BH device ID
02h set sampling rate
BH 00h 10/second
01h 20/second
02h 40/second
03h 60/second
04h 80/second
05h 100/second
06h 200/second
03h set resolution
BH 00h one count per mm
01h two counts per mm
02h four counts per mm
03h eight counts per mm
04h get type
return BH device ID
05h initialize
BH data package size (1 - 8 bytes)
06h get/set scaling factor
BH 00h return device status
return BL status
bit 0: right button pressed
bit 1: reserved
bit 2: left button pressed
bit 3: reserved
bit 4: 0=1:1 scaling, 1=2:1 scaling
bit 5: device enabled
bit 6: 0=stream mode, 1=remote mode
bit 7: reserved
CL resolution (see function 03h)
DL sample rate, reports per second
01h set scaling at 1:1
02h set scaling at 2:1
07h set device handler address
ES:BX user device handler
return AL 00h
return CF set on error
AH status
00h successful
01h invalid function
02h invalid input
03h interface error
04h need to resend
05h no device handler installed
note The values in BH for those functions that take it as input are stored
in different locations for each subfunction
Function 0C3h Enable/Disable Watchdog Timeout (PS/2 50+)
entry AH 0C3h
AL 00h disable
01h enable
BX timer counter
return CF set on error
note The watchdog timer generates an NMI
Function 0C4h Programmable Option Select (PS/2 50+)
entry AH 04Ch
AL 00h return base POS register address
01h enable slot
BL slot number
02h enable adapter
return CF set on error
DX base POS register address (if function 00h)
Function 0DEh DesQview Services (DesQview)
entry AH 0DEh
AL 00h Get Program Name
return AX offset into DESQVIEW.DVO of current
program's record:
byte length of name
n bytes name
2 bytes keys to invoke program (second
= 00h if only one key used)
word ? (I see 0 always)
byte end flag: 00h for all but last
entry, which is 0FFh
01h Update "Open Window" Menu
return none
note Reads DESQVIEW.DVO, disables Open menu if file
not in current directory
02h unimplemented in DV 2.0x
return nothing (NOP in DV 2.0x)
03h unimplemented in DV 2.0x
return nothing (NOP in DV 2.0x)
04h Get Available Common Memory
return BX bytes of common memory available
CX largest block available
DX total common memory in bytes
05h Get Available Conventional Memory
return BX K of memory available
CX largest block available
DX total conventional memory in K
06h Get Available Expanded Memory
return BX K of expanded memory available
CX largest block available
DX total expanded memory in K
07h APPNUM Get Current Program's Number
return AX number of program as it appears on the
"Switch Windows" menu
08h GET (unknown)
return AX 0 unknown
1 unknown
09h unimplemented in DV 2.00
return nothing (NOP in DV 2.00)
0Ah DBGPOKE Display Character on Status Line
BL character
return character displayed, next call will display in
next position (which wraps back to the start of
the line if off the right edge of screen)
note 1) Displays character on bottom line of *physical*
screen, regardless of current size of window
(even entirely hidden)
2) Does not know about graphics display modes,
just pokes the characters into display memory
0Bh APILEVEL Define Minimum API Level Required
BL API level
>2 pops up "You need a newer version" error
window in DV 2.00
BH unknown
return AX maximum API level?
0Ch GETMEM Allocate "System" Memory
BX number of bytes
return ES:DI pointer to allocated block
0Dh PUTMEM Deallocate "System" Memory
ES:DI pointer to previously allocated block
return nothing
0Eh Find Mailbox by Name (DV 2.0+)
ES:DI pointer to name to find
CX length of name
return BX 0 not found
1 found
DS:SI object handle
0Fh Enable DesQview Extensions (DV 2.0+)
return AX and BX destroyed (seems to be bug, weren't
saved & restored)
note 1) Sends a manager stream with opcodes AEh, BDh,
and BFh to task's window
2) Enables an additional mouse mode
10h PUSHKEY PUT KEY INTO KEYBOARD INPUT STREAM (DV 2.0+)
BH scan code
BL character
return BX unknown (sometimes, but not always,
same as BX passed in)
note A later read will get the keystroke as if it
had been typed by the user
11h ENABLE/DISABLE AUTO JUSTIFICATION OF WINDOW (DV 2.0+)
BL 0 viewport will not move automatically
nonzero viewport will move to keep cursor
visible
return none
12h unknown (DV 2.0+)
BX 0 clear something?
nonzero set something?
return none
Interrupt 16h Keyboard I/O
(0:0058h) access the keyboard
Function 00h Get Keyboard Input - read the next character in keyboard buffer,
if no key ready, wait for one.
entry AH 00h
return AH scan code
AL ASCII character
Function 01h Check Keystroke Buffer - Do Not Clear
entry AH 01h
return ZF 0 (clear) if character in buffer
1 (set) if no character in buffer
AH scan code of character (if ZF=0)
AL ASCII character if applicable
note Keystroke is not removed from buffer
Function 02h Shift Status - fetch bit flags indicating shift status
entry AH 02h
return AL bit codes (same as [0040:0017])
bit 7 Insert state
bit 6 CapsLock state
bit 5 NumLock state
bit 4 ScrollLock state
bit 3 Alt key
bit 2 Control key
bit 1 Left shift (left caps-shift key)
bit 0 Right shift (right caps-shift key)
note other codes found at [0040:0018]
bit 7 Insert shift (Ins key)
bit 6 Caps shift (CapsLock key)
bit 5 Num shift (NumLock key)
bit 4 Scroll shift (ScrollLock key)
bit 3 Hold state (Ctrl-NumLock is in effect)
Function 03h Keyboard - Set Repeat Rate (PCjr, AT, XT/286, PS/2)
entry AH 03h
AL 00h reset typematic (PCjr)
01h increase initial delay (PCjr)
02h increase continuing delay (PCjr)
03h increase both delays (PCjr)
04h turn off typematic (PCjr)
05h set typematic rate (AT, PS/2)
BH 00h-03h for delays of 250ms, 500ms, 750ms, or 1s
BL 00h-1Fh for typematic rates of 30cps down to 2cps
Function 04h Keyboard Click Toggle (PCjr and Convertible)
entry AH 04h
AL 00h for click off
01h for click on
Function 05h Keyboard Buffer Write (AT or PS/2 with enhanced kbd)
(XT/286, PS/2, AT with "Enhanced" keyboard)
entry AH 05h
CH scan code
CL ASCII character
return AL 01h if buffer full
Function 10h Get Enhanced Keystroke And Read (F11, F12 Enhanced Keyb'd)
(XT/286, PS/2, AT with "Enhanced" keyboard)
entry AH 10h
return AH scan code
AL ASCII character if applicable
Function 11h Check Enhanced Keystroke (F11-F12 on enhanced keyboard)
(XT/286, PS/2, AT with "Enhanced" keyboard)
entry AH 11h
return ZF 0 (clear) if key pressed
1 if buffer empty
AH scan code (when ZF=0)
AL ASCII character if applicable (when ZF=0)
note Keystroke is not removed from buffer
Function 12h Extended Get Shift Status (F11, F12 Enhanced keyboard)
entry AH 12h
return AL bit
0 right Shift key depressed
1 left Shift key depressed
2 Control key depressed
3 Alt key depressed
4 ScrollLock state active
5 NumLock state active
6 CapsLock state active
7 insert state is active
AH 0 left Control key pressed
1 left Alt key depressed
2 right Control key pressed
3 right Alt key depressed
4 Scroll Lock key depressed
5 NumLock key depressed
6 CapsLock key depressed
7 SysReq key depressed
Function 0F0h Set CPU speed (Compaq 386)
entry AH 0F0h set speed
return unknown
note used by Compaq DOS MODE command.
parameters not availible
Interrupt 17h Printer
(0:005Ch) access the parallel printer(s)
AH is changed. All other registers left alone.
Function 00h Print Character/send AL to printer DX (0, 1, or 2)
entry AH 00h
AL character
DX printer to be used (0,1,2)
return AH status byte
bit
0 time out
1 unused
2 unused
3 I/O error
4 selected
5 out of paper
6 acknowledge
7 not busy
Function 01h Initialize Printer - set init line low, send 0Ch to printer DX
entry AH 01h
DX printer port to be initialized (0,1,2)
return status as below
Function 02h Printer Status - read status of printer DX into AH
entry AH 02h
DX printer port to be used (0,1,2)
return AH bit flags bit 7 0 = printer is busy
bit 6 ACKnowledge line state
bit 5 out-of-paper line state
bit 4 printer selected line state
bit 3 I/O error
bit 2 unused
bit 1 unused
bit 0 time-out error
Interrupt 18h ROM BASIC
(0:0060h) Execute ROM BASIC at address 0F600h:0000h
note 1) Often reboots a compatible
Interrupt 19h Bootstrap Loader
(0:0064h) Reads track 0, sector 1 into address 0000h:7C00h, then transfers
control to that address. If no diskette drive available,
transfers to ROM-BASIC or displays loader error message.
Causes reboot of disk system if invoked while running.
(no memory test performed).
Interrupt 1Ah Time of Day
(0:0068h) access the PC internal clock
Function 00h Read System Time Counter
entry AH 00h
return CX high word of clock count
DX low word of clock count
AL 00h if clock was read or written (via AH=0,1) within the current
24-hour period. Otherwise, AL > 0
Function 01h Set Clock - set # of 55ms clock ticks in system time counter
entry AH 01h
CX:DX high word/low word count of timer ticks
return none
note 1) The clock ticks are incremented by timer interrupt at 18.2065 times
per second or 54.9254milliseconds/count. Therefore:
counts per second = 18 (12h)
counts per minute = 1092 (444h)
counts per hour = 65543 (10011h)
counts per day = 1573040 (1800B0h)
2) counter is zeroed when system is rebooted
2) IBM and Microsoft recommend using int 21 Fn 4Ch. Using int 20 is
officially frowned upon since the introduction of DOS 2.0
Function 02h Read Real Time Clock Time (AT and after)
entry AH 02h
return CH hours in BCD
CL minutes in BCD
DH seconds in BCD
DL 1 (set) if daylight savings time option
CF 1 (set) if clock not operating
Function 03h Set Real Time Clock Time (AT and after)
entry AH 03h
CH hours in BCD
CL minutes in BCD
DH seconds in BCD
DL 0 (clear) if standard time
1 (set) if daylight savings time option
return none
Function 04h Read Real Time Clock Date (AT and after)
entry AH 04h
return CH century in BCD (19 or 20)
CL year in BCD
DH month in BCD
DL day in BCD
CF 1 (set) if clock not operating
Function 05h Set Real Time Clock Date (AT and after)
entry AH 05h
CH century in BCD (19 or 20)
CL year in BCD
DH month in BCD
DL day in BCD
return none
Function 06h Set Real Time Clock Alarm (AT and after)
entry AH 06h
CH hours in BCD
CL minutes in BCD
DH seconds in BCD
return CF set if alarm already set or clock inoperable
note Int 4Ah occurs at specified alarm time every 24hrs until reset
Function 07h Reset Real Time Clock Alarm (AT and after)
entry AH 07h
return none
Function 08h Set Real Time Clock Activated Power On Mode (Convertible)
entry AH 08h
CH hours in BCD
CL minutes in BCD
DH seconds in BCD
Function 09h Read Real Time Clock Alarm Time and Status
(Convertible and PS/2 Model 30)
entry AH 09h
return CH hours in BCD
CL minutes in BCD
DH seconds in BCD
DL alarm status:
00h if alarm not enabled
01h if alarm enabled but will not power up system
02h if alarm will power up system
Function 0Ah Read System-Timer Day Counter (XT-2 [640k motherboard], PS/2)
entry AH 0Ah
return CF set on error
CX count of days since Jan 1,1980
Function 0Bh Set System-Timer Day Counter (XT-2 [640k motherboard], PS/2)
entry AH 0Bh
CX count of days since Jan 1,1980
return CF set on error
Function 80h Set Up Sound Multiplexor (PCjr) (Tandy 1000?)
entry AH 80h
AL 00h source is 8253 channel 2
01h source is cassette input
02h source is I/O channel "audio in"
03h source is TI sound generator chip
Interrupt 1Bh Control-Break
(0:006Ch) This interrupt is called when the keyboard scanner of the IBM
machines detects Ctrl and Break pressed at the same time.
note 1) If the break occurred while processing an interrupt, one or more
end of interrupt commands must be send to the 8259 Programmable
Interrupt Controller.
2) All I/O devices should be reset in case an operation was underway at
the time.
3) It is normally pointed to an IRET during system initialization so that
it does nothing, but some programs change it to return a ctrl-C scan
code and thus invoke int 23h.
Interrupt 1Ch Timer Tick
(0:0070h)
note 1) Taken 18.2065 times per second
2) Normally vectors to dummy IRET unless PRINT.COM has been installed.
3) If an application moves the interrupt pointer, it is the responsibility
of that application to save and restore all registers that may be
modified.
Interrupt 1Dh Vector of Video Initialization Parameters.
(0:0074h) This doubleword address points to 3 sets of 16-bytes containing
data to initialize for video modes for video modes 0 & 1 (40
column), 2 & 3 (80 column), and 4, 5 & 6 (graphics) on the
Motorola 6845 CRT controller chip.
6845 registers:
R0 horizontal total (horizontal sync in characters)
R1 horizontal displayed (characters per line)
R2 horizontal sync position (move display left or right)
R3 sync width (vertical and horizontal pulse: 4-bits each)
R4 vertical total (total character lines)
R5 vertical adjust (adjust for 50 or 60 Hz refresh)
R6 vertical displayed (lines of chars displayed)
R7 vertical sync position (lines shifted up or down)
R8 interlace (bits 4 and 5) and skew (bits 6 and 7)
R9 max scan line addr (scan lines per character row)
R10 cursor start (starting scan line of cursor)
R11 cursor stop (ending scan line of cursor)
R12 video memory start address high byte (6-bits)
R13 video memory start address low byte (8-bits)
R14 cursor address high byte (6-bits)
R15 cursor address low byte (8-bits)
6845 Video Init Tables:
table for modes 0 and 1 \
table for modes 2 and 3 \ each table is 16 bytes long and
table for modes 4,5, and 6 / contains values for 6845 registers
table for mode 7 /
4 words: size of video RAM for modes 0/1, 2/3, 4/5, and 6/7
8 bytes: number of columns in each mode
8 bytes: video controller mode byte for each mode
note 1) There are 4 separate tables, and all 4 must be initialized if all
video modes will be used.
2) The power-on initialization code of the computer points this vector
to the ROM BIOS video routines.
3) IBM recommends that is this table needs to be modified, it should be
copied into RAM and only the nescessary changes made.
Interrupt 1Eh Vector of Diskette Controller Parameters
(0:0078h) Dword address points to data base table that is used by BIOS.
Default location is at 0F000:0EFC7h. 11-byte table format:
bytes:
00h 4-bit step rate, 4-bit head unload time
01h 7-bit head load time, 1-bit DMA flag
02h 54.9254 ms counts - delay till motor off (37-38 typ)
03h sector size:
00h 128 bytes
01h 256 bytes
02h 512 bytes
03h 1024 bytes
04h last sector on track (8 or 9 typical)
05h gap between sectors on read/write (42 typical)
06h data length for DMA transfers (0FFh typical)
07h gap length between sectors for format (80 typical)
08h sector fill byte for format (0F6h typical)
09h head settle time (in milliseconds) (15 to 25 typical)
DOS 1.0 0
DOS 2.10 15
DOS 3.1 1
10h motor start time (in 1/8 second intervals) (2 to 4 typ.)
DOS 2.10 2
note 1) This vector is pointed to the ROM BIOS diskette tables on system
initialization
2) IBM recommends that is this table needs to be modified, it should be
copied into RAM and only the nescessary changes made.
Interrupt 1Fh Pointer to Graphics Character Extensions (Graphics Set 2)
(0:007Ch) This is the pointer to data used by the ROM video routines to
display characters above ASCII 127 while in CGA medium and high
res graphics modes.
note 1) Doubleword address points to 1K table composed of 28 8-byte character
definition bit-patterns. First byte of each entry is top row, last byte
is bottom row.
2) The first 128 character patterns are located in system ROM.
3) This vector is set to 000:0 at system initialization
4) Used by DOS' external GRAFTABL command
Interrupt 20h PROGRAM TERMINATE
(0:0080h)
Issue int 20h to exit from a program. This vector transfers to the logic in
DOS to restore the terminate address, the Ctrl-Break address,and the critical
error exit address to the values they had on entry to the program. All the file
buffers are flushed and all handles are closed. You should close all files
changed in length (see function calls 10h and 3Eh) before issuing this
interrupt. If the changed file is not closed, its length, time, and date are
not recorded correctly in the directory.
For a program to pass a completion code or an error code when terminating, it
must use either function call 4Ch (Terminate a Process) or 31h (Terminate
Process and Stay Resident). These two methods are preferred over using
int 20h and the codes returned by them can be interrogated in batch processing.
Important: Before you issue an interrupt 20h, your program must ensure that
the CS register contains the segment of its program segment prefix.
Interrupt 20h DOS - Terminate Program
entry no parameters
return none
Interrupt 20h Minix - Send/Receive Message
entry AX process ID of other process
BX pointer to message
CX 1 send
2 receive
3 send&receive
note The message contains the system call number (numbered as in V7 Unix)
and the call parameters
CHAPTER 4
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
DOS INTERRUPTS AND FUNCTION CALLS
CONTENTS
DOS Registers ........................................................... 4-
Interrupts .............................................................. 4-
20h Program Terminate ........................................... 4-
21h Function Request ............................................ 4-
Function Calls .............................................. 4-
Listing of Function Calls ................................... 4-
00h Program Terminate ................................... 4-
01h Keyboard Input ...................................... 4-
02h Display Output ...................................... 4-
03h Auxiliary Input ..................................... 4-
04h Auxiliary Output .................................... 4-
05h Printer Output ...................................... 4-
06h Direct Console I/O .................................. 4-
07h Direct Console Input Without Echo ................... 4-
08h Console Input Without Echo ...........................4-
09h Print String .........................................4-
0Ah Buffered Keyboard Input ..............................4-
0Bh Check Standard Input Status ..........................4-
0Ch Clear Keyboard Buffer and Invoke a Kbd Function ..... 4-
0Dh Disk Reset .......................................... 4-
0Eh Select Disk ......................................... 4-
0Fh Open File ........................................... 4-
10h Close File .......................................... 4-
11h Search for First Entry .............................. 4-
12h Search for Next Entry ............................... 4-
13h Delete File ......................................... 4-
14h Sequential Read ..................................... 4-
15h Sequential Write .................................... 4-
16h Create File ......................................... 4-
17h Rename File ......................................... 4-
18h ** Unknown .......................................... 4-
19h Current Disk ........................................ 4-
1Ah Set Disk Transfer Address ........................... 4-
1Bh Allocation Table Information ........................ 4-
1Ch Allocation Table Information for Specific Device .... 4-
1Dh ** Unknown .......................................... 4-
1Eh ** Unknown .......................................... 4-
1Fh ** Read DOS Disk Block (default drive) .............. 4-
20h ** Unknown .......................................... 4-
21h Random Read ......................................... 4-
22h Random Write ........................................ 4-
23h File Size ........................................... 4-
24h Set Relative Record Field ........................... 4-
25h Set Interrupt Vector ................................ 4-
26h Create New Program Segment .......................... 4-
27h Random Block Read ................................... 4-
28h Random Block Write .................................. 4-
29h Parse Filename ...................................... 4-
2Ah Get Date ............................................ 4-
2Bh Get Date ............................................ 4-
2Ch Get Time ............................................ 4-
2Dh Set Time ............................................ 4-
2Eh Set/Reset Verify Switch ............................. 4-
2Fh Get Disk Transfer Address (DTA) ..................... 4-
30h Get DOS Version Number .............................. 4-
31h Terminate Process and Stay Resident ................. 4-
32h ** Read DOS Disk Block .............................. 4-
33h Ctrl-Break Check .................................... 4-
34h ** Return INDOS Flag ................................ 4-
35h Get Vector .......................................... 4-
36h Get Disk Free Space ................................. 4-
37h ** Get/Set Switch Character (SWITCHAR) .............. 4-
38h Return Country Dependent Information ................ 4-
39h Create Subdirectory (MKDIR) ......................... 4-
3Ah Remove Subdirectory (RMDIR) ......................... 4-
3Bh Change Durrent Directory (CHDIR) .................... 4-
3Ch Create a File (CREAT) ............................... 4-
3Dh Open a File ......................................... 4-
3Eh Close a File Handle ................................. 4-
3Fh Read From a File or Device .......................... 4-
40h Write to a File or Device ........................... 4-
41h Delete a File from a Specified Directory (UNLINK) ... 4-
42h Move File Read/Write Pointer (LSEEK) ................ 4-
43h Change File Mode (CHMOD) ............................ 4-
44h I/O Control for Devices (IOCTL) ..................... 4-
45h Duplicate a File Handle (DUP) ....................... 4-
46h Force a Duplicate of a Handle (FORCDUP) ............. 4-
47h Get Current Directory ............................... 4-
48h Allocate Memory ..................................... 4-
49h Free Allocated Memory ............................... 4-
4Ah Modify Allocated Memory Blocks (SETBLOCK) ........... 4-
4Bh Load or Execute a Program (EXEC) .................... 4-
4Ch Terminate a Process (EXIT) .......................... 4-
4Dh Get Return Code of a Subprocess (WAIT) .............. 4-
4Eh Find First Matching File (FIND FIRST) ............... 4-
4Fh Find Next Matching File (FIND NEXT) ................. 4-
50h ** Set PSP .......................................... 4-
51h ** Get PSP .......................................... 4-
52h ** IN-VARS .......................................... 4-
53h ** Translate BPB .................................... 4-
54h Get Verify Setting .................................. 4-
55h ** Create Child PSP ................................. 4-
56h Rename a File ....................................... 4-
57h Get or Set Timestamp of a File ...................... 4-
58h ** Get/Set Allocation Strategy (DOS 3.x) ............ 4-
59h Get Extended Error Code ............................. 4-
5Ah Create Unique Filename .............................. 4-
5Bh Create a New File ................................... 4-
5Ch Lock/Unlock File Access ............................. 4-
5Dh ** Network - Partial ................................ 4-
5Eh ** Network Printer .................................. 4-
5Fh ** Network Redirection .............................. 4-
60h ** Parse Pathname ................................... 4-
61h ** Unknown .......................................... 4-
62h Get Program Segment Prefix (PSP) Address ............ 4-
63h ** Get Lead Byte Table (DOS 2.25) ................... 4-
64h ** Unknown .......................................... 4-
65h ** Get Extended Country Information (DOS 3.3) ....... 4-
66h ** Get/Set Global Code Page Table (DOS 3.3) ......... 4-
67h ** Set Handle Count (DOS 3.3) ....................... 4-
68h ** Commit File (DOS 3.3) ............................ 4-
Calling the DOS Services .................................... 4-
22h Terminate Address ........................................... 4-
23h Ctrl-Break Exit Address ..................................... 4-
24h Critical Error Handler Vector ............................... 4-
25h Absolute Disk Read .......................................... 4-
26h Absolute Disk Write ......................................... 4-
27h Terminate and Stay Resident ................................. 4-
28h ** DOS Idle Interrupt ....................................... 4-
29h ** Quick Screen Output ...................................... 4-
2Ah Microsoft Networks Session Layer Interrupt .................. 4-
2Bh ** Unknown .................................................. 4-
2Ch ** Unknown .................................................. 4-
2Dh ** Unknown .................................................. 4-
2Eh ** Alternate EXEC ........................................... 4-
2Fh Multiplex Interrupt ......................................... 4-
DOS REGISTERS
DOS uses the following registers, pointers, and flags when it executes
interrupts and function calls:
GENERAL REGISTERS register definition
AX accumulator (16 bit)
AH accumulator high-order byte (8 bit)
AL accumulator low order byte (8 bit)
BX base (16 bit)
BH base high-order byte (8 bit)
BL base low-order byte (8 bit)
CX count (16 bit)
CH count high order byte (8 bit)
CL count low order byte (8 bit)
DX data (16 bit)
DH date high order byte (8 bit)
DL data low order byte (8 bit)
FLAGS AF, CF, DF, IF, OF, PF, SF, TF, ZF
POINTERS register definition
SP stack pointer (16 bit)
BP base pointer (16 bit)
IP instruction pointer (16 bit)
SEGMENT REGISTERS register definition
CS code segment (16 bit)
DS data segment (16 bit)
SS stack segment (16 bit)
ES extra segment (16 bit)
INDEX REGISTERS register definition
DI destination index (16 bit)
SI stack index (16 bit)
INTERRUPTS
Microsoft recommends that a program wishing to examine or set the contents of
any interrupt vector use the DOS function calls 35h and 25h provided for those
purposes and avoid referencing the interrupt vector locations directly.
DOS reserves interrupt numbers 20h to 3Fh for its own use. This means absolute
memory locations 80h to 0FFh are reserved by DOS. The defined interrupts are as
follows with all values in hexadecimal.
Interrupt 21h FUNCTION CALL REQUEST
(0:0084h)
DOS provides a wide variety of function calls for character device I/O, file
management, memory management, date and time functions,execution of other
programs, and more. They are grouped as follows:
call description
00h program terminate
01h-0Ch character device I/O, CP/M compatibility format
0Dh-24h file management, CP/M compatibility format
25h-26h nondevice functions, CP/M compatibility format
27h-29h file management, CP/M compatibility format
2Ah-2Eh nondevice functions, CP/M compatibility format
2Fh-38h extended functions
39h-3Bh directory group
3Ch-46h extended file management
47h directory group
48h-4Bh extended memory management
54h-57h extended functions
5Eh-5Fh networking
60h-62h extended functions
63h-66h enhanced foreign language support
List of DOS services: * = undocumented
00h terminate program
01h get keyboard input
02h display character to STDIO
03h get character from STDAUX
04h output character to STDAUX
05h output character to STDPRN
06h direct console I/O - keyboard to screen
07h get char from std I/O without echo
08h get char from std I/O without echo, checks for ^C
09h display a string to STDOUT
0Ah buffered keyboard input
0Bh check STDIN status
0Ch clear keyboard buffer and invoke keyboard function
0Dh flush all disk buffers
0Eh select disk
0Fh open file with File Control Block
10h close file opened with File Control Block
11h search for first matching file entry
12h search for next matching file entry
13h delete file specified by File Control Block
14h sequential read from file specified by File Control Block
15h sequential write to file specified by File Control Block
16h find or create firectory entry for file
17h rename file specified by file control block
18h* unknown
19h return current disk drive
1Ah set disk transfer area (DTA)
1Bh get current disk drive FAT
1Ch get disk FAT for any drive
1Dh* unknown
1Eh* unknown
1Fh* read DOS disk block, default drive
20h* unknown
21h random read from file specified by FCB
22h random write to file specified by FCB
23h return number of records in file specified by FCB
24h set relative file record size field for file specified by FCB
25h set interrupt vector
26h create new Program Segment Prefix (PSP)
27h random file block read from file specified by FCB
28h random file block write to file specified by FCB
29h parse the command line for file name
2Ah get the system date
2Bh set the system date
2Ch get the system time
2Dh set the system time
2Eh set/clear disk write VERIFY
2Fh get the Disk Transfer Address (DTA)
30h get DOS version number
31h TSR, files opened remain open
32h* read DOS Disk Block
33h get or set Ctrl-Break
34h* INDOS Critical Section Flag
35h get segment and offset address for an interrupt
36h get free disk space
37h* get/set option marking character (SWITCHAR)
38h return country-dependent information
39h create subdirectory
3Ah remove subdirectory
3Bh change current directory
3Ch create and return file handle
3Dh open file and return file handle
3Eh close file referenced by file handle
3Fh read from file referenced by file handle
40h write to file referenced by file handle
41h delete file
42h move file pointer (move read-write pointer for file)
43h set/return file attributes
44h device IOCTL (I/O control) info
45h duplicate file handle
46h force a diuplicate file handle
47h get current directory
48h allocate memory
49h release allocated memory
4Ah modify allocated memory
4Bh load or execute a program
4Ch terminate prog and return to DOS
4Dh get return code of subprocess created by 4Bh
4Eh find first matching file
4Fh fine next matching file
50h* set new current Program Segment Prefix (PSP)
51h* puts current PSP into BX
52h* pointer to the DOS list of lists
53h* translates BPB (Bios Parameter Block, see below)
54h get disk verification status (VERIFY)
55h* create PSP: similar to function 26h
56h rename a file
57h get/set file date and time
58h get/set allocation strategy (DOS 3.x)
59h get extended error information
5Ah create a unique filename
5Bh create a DOS file
5Ch lock/unlock file contents
5Dh* network
5Eh* network printer
5Fh* network redirection
60h* parse pathname
61h* unknown
62h get program segment prefix (PSP)
63h* get lead byte table (DOS 2.25)
64h* unknown
65h* get extended country information (DOS 3.3)
66h* get/set global code page table (DOS 3.3)
67h* set handle count (DOS 3.3)
68h* commit file (DOS 3.3)
CALLING THE DOS SERVICES
The DOS services are invoked by placing the number of the desired function in
register AH, subfunction in AL, setting the other registers to any specific
requirements of the function, and invoking int 21h.
On return, the requested service will be performed if possible. Most codes
will return an error; some return more information. Details are contained in
the listings for the individual functions. Extended error return may be
obtained by calling function 59h (see 59h).
Register settings listed are the ones used by DOS. Some functions will return
with garbage values in unused registers. Do not test for values in unspecified
registers; your program may exhibit odd behavior.
DS:DX pointers are the data segment register (DS) indexed to the DH and DL
registers (DX). DX always contains the offset address, DS contains the segment
address.
The File Control Block services (FCB services) were part of DOS 1.0. Since
the release of DOS 2.0, Microsoft has recommended that these services not be
used. A set of considerably more enhanced services (handle services) were
introduced with DOS 2.0. The handle services provide support for wildcards and
subdirectories, and enhanced error detection via function 59h.
The data for the following calls was compiled from various Intel, Microsoft,
IBM, and other publications. There are many subtle differences between MSDOS
and PCDOS and between the individual versions. Differences between the
versions are noted as they occur.
There are various ways of calling the DOS functions. For all methods, the
function number is loaded into register AH, subfunctions and/or parameters are
loaded into AL or other registers, and call int 21 by one of the following
methods:
A) call interrupt 21h directly
B) perform a long call to offset 50h in the program's PSP.
1) This method will not work under DOS 1.x
C) place the function number in CL and perform an intrasegment call to
location 05h in the current code segment. This location contains a long
call to the DOS function dispatcher.
1) IBM recommends this method be used only when using existing programs
written for different calling conventions. This method should be avoided
unless you some specific use for it
2) AX is always destroyed by this method
3) This method is valid only for functions 00h-24h.
INT 21H DOS services
Function (hex)
- Indicates Functions not documented in the IBM DOS Technical Reference.
Note some functions have been documented in other Microsoft or licensed OEM
documentation.
00h Terminate Program
Ends program, updates, FAT, flushes buffers, restores registers
entry AH 00h
CS segment address of PSP
return none
note 1) Program must place the segment address of the PSP control block in CS
before calling this function.
2) The terminate, ctrl-break,and critical error exit addresses (0Ah, 0Eh,
12h) are restored to the values they had on entry to the terminating
program, from the values saved in the program segment prefix at
locations PSP:000Ah, PSP:000Eh, and PSP:0012h.
3) All file buffers are flushed and the handles opened by the process are
closed.
4) Any files that have changed in length and are not closed are not
recorded properly in the directory.
5) Control transfers to the terminate address.
6) This call performs exactly the same function as int 20h.
7) All memory used by the program is returned to DOS.
01h Get Keyboard Input
Waits for char at STDIN (if nescessary), echoes to STDOUT
entry AH 01h
return AL char from STDIN (8 bits)
note 1) Checks char for Ctrl-C, if char is Ctrl-C, executes int 23h.
2) For function call 06h, extended ASCII codes require two function calls.
The first call returns 00h as an indicator that the next call will be an
extended ASCII code.
3) Input and output are redirectable. If redirected, there is no way to
detect EOF.
02h Display Output
Outputs char in DL to STDOUT
entry AH 02h
DL 8 bit data (usually ASCII character)
return none
note 1) If char is 08 (backspace) the cursor is moved 1 char to the left
(nondestructive backspace).
2) If Ctrl-C is detected after input, int 23h is executed.
3) Input and output are redirectable. If redirected, there is no way to
detect disk full.
03h Auxiliary Input
Get (or wait until) character from STDAUX
entry AH 03h
return AL char from auxiliary device
note 1) AUX, COM1, COM2 is unbuffered and not interrupt driven
2) This function call does not return status or error codes. For greater
control it is recommended that you use ROM BIOS routine (int 14h) or
write an AUX device driver and use IOCTL.
3) At startup, PC-DOS initializes the first auxiliary port (COM1) to 2400
baud, no parity, one stop bit, and an 8-bit word. MSDOS may differ.
4) If Ctrl-C is has been entered from STDIN, int 23h is executed.
04h Auxiliary Output
Write character to STDAUX
entry AH 04h
DL char to send to AUX
return none
note 1) This function call does not return status or error codes. For greater
control it is recommended that you use ROM BIOS routine (int 14h) or
write an AUX device driver and use IOCTL.
2) If Ctrl-C is has been entered from STDIN, int 23h is executed.
3) Default is COM1 unless redirected by DOS.
4) If the device is busy, this function will wait until it is ready.
05h Printer Output
Write character to STDPRN
entry AL 05h
DL character to send
return none
note 1) If Ctrl-C is has been entered from STDIN, int 23h is executed.
2) Default is PRN or LPT1 unless redirected with the MODE command.
3) If the printer is busy, this function will wait until it is ready.
06h Direct Console I/O
Get character from STDIN; echo character to STDOUT
entry AH 06h
DL 0FFh for console input, or 00h-0FEh for console output
return ZF zero flag set (1) = no character
zero flag clear (0) = character recieved
AL character
note 1) Extended ASCII codes require two function calls. The first call returns
00h to indicate the next call will return an extended code.
2) If DL is not 0FFh, DL is assumed to have a valid character that is
output to STDOUT.
3) This function does not check for Ctrl-C or Ctrl-PrtSc.
4) Does not echo input to screen
5) If I/O is redirected, EOF or disk full cannot be detected.
07h Direct Console Input Without Echo (does not check BREAK)
Get or wait for char at STDIN, returns char in AL
entry AH 07h
return AL character from standard input device
note 1) Extended ASCII codes require two function calls. The first call returns
00h to indicate the next call will return an extended code.
2) No checking for Ctrl-C or Ctrl-PrtSc is done.
3) Input is redirectable.
08h Console Input Without Echo (checks BREAK)
Get or Wait for char at STDIN, return char in AL
entry AH 08h
return AL char from standard input device
note 1) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h.
2) For function call 08h, extended ASCII characters require two function
calls. The first call returns 00h to signify an extended ASCII code.
The next call returns the actual code.
3) Input is redirectable. If redirected, there is no way to check EOF.
09h Print String
Outputs Characters in the Print String to the STDOUT
entry AH 09h
DS:DX pointer to the Character String to be displayed
return none
note 1) The character string in memory must be terminated by a $ (24h)
The $ is not displayed.
2) Output to STDOUT is the same as function call 02h.
0Ah Buffered Keyboard Input
Reads characters from STDIN and places them in the buffer beginning
at the third byte.
entry AH 0Ah
DS:DX pointer to an input buffer
return none
note 1) Min buffer size = 1, max = 255
2) Char is checked for ctrl-C. If ctrl-C is detected, executes int 23h.
3) Format of buffer DX:
byte contents
1 Maximum number of chars the buffer will take, including CR.
Reading STDIN and filling the buffer continues until a carriage
return (<Enter> or 0Dh) is read. If the buffer fills to one less
than the maximum number the buffer can hold, each additional
number read is ignored and ASCII 7 (BEL) is output to the
display until a carriage return is read. (you must set this
value)
2 Actual number of characters received, excluding the carriage
return, which is always the last character. (the function sets
this value)
3-n Characters received are placed into the buffer starting here.
Buffer must be at least as long as the number in byte 1.
4) Input is redirectable. If redirected, there is no way to check EOF.
5) The string may be edited with the standard DOS editing commands as it
is being entered.
6) Extended ASCII characters are stored as 2 bytes, the first byte being
zero.
0Bh Check Standard Input (STDIN) status
Checks for character availible at STDIN
entry AH 0Bh
return AL 0FFh if a character is availible from STDIN
00h if no character is availible from STDIN
note 1) Checks for Ctrl-C. If Ctrl-C is detected, int 23h is executed
2) Input can be redirected.
3) Checks for character only, it is not read into the application
0Ch Clear Keyboard Buffer & Invoke a Keyboard Function (FCB)
Dumps buffer, executes function in AL (01h,06h,07h,08h,0Ah only)
entry AH 0Ch
AL function number (must be 01h, 06h, 07h, 08h, or 0Ah)
return AL 00h buffer was flushed, no other processing performed
other any other value has no meaning
note 1) Forces system to wait until a character is typed.
2) Flushes all typeahead input, then executes function specified by AL (by
moving it to AH and repeating the int 21 call).
3) If AL contains a value not in the list above, the keyboard buffer is
flushed and no other action is taken.
0Dh Disk Reset
Flushes all currently open file buffers to disk
entry AH 0Dh
return none
note 1) Does not close files. Does not update directory entries; files changed
in size but not closed are not properly recorded in the directory
2) Sets DTA address to DS:0080h
3) Should be used before a disk change, Ctrl-C handlers, and to flush
the buffers to disk.
0Eh Select Disk
Sets the drive specified in DL (if valid) as the default drive
entry AL 0Eh
DL new default drive number (0=A:,1=B:,2=C:,etc.)
return AL total number of logical drives (not nescessarily physical)
note 1) For DOS 1.x and 2.x, the minimum value for AL is 2.
2) For DOS 3.x, the minimum value for AL is 5.
3) The drive number returned is not nescessarily a valid drive.
4) For DOS 1.x: 16 logical drives are availible. A-P.
For DOS 2.x: 63 logical drives are availible. (Letters are only used for
the first 26 drives. If more than 26 logical drives are
used, further drive letters will be other ASCII characters
ie {,], etc.
For DOS 3.x: 26 logical drives are availible. A-Z.
0Fh Open Disk File (FCB)
Searches current directory for specified filename and opens it
entry AH 0Fh
DS:DX pointer to an unopened FCB
return AL 00h if file found
0FFh if file not not found
note 1) If the drive code was 0 (default drive) it is changed to the actual
drive used (1=A:,2=B:,3=C:, etc). This allows changing the default drive
without interfering with subsequent operations on this file.
2) The current block field (FCB bytes C-D, offset 0Ch) is set to zero.
3) The size of the record to be worked with (FCB bytes E-F, offset 0Eh) is
set to the system default of 80h. The size of the file (offset 10h) and
the date (offset 14h) are set from information obtained in the root
directory. You can change the default value for the record size (FCB
bytes E-F) or set the random record size and/or current record field.
Perform these actions after the open but before any disk operations.
4) The file is opened in compatibility mode.
5) Microsoft recommends handle function call 3Dh be used instead.
6) This call is also used by the APPEND command in DOS 3.2+
7) Before performing a sequential disk operation on the file, you must
set the Current Record field (offset 20h). Before performing a random
disk operation on the file, you must set the Relative Record field
(offset 21h). If the default record size of 128 bytes is incorrect, set
it to the correct value.
10h Close File (FCB)
Closes a File After a File Write
entry AH 10h
DS:DX pointer to an opened FCB
return AL 00h if the file is found and closed
0FFh if the file is not found in the current directory
note 1) This function call must be done on open files that are no longer needed,
and after file writes to insure all directory information is updated.
2) If the file is not found in its correct position in the current
directory, it is assumed that the diskette was changed and AL returns
0FFh. This error return is reportedly not completely reliable with DOS
version 2.x.
3) If found, the directory is updated to reflect the status in the FCB, the
buffers to that file are flushed, and AL returns 00h.
11h Search For First Matching Entry (FCB)
Searches current disk & directory for first matching filename
entry AH 11h
DS:DX pointer to address of FCB
return AL 00h successful match
0FFh no matching filename found
note 1) The FCB may contain the wildcard character ? under Dos 2.x, and ? or *
under 3.x.
2) The original FCB at DS:DX contains information to continue the search
with function 12h, and should not be modified.
3) If a matching filename is found, AL returns 00h and the locations at the
Disk Transfer Address are set as follows:
a) If the FCB provided for searching was an extended FCB, then the first
byte at the disk transfer address is set to 0FFh followed by 5 bytes
of zeroes, then the attribute byte from the search FCB, then the
drive number used (1=A, 2=B, etc) then the 32 bytes of the directory
entry. Thus, the disk transfer address contains a valid unopened FCB
with the same search attributes as the search FCB.
b) If the FCB provided for searching was a standard FCB, then the first
byte is set to the drive number used (1=A,2=b,etc), and the next 32
bytes contain the matching directory entry. Thus, the disk transfer
address contains a valid unopened normal FCB.
4) If an extended FCB is used, the following search pattern is used:
a) If the FCB attribute byte is zero, only normal file entries are
found. Entries for volume label, subdirectories, hidden or system
files, are not returned.
b) If the attribute byte is set for hidden or system files, or
subdirectory entries, it is to be considered as an inclusive search.
All normal file entries plus all entries matching the specified
attributes are returned. To look at all directory entries except the
volume label, the attribute byte may be set to hidden + system +
directory (all 3 bits on).
c) If the attribute field is set for the volume label, it is considered
an exclusive search, and ONLY the volume label entry is returned.
5) This call is also used by the APPEND command in DOS 3.2+
12h Search For Next Entry Using FCB (FCB)
Search for next matching filename
entry AH 12h
DS:DX pointer to the unopened FCB specified from the previous Search
First (11h) or Search Next (12h)
return AL 00h if matching filename found
0FFh if matching filename was not found
note 1) After a matching filename has been found using function call 11h,
function 12h may be called to find the next match to an ambiguous
request. For DOS 2.x, ?'s are allowed in the filename. For DOS 3.x,
global (*) filename characters are allowed.
2) The DTA contains info from the previous Search First or Search Next.
3) All of the FCB except for the name/extension field is used to keep
information nescessary for continuing the search, so no disk operations
may be performed with this FCB between a previous function 11h or 12h
call and this one.
4) If the file is found, an FCB is created at the DTA address and set up to
open or delete it.
13h Delete File Via FCB (FCB)
Deletes file specified in FCB from current directory
entry AH 13h
DS:DX pointer to address of FCB
return AL 00h file deleted
0FFh if file not found or was read-only
note 1) All matching current directory entries are deleted. The global filename
character "?" is allowed in the filename.
2) Will not delete files with read-only attribute set
3) Close open files before deleting them.
4) Requires Network Access Rights
14h Sequential Disk File Read (FCB)
Reads record sequentially from disk via FCB
entry AH 14h
DS:DX pointer to an opened FCB
return AL 00h successful read
01h end of file (no data read)
02h Data Transfer Area too small for record size specified
or segment overflow
03h partial record read, EOF found
note 1) The record size is set to the value at offset 0Eh in the FCB.
2) The record pointed to by the Current Block (offset 0Ch) and the Current
Record (offset 20h) fields is loaded at the DTA, then the Current Block
and Current Record fields are incremented.
3) The record is read into memory at the current DTA address as specified
by the most recent call to function 1Ah. If the size of the record and
location of the DTA are such that a segment overflow or wraparound would
occur, the error return is set to AL=02h
4) If a partial record is read at the end of the file, it is passed to the
requested size with zeroes and the error return is set to AL=03h.
15h Sequential Disk Write (FCB)
Writes record specified by FCB sequentially to disk
entry AH 15h
DS:DX pointer to address of FCB
return AL 00h successful write
01h diskette full, write canceled
02h disk transfer area (DTA) too small or segment wrap
note 1) The data to write is obtained from the disk transfer area
2) The record size is set to the value at offset 0Eh in the FCB.
3) This service cannot write to files set as read-only
4) The record pointed to by the Current Block (offset 0Ch) and the Current
Record (offset 20h) fields is loaded at the DTA, then the Current Block
and Current Record fields are incremented.
5) If the record size is less than a sector, the data in the DTA is written
to a buffer; the buffer is written to disk when it contains a full
sector of data, the file is closed, or a Reset Disk (function 0Dh) is
issued.
6) The record is written to disk at the current DTA address as specified
by the most recent call to function 1Ah. If the size of the record and
location of the DTA are such that a segment overflow or wraparound would
occur, the error return is set to AL=02h
16h Create A Disk File (FCB)
Search and open or create directory entry for file
entry AH 16h
DS:DX pointer to an FCB
return AL 00h successful creation
0FFh no room in directory
note 1) If a matching directory entry is found, the file is truncated to zero
bytes.
2) If there is no matching filename, a filename is created.
3) This function calls function 0Fh (Open File) after creating or
truncating a file.
4) A hidden file can be created by using an extended FCB with the attribute
byte (offset FCB-1) set to 2.
17h Rename File Specified by File Control Block (FCB) (FCB)
Renames file in current directory
entry AH 17h
DS:DX pointer to an FCB (see note 4)
return AL 00h successfully renamed
0FFh file not found or filename already exists
note 1) This service cannot rename read-only files
2) The "?" wildcard may be used.
3) If the "?" wildcard is used in the second filename, the corresponding
letters in the filename of the directory entry are not changed.
4) The FCB must have a drive number, filename, and extension in the usual
position, and a second filename starting 6 bytes after the first, at
offset 11h.
5) The two filenames cannot have the same name.
6) FCB contains new name starting at byte 17h.
18h Internal to DOS
* Unknown
entry AH 18h
return AL 0
19h Get Current Disk Drive
Return designation of current default disk drive
entry AH 19h
return AL current default drive (0=A, 1=B,etc.)
note Some other DOS functions use 0 for default, 1=A, 2=B, etc.
1Ah Set Disk Transfer Area Address (DTA)
Sets DTA address to the address specified in DS:DX
entry AH 1Ah
DS:DX pointer to buffer
return none
note 1) The default DTA is 128 bytes at offset 80h in the PSP. DOS uses the
DTA for all file I/O.
2) Registers are unchanged.
3) No error codes are returned.
2) Disk transfers cannot wrap around from the end of the segment to the
beginning or overflow into another segment.
1Bh Get Current Drive File Allocation Table Information
Returns information from the FAT on the current drive
entry AH 1Bh
exit AL number of sectors per allocation unit (cluster)
DS:BX address of the current drive's media descriptor byte
CX number of bytes per sector
DX number of allocation units (clusters) for default drive
note 1) Save DS before calling this function.
2) This call returned a pointer to the FAT in DOS 1.x. Beginning with
DOS 2.00, it returns a pointer only to the table's ID byte.
3) IBM recommends programmers avoid this call and use int 25h instead.
1Ch Get File Allocation Table Information for Specific Device
Returns information on specified drive
entry AH 1Ch
DL drive number (1=A, 2=B, 3=C, etc)
return AL number of sectors per allocation unit (cluster)
DS:BX address of media descriptor byte for drive in DL
CX sector size in bytes
DX number of allocation units (clusters)
note 1) DL = 0 for default.
2) Save DS before calling this function.
3) Format of media-descriptor byte:
bits: 0 0 (clear) not double sided
1 (set) double sided
1 0 (clear) not 8 sector
1 (set) 8 sector
2 0 (clear) nonremovable device
1 (set) removable device
3-7 always set (1)
4) This call returned a pointer to the FAT in DOS 1.x. Beginning with
DOS 2.00, it returns a pointer only to the table's ID byte.
5) IBM recommends programmers avoid this call and use int 25h instead.
1Dh Not Documented by Microsoft
* Unknown
entry AH 1Dh
return AL 0
1Eh Not Documented by Microsoft
* Unknown
entry AH 1Eh
return AL 0
note Apparently does nothing
1Fh Get Default Drive Parameter Block
* Same as function call 32h (below), except that the table is accessed from
the default drive
entry AH 1Fh
other registers unknown
return AL 00h no error
0FFh error
DS:BX points to DOS Disk Parameter Block for default drive.
note 1) Unknown vector returned in ES:BX.
2) For DOS 2.x and 3.x, this just invokes function 32h (undocumented,
Read DOS Disk Block) with DL=0
20h Unknown
* Internal - does nothing?
entry AH 20h
return AL 0
21h Random Read from File Specified by File Control Block (FCB)
Reads one record as specified in the FCB into the current DTA.
entry AH 21h
DS:DX address of the opened FCB
return AL 00h successful read operation
01h end of file (EOF), no data read
02h DTA too small for the record size specified
03h end of file (EOF), partial data read
note 1) The current block and current record fields are set to agree with the
random record field. Then the record addressed by these fields is read
into memory at the current Disk Transfer Address.
2) The current file pointers are NOT incremented this function.
3) If the DTA is larger than the file, the file is padded to the requested
length with zeroes.
22h Random Write to File Specified by FCB (FCB)
Writes one record as specified in the FCB to the current DTA
entry AH 22h
DS:DX address of the opened FCB
return AL 00h successful write operation
01h disk full; no data written (write was canceled)
02h DTA too small for the record size specified (write was
canceled)
note 1) This service cannot write to read-only files.
2) The record pointed to by the Current Block (offset 0Ch) and the Current
Record (offset 20h) fields is loaded at the DTA, then the Current Block
and Current Record fields are incremented.
3) If the record size is less than a sector, the data in the DTA is written
to a buffer; the buffer is written to disk when it contains a full
sector of data, the file is closed, or a Reset Disk (function 0Dh) is
issued.
4) The current file pointers are NOT incremented this function.
5) The record is written to disk at the current DTA address as specified
by the most recent call to function 1Ah. If the size of the record and
location of the DTA are such that a segment overflow or wraparound would
occur, the error return is set to AL=02h
23h Get File Size (FCB)
Searches current subdirectory for matching file, returns size in FCB
entry AH 23h
DS:DX address of an unopened FCB
return AL 00h file found
0FFh file not found
note 1) Record size field (offset 0Eh) must be set before invoking this function
2) The disk directory is searched for the matching entry. If a matching
entry is found, the random record field is set to the number of records
in the file. If the value of the Record Size field is not an even
divisor of the file size, the value set in the relative record field is
rounded up. This gives a returned value larger than the actual file size
3) This call is used by the APPEND command in DOS 3.2+
24h Set Relative Record Field (FCB)
Set random record field specified by an FCB
entry AH 24h
DS:DX address of an opened FCB
return Random Record Field of FCB is set to be same as Current Block
and Current Record.
note 1) You must invoke this function before performing random file access.
2) The relative record field of FCB (offset 21h) is set to be same as the
Current Block (offset 0Ch) and Current Record (offset 20h).
3) No error codes are returned.
4) The FCB must already be opened.
25h Set Interrupt Vector
Sets the address of the code DOS is to perform each time the specified
interrupt is invoked.
entry AH 25h
AL int number to reassign the handler to
DS:DX address of new interrupt vector
return none
note 1) Registers are unchanged.
2) No error codes are returned.
3) The interrupt vector table for the interrupt number specified in AL
is set to the address contained in DS:DX. Use function 35h (Get Vector)
to get the contents of the interrupt vector and save it for later use.
4) When you use function 25 to set an interrupt vector, DOS 3.2 doesn't
point the actual interrupt vector to what you requested. Instead, it
sets the interrupt vector to point to a routine inside DOS, which does
this:
1. Save old stack pointer
2. Switch to new stack pointer allocated from DOS's stack pool
3. Call your routine
4. Restore old stack pointer
The purpose for this was to avoid possible stack overflows when there
are a large number of active interrupts. IBM was concerned (this was an
IBM change, not Microsoft) that on a Token Ring network there would be
a lot of interrupts going on, and applications that hadn't allocated
very much stack space would get clobbered.
26h Create New Program Segment Prefix (PSP)
This service copies the current program-segment prefix to a new memory
location for the creation of a new program or overlay. Once the new PSP is
in place, a DOS program can read a DOS COM or overlay file into the memory
location immediately following the new PSP and pass control to it.
entry AH 26h
DX segment number for the new PSP
return none
note 1) Microsoft recommends you use the newer DOS service 4Bh (EXEC) instead.
2) The entire 100h area at location 0 in the current PSP is copied into
location 0 of the new PSP. The memory size information at location 6
in the new segment is updated and the current termination, ctrl-break,
and critical error addresses from interrupt vector table entries for
ints 22h, 23h, and 24 are saved in the new program segment starting at
0Ah. They are restored from this area when the program terminates.
3) Current PSP is copied to specified segment
27h Random Block Read From File Specified by FCB (FCB)
Similar to 21h (Random Read) except allows multiple files to be read.
entry AH 27h
CX number of records to be read
DS:DX address of an opened FCB
return AL 00h successful read
01h end of file, no data read
02h DTA too small for record size specified (read canceled)
03h end of file
CX actual number of records read (includes partial if AL=03h)
note 1) The record size is specified in the FCB. The service updates the Current
Block (offset 0Ch) and Current Record (offset 20h) fields to the next
record not read.
2) If CX contained 0 on entry, this is a NOP.
3) If the DTA is larger than the file, the file is padded to the requested
length with zeroes.
4) This function assumes that the FCB record size field (0Eh) is correctly
set. If not set by the user, the default is 128 bytes.
5) The record is written to disk at the current DTA address as specified
by the most recent call to function 1Ah. If the size of the record and
location of the DTA are such that a segment overflow or wraparound would
occur, the error return is set to AL=02h
28h Random Block Write to File Specified in FCB (FCB)
Similar to 27h (Random Write) except allows multiple files to be read.
entry AH 28h
CX number of records to write
DS:DX address of an opened FCB
return AL 00h successful write
01h disk full, no data written
02h DTA too small for record size specified (write canceled)
CX number of records written
note 1) The record size is specified in the FCB.
2) This service allocates disk clusters as required.
3) This function assumes that the FCB Record Size field (offset 0Eh) is
correctly set. If not set by the user, the default is 128 bytes.
4) The record size is specified in the FCB. The service updates the Current
Block (offset 0Ch) and Current Record (offset 20h) fields to the next
record not read.
5) The record is written to disk at the current DTA address as specified
by the most recent call to function 1Ah. If the size of the record and
location of the DTA are such that a segment overflow or wraparound would
occur, the error return is set to AL=02h
6) If called with CX=0, no records are written, but the FCB's File Size
entry (offset 1Ch) is set to the size specified by the FCB's Relative
Record field (offset 21h).
29h Parse the Command Line for Filename (FCB)
Parses a text string into the fields of a File Control Block
entry AH 29h
DS:SI pointer to string to parse
ES:DI pointer to memory buffer to fill with unopened FCB
AL bit mask to control parsing
bit 0 = 0: parsing stops if file seperator found
1: causes service to scan past leading chars such as
blanks. Otherwise assumes the filename begins in
the first byte
1 = 0: drive number in FCB set to default (0) if string
contains no drive number
1: drive number in FCB not changed
2 = 0: filename in FCB set to 8 blanks if no filename in
string
1: filename in FCB not changed if string does not
contain a filename
3 = 0: extension in FCB set to 3 blanks if no extension in
string
1: extension left unchanged
4-7 must be zero
return AL 00h no wildcards in name or extension
01h wildcards appeared in name or extension
0FFh invalid drive specifier
DS:SI pointer to the first character after the parsed string
ES:DI pointer to the unopened FCB
note 1) If the * wildcard characters are found in the command line, this service
will replace all subsequent chars in the FCB with question marks.
2) This service uses the characters as filename separators
DOS 1 : ; . , + / [ ] = " TAB SPACE
DOS 2,3 : ; . , + = TAB SPACE
3) This service uses the characters
: ; . , + < > | / \ [ ] = " TAB SPACE
or any control characters as valid filename separators
4) A filename cannot contain a filename terminator. If one is encountered,
all processing stops. The handle functions will allow use of some of
these characters.
5) If no valid filename was found on the command line, ES:DI +1 points
to a blank (ASCII 32).
6) This function cannot be used with filespecs which include a path
7) Parsing is in the form D:FILENAME.EXT. If one is found, a corresponding
unopened FCB is built at ES:DI
2Ah Get Date
Returns day of the week, year, month, and date
entry AH 2Ah
return CX year (1980-2099)
DH month (1-12)
DL day (1-31)
AL weekday 00h Sunday
01h Monday
02h Tuesday
03h Wednesday
04h Thursday
05h Friday
06h Saturday
note 1) Date is adjusted automatically if clock rolls over to the next day,
and takes leap years and number of days in each month into account.
2) Although DOS cannot set an invalid date, it can read one, such as
1/32/80, etc.
3) DesQview also accepts CX = 4445h and DX = 5351h, i.e. 'DESQ' as valid
4) DOS will accept CH=0 (midnight) as a valid time, but if a file's time
is set to exactly midnight the time will not be displayed by the DIR
command.
2Bh Set Date
set current system date
entry AH 2Bh
CX year (1980-2099)
DH month (1-12)
DL day (1-31)
return AL 00h no error (valid date)
0FFh invalid date specified
note 1) On entry, CX:DX must have a valid date in the same format as returned
by function call 2Ah
2) DOS 3.3 also sets CMOS clock
2Ch Get Time
Get current system time from CLOCK$ driver
entry AH 2Ch
return CH hours (0-23)
CL minutes (0-59)
DH seconds (0-59)
DL hundredths of a second (0-99)
note 1) Time is updated every 5/100 second.
2) The date and time are in binary format
2Dh Set Time
Sets current system time
entry AH 2Dh
CH hours (0-23)
CL minutes (0-59)
DH seconds (0-59)
DL hundredths of seconds (0-99)
return AL 00h if no error
0FFh if bad value sent to routine
note 1) DOS 3.3 also sets CMOS clock
2) CX and DX must contain a valid time in binary
2Eh Set/Reset Verify Switch
Set verify flag
entry AH 2Eh
AL 00 to turn verify off (default)
01 to turn verify on
return none
note 1) This is the call invoked by the DOS VERIFY command
2) Setting of the verify switch can be obtained by calling call 54h
3) This call is not supported on network drives
4) DOS checks this flag each time it accesses a disk
2Fh Get Disk Transfer Address (DTA)
Returns current disk transfer address used by all DOS read/write operations
entry AH 2Fh
return ES:BX address of DTA
note 1) The DTA is set by function call 1Ah
2) Default DTA address is a 128 byte buffer at offset 80h in that program's
Program Segment Prefix
30h Get DOS Version Number
Return DOS version and/or user number
entry AH 30h
return AH minor version number (i.e., DOS 2.10 returns AX = 0A02h)
AL major version number
BH OEM ID number
00h IBM
16h DEC
BL:CX 24-bit user serial number
note 1) If AL returns a major version number of zero, the DOS version is
below 1.28 for MSDOS and below 2.00 for PCDOS.
2) IBM PC-DOS always returns 0000h in BX and CX.
31h Terminate Process and Stay Resident
KEEP, or TSR
entry AH 31h
AL exit code
DX program memory requirement in 16 byte paragraphs
return AX return code (retrieveable by function 4Dh)
note 1) Files opened by the application are not closed when this call is made
2) Memory can be used more efficiently if the block containing the copy of
the DOS environment is deallocated before terminating. This can be done
by loading ES with the segment contained in 2Ch of the PSP and issuing
function call 49h (Free Allocated Memory).
3) Unlike int 27h, more than 64k may be made resident with this call
32h Read DOS Disk Block
* Retrieve the pointer to the drive parameter block for a drive
entry AH 32h
DL drive (0=default, 1=A:, etc.).
return AL 00h if drive is valid
0FFh if drive is not valid
DS:BX points to DOS Drive Parameter Table. Format of block:
Bytes Type Value
00h byte Drive: 0=A:, 1=B:, etc.
01h byte Unit within drive (0, 1, 2, etc.)
02h-03h word Bytes per sector
04h byte Sectors per cluster - 1
05h byte Cluster to sector shift (i.e., how far to shift-
left the bytes/sector to get bytes/cluster)
06h-07h word Number of reserved (boot) sectors
08h byte Number of FATs
09h-0Ah word Number of root directory entries
0Bh-0Ch word Sector # of 1st data. Should be same as # of
sectors/track.
0Dh-0Eh word # of clusters + 1 (=last cluster #)
0Fh byte Sectors for FAT
10h-11h word First sector of root directory
12h-15h dword Address of device driver header for this drive
16h byte Media Descriptor Byte for this drive
17h byte Zero if disk has been accessed
18h-1Bh dword address of next DOS Disk Block (0FFFFh means
last in chain)
22h byte Current Working Directory (2.0 only) (64 bytes)
note 1) Use [BX+0D] to find no. of clusters (>1000H, 16-bit FAT; if not, 12-bit
(exact dividing line is probably a little below 1000h to allow for
bad sectors, EOF markers, etc.)
2) Short article by C.Petzold, PC Magazine Vol.5,no.8, and the article
"Finding Disk Parameters" in the May 1986 issue of PC Tech Journal.
3) This call is mostly supported in OS/2 1.0's DOS Compatibility Box. The
dword at 12h will not return the address of the next device driver when
in the Compatibility Box.
33h Control-Break Check
Get or set control-break checking at CON
entry AH 33h
AL 00h to test for break checking
01h to set break checking
DL 00h to disable break checking
01h to enable break checking
02h internal, called by PRINT.COM (DOS 3.1)
return DL 00h if break=off
01h if break=on
AL 0FFh error
34h Return INDOS Flag
* Returns ES:BX pointing to Critical Section Flag, byte indicating whether
it is safe to interrupt DOS.
entry AH 34h
return ES:BX points to DOS "critical section flag"
note 1) If byte is 0, it is safe to interrupt DOS. This was mentioned in some
documentation by Microsoft on a TSR standard, and PC Magazine reports
it functions reliably under DOS versions 2.0 through 3.3. Chris
Dunford (of CED fame) and a number of anonymous messages on the BBSs
indicate it may not be totally reliable.
2) The byte at ES:BX+1 is used by the Print program for this same purpose,
so it's probably safer to check the WORD at ES:BX.
3) Reportedly, examination of DOS 2.10 code in this area indicates that the
byte immediately following this "critical section flag" must be 00h to
permit the PRINT.COM interrupt to be called. For DOS 3.0 and 3.1 (except
Compaq DOS 3.0), the byte before the "critical section flag" must be
zero; for Compaq DOS 3.0, the byte 01AAh before it must be zero.
4) In DOS 3.10 this reportedly changed to word value, with preceding byte.
5) This call is supported in OS/2 1.0's DOS Compatibility Box
6) Gordon Letwin of Microsoft discussed this call on ARPAnet in 1984. He
stated:
a) this is not supported under any version of the DOS
b) it usually works under DOS 2, but there may be circumstances
when it doesn't (general disclaimer, don't know of a specific
circumstance)
c) it will usually not work under DOS 3 and DOS 3.1; the DOS is
considerably restructured and this flag takes on additional
meanings and uses
d) it will fail catastrophically under DOS 4.0 and forward.
Obviously this information is incorrect since the call works fine
through DOS 3.3. Microsoft glasnost?
35h Get Vector
Get interrupt vector
entry AH 35h
AL interrupt number (hexadecimal)
return ES:BX address of interrupt vector
note Use function call 25h to set the interrupt vectors
36h Get Disk Free Space
get information on specified drive
entry AH 36h
DL drive number (0=default, 1=A:, 2=B:, etc)
return AX number of sectors per cluster
0FFFFh means drive specified in DL is invalid
BX number of availible clusters
CX bytes per sector
DX clusters per drive
note 1) Mult AX * CX * BX for free space on disk
2) Mult AX * CX * DX for total disk space
3) Function 36h returns an incorrect value after an ASSIGN command. Prior
to ASSIGN, the DX register contains 0943h on return, which is the free
space in clusters on the HC diskette. After ASSIGN, even with no
parameters, 0901h is returned in the DX register; this is an incorrect
value. Similar results occur with DD diskettes on a PC-XT or a PC-AT.
This occurs only when the disk is not the default drive. Results are as
expected when the drive is the default drive. Therefore, the
circumvention is to make the desired drive the default drive prior to
issuing this function call.
4) Int 21h, function call 36h returns an incorrect value after an ASSIGN
command. Prior to ASSIGN, the DX register contains 0943h on return,
which is the free space in clusters on the HC diskette. After ASSIGN,
even with no parameters, 0901h is returned in the DX register; this is
an incorrect value. Similar results occur with DD diskettes on a PC-XT
or a PC-AT. This occurs only when the disk is not the default drive.
Results are as expected when the drive is the default drive. Therefore,
the circumvention is to make the desired drive the default drive prior
to issuing this function call.
5) This function supercedes functions 1Bh and 1Ch.
37h SWITCHAR / AVAILDEV
* Get/set option marking character (is usually "/"), and device type
entry AH 37h
AL 00h read switch character (returns current character in DL)
01h set character in DL as new switch character
(DOS 2.x) 02h read device availability (as set by function AL=3) into
DL. A 0 means devices that devices must be accessed in
file I/O calls by /dev/device. A non-zero value means
that devices are accessible at every level of the
directory tree (e.g., PRN is the printer and not a file
PRN).
AL=2 to return flag in DL, AL=3 to set from DL (0 = set,
1 = not set).
(DOS 2.x) 03h get device availability, where:
DL 00h means /dev/ must precede device names
01h means /dev/ need not precede device names
return DL switch character (if AL=0 or 1)
device availability flag (if AL=2 or 3)
AL 0FFh the value in AL was not in the range 0-3.
note 1) Functions 2 & 3 appear not to be implemented for DOS 3.x.
2) It is documented on page 4.324 of the MS-DOS (version 2) Programmer's
Utility Pack (Microsoft - published by Zenith).
3) Works on all versions of IBM PC-DOS from 2.0 through 3.3.1.
4) The SWITCHAR is the character used for "switches" in DOS command
arguments (defaults to '/', as in "DIR/P"). '-' is popular to make a
system look more like UNIX; if the SWITCHAR is anything other than '/',
then '/' may be used instead of '\' for pathnames
5) Ignored by XCOPY, PKARC, LIST
6) SWITCHAR may not be set to any character used in a filename
7) In DOS 3.x you can still read the "AVAILDEV" byte with subfunction 02h
but it always returns 0FFh even if you try to change it to 0 with
subfunction 03h.
8) AVAILDEV=0 means that devices must be referenced in an imaginary
subdirectory "\dev" (similar to UNIX's /dev/*); a filename "PRN.DAT"
can be created on disk and manipulated like any other. If AVAILDEV != 0
then device names are recognized anywhere (this is the default):
"PRN.DAT" is synonymous with "PRN:".
9) These functions reportedly are not supported in the same fashion in
various implementations of DOS.
38h Return Country Dependent Information (PCDOS 2.0, 2.1, MSDOS 2.00 only)
get country-dependent information
entry AH 38h
AL function code (must be 0 in DOS 2.x)
DS:DX pointer to 32 byte memory area
return AX error code if CF set
DS:DX country data if CF not set
word date/time format
0 = USA standard H:M:S M/D/Y
1 = European standard H:M:S D/M/Y
2 = Japanese standard H:M:S D:M:Y
byte ASCIIZ string currency symbol followed by byte of zeroes
byte ASCIIZ string thousands separator followed by byte of
zeroes
byte ASCIIZ string decimal separator followed by byte of
zeroes
24 bytes reserved
38h Get Country Dependent Information (PCDOS 3.x+, MSDOS 2.01+)
get country-dependent information
entry AH 38h
AL function code
00h to get current country information
code country code to get information for, for countries
with codes less than 255
0FFh to get country information for countries with a code
greater than 255
BX 16 bit country code if AL=0FFh
DS:DX pointer to the memory buffer where the data will be returned
return CF 0 (clear) function completed
1 (set) error
AX error code if CF set
2 invalid country code (no table for it)
BX country code (usually international telephone code)
DS:DX country data if CF not set
word date/time format
0 = USA standard H:M:S M/D/Y
1 = European standard H:M:S D/M/Y
2 = Japanese standard H:M:S D:M:Y
5 bytes currency symbol null terminated
2 bytes thousands separator null terminated
2 bytes decimal separator null terminated
2 bytes date separator null terminated
2 bytes time separator null terminated
byte bit field currency format
bit 0 = 0 if currency symbol precedes the value
1 if currency symbol is after the value
bit 1 = 0 no spaces between value and currency symbol
1 one space between value and currency symbol
bits 2-7 not defined by Microsoft
byte number of significant decimal digits in currency
(number of places to right of decimal point)
byte time format
bit 0 = 0 12 hour clock
bit 0 = 1 24 hour clock
2 words case map call address
entry AL ASCII code of character to be converted to
uppercase
return AL ASCII code of the uppercase input character
2 bytes data list separator null terminated
5 words reserved
note 1) When an alternate keyboard handler is invoked, the keyboard routine is
loaded into user memory starting at the lowest portion of availible
user memory. The BIOS interrupt vector that services the keyboard is
redirected to the memory area where the new routine resides. Each new
routine takes up about 1.6K of memory and has lookup tables that return
values unique to each language. (KEYBxx in the DOS book)
Once the keyboard interrupt vector is changed by the DOS keyboard
routine, the new routine services all calls unless the system is
returned to the US format by the ctrl-alt-F1 keystroke combination. This
does not change the interrupt vector back to the BIOS location; it
merely passes the table lookup to the ROM locations.
2) Ctrl-Alt-F1 will only change systems with US ROMS to the US layout.
Some systems are delivered with non-US keyboard handler routines in ROM
3) Case mapping call: the segment/offset of a FAR procedure that performs
country-specific lower-to-upper case mapping on ASCII characters 80h to
0FFh. It is called with the character to be mapped in AL. If there is
an uppercase code for the letter, it is returned in AL, if there is no
code or the function was called with a value of less than 80h AL is
returned unchanged.
38h Set Country Dependent Information
set country-dependent information
entry AH 38h
AL code country code to set information for, for countries
with codes less than 255
0FFh to set country information for countries with a code
greater than 255
BX 16 bit country code if AL=0FFh
DX 0FFFFh
return CF clear successful
set if error
AX error code if CF flag set
39h Create Subdirectory (MKDIR)
Makes a subdirectory along the indicated path
entry AH 39h
DS:DX address of ASCIIZ pathname string
return flag CF 0 successful
1 error
AX error code if any (3, 5)
note 1) The ASCIIZ string may contain drive and subdirectory.
2) Drive may be any valid drive (not nescessarily current drive)
3) The pathname cannot exceed 64 characters
3Ah Remove Subdirectory (RMDIR)
remove a directory entry
entry AH 3Ah
DS:DX address of ASCIIZ pathname string
return CF clear successful
set AX error code if any (3, 5, 16)
note 1) The ASCIIZ string may contain drive and subdirectory.
2) Drive may be any valid drive (not nescessarily current drive)
3) The pathname cannot exceed 64 characters
3Bh Change Current Directory
(CHDIR)
entry AH 3Bh
DS:DX address of ASCIIZ string
return flag CF 0 successful
1 error
AX error code if any (3)
note 1) The pathname cannot exceed 64 characters
2) The ASCIIZ string may contain drive and subdirectory.
3) Drive may be any valid drive (not nescessarily current drive)
3Ch Create A File (CREAT)
create a file with handle
entry AH 3Ch
CX attributes for file
00h normal
01h read only
02h hidden
03h system
DS:DX address of ASCIIZ filename string
return flag CF 0 successful creation
1 error
AX 16 bit file handle
or error code (3, 4, 5)
note 1) The ASCIIZ string may contain drive and subdirectory.
2) Drive may be any valid drive (not nescessarily current drive)
3) If the volume label or subdirectory bits are set in CX, they are ignored
4) The file is opened in read/write mode
5) If the file does not exist, it is created. If one of the same name
exists, it is truncated to a length of 0.
3Dh Open A File
Open disk file with handle
entry AH 3Dh
AL access code byte
(DOS 2.x) bits 0-2 file attribute
000 read only
001 write only
010 read/write
bits 3-7 should be set to zero
(DOS 3.x) bits 0-2 file attribute
000 read only
001 write only
010 read/write
bit 3 reserved
0 should be set to zero
bits 4-6 sharing mode (network)
000 compatibility mode (the way FCBs open files)
001 read/write access denied (exclusive)
010 write access denied
011 read access denied
100 full access permitted
bit 7 inheritance flag
0 file inherited by child process
1 file private to child process
DS:DX address of ASCIIZ pathname string
return flag CF set on error
AX error code
1 error
AX 16 bit file handle
or error code (1, 2, 4, 5, 0Ch)
note 1) Opens any normal, system, or hidden file
2) Files that end in a colon are not opened
3) The rear/write pointer is set at the first byte of the file and the
record size of the file is 1 byte (the read/write pointer can be changed
through function call 42h). The returned file handle must be used for
all subsequent input and output to the file.
4) If the file handle was inherited from a parent process or was
duplicated by DUP or FORCEDUP, all sharing and access restrictions are
also inherited.
5) A file sharing error (error 1) causes an int 24h to execute with an
error code of 2
3Eh Close A File Handle
Close a file and release handle for reuse
entry AH 3Eh
BX file handle
return flag CF 0 successful close
1 error
AX error code if error (6)
note 1) When executed, the file is closed, the directory is updated, and all
buffers for that file are flushed. If the file was changed, the time
and date stamps are changed to current
2) If called with the handle 00000, it will close STDIN (normally the
keyboard).
3Fh Read From A File Or Device
Read from file with handle
entry AH 3Fh
BX file handle
CX number of bytes to read
DS:DX address of buffer
return flag CF 0 successful read
1 error
AX 0 pointer was already at end of file
or number of bytes read
or error code (5, 6)
note 1) This function attempts to transfer the number of bytes specified in CX
to a buffer location. It is not guaranteed that all bytes will be read.
If AX < CX a partial record was read.
2) If performed from STDIN (file handle 0000), the input can be redirected
3) If used to read the keyboard, it will only read to the first CR
4) The file pointer is incremented to the last byte read.
40h Write To A File Or Device
Write to file with handle
entry AH 40h
BX file handle
CX number of bytes to write
DS:DX address of buffer
return flag CF 0 successful write
1 error
AX number of bytes written
or error code (5, 6)
note 1) This call attempts to transfer the number of bytes indicated in CX
from a buffer to a file. If CX and AX do not match after the write,
an error has taken place; however no error code will be returned for
this problem. This is usually caused by a full disk.
2) If the write is performed to STDOUT (handle 0001), it may be redirected
3) To truncate the file at the current position of the file pointer, set
the number of bytes in CX to zero before calling int 21h. The pointer
can be moved to any desired position with function 42h.
4) This function will not write to a file or device marked read-only.
41h Delete A File From A Specified Subdirectory
(UNLINK)
entry AH 41h
DS:DX pointer to ASCIIZ filespec to delete
return CF 0 successful
1 error
AX error code if any (2, 5)
note 1) This function will not work on a file marked read-only
2) Wildcards are not accepted
42h Move a File Read/Write Pointer
(LSEEK)
entry AH 42h
AL method code
00h offset from beginning of file
01h offset from present location
02h offset from end of file
BX file handle
CX most significant half of offset
DX least significant half of offset
return AX low offset of new file pointer
DX high offset of new file pointer
CF 0 successful move
1 error
AX error code (1, 6)
note 1) If pointer is at end of file, reflects file size in bytes.
2) The value in DX:AX is the absolute 32 bit byte offset from the beginning
of the file
43h Get/Set file attributes
(CHMOD)
entry AH 43h
AL 00h get file attributes
01h set file attributes
CX file attributes to set
bit 0 read only
1 hidden file
2 system file
3 volume label
4 subdirectory
5 written since backup
DS:DX pointer to full ASCIIZ file name
return CF set if error
AX error code (1, 2, 3, 5)
CX file attributes on get
attributes:
01h read only
02h hidden
04h system
0FFh archive
44h I/O Control for Devices (IOCTL)
Get or set device information
entry AH 44h
AL 00h get device information (from DX)
BX file or device handle
return DX device info
If bit 7 set: (character device)
bit 0: console input device
1: console output device
2: NUL device
3: CLOCK$ device
4: device is special
5: binary (raw) mode
6: Not EOF
12: network device (DOS 3.x)
14: can process IOCTL control
strings (func 2-5)
If bit 7 clear: (file)
bits 0-5 are block device number
6: file has not been written
12: Network device (DOS 3.x)
15: file is remote (DOS 3.x)
01h set device information (DH must be zero for this call)
DX bits:
0 1 console input device
1 1 console output device
2 1 null device
3 1 clock device
4 1 reserved
5 0 binary mode - don't check for control chars
1 cooked mode - check for control chars
6 0 EOF - End Of File on input
7 device is character device if set, if not, EOF
is 0 if channel has been written, bits 0-5 are
block device number
12 network device
14 1 can process control strings (AL 2-5, can only be
read, cannot be set)
15 n reserved
02h read CX bytes to device in DS:DX from BX control chan
03h Write Device Control String
BX device handle
CX number of bytes to write
DS:DX pointer to buffer
return AX number of bytes written
04h read from block device (drive number in BL)
BL drive number (0=default)
CX number of bytes to read
DS:DX pointer to buffer
return AX number of bytes read
05h write to block device (drive number in BL)
AX number of bytes transfered
06h get input handle status
07h get output handle status
AX 0FFh for ready
00h for not ready
08h removable media bit (DOS 3.x)
return AX 00h device is removable
01h device is nonremovable
0Fh invalid drive specification
09h test local or network device in BL (DOS 3.x)
BL drive number (0=default)
return DX attribute word, bit 12 set if device is
remote
0Ah is handle in BX local or remote? (DOS 3.x)
BX file handle
return DX (attribute word) bit 15 set if file is remote
0Bh change sharing retry count to DX (default=3), (DOS 3.x)
CX delay (default 1)
DX retry count (default 3)
0Ch general IOCTL (DOS 3.3 [3.2?]) allows a device driver to
prepare, select, refresh, and query Code Pages
0Dh Block Device Request (DOS 3.3+)
BL drive number (0=default)
CH major subfunction
CL minor subfunction
40h set device parameters
41h write logical device track
42h format and verify logical device track
60h get device parameters
61h read logical device track
62h verify logical device track
DS:DX pointer to parameter block
0Eh GET LOGICAL DEVICE (DOS 3.3+)
BL drive number (0=default)
return AL=0 block device has only one logical drive
assigned 1..n the last letter used to reference
the device (1=A:,etc)
0Fh Set Logical Device (DOS 3.3+)
BL drive number: 0=default, 1=A:, 2=B:, etc.
BX file handle
CX number of bytes to read or write
DS:DX data or buffer
DX data
return AX number of bytes transferred
or error code (call function 59h for extended error codes)
or status
00h not ready
0FFh ready
CF set if error
45h Duplicate a File Handle (DUP)
Create duplicate handle
entry AH 45h
BX file handle to duplicate
return CF clear AX duplicate handle
set AX error code (4, 6)
note 1) If you move the pointed of one handle, the pointer of the other will
also be moved.
2) The handle in BX must be open
46h Force Duplicate of a Handle (FORCEDUP or CDUP)
forces handle in CX to refer to the same file at the same position as BX
entry AH 46h
BX existing file handle
CX new file handle
return CF clear both handles now refer to existing file
set error
AX error code (4, 6)
note 1) If CX was an open file, it is closed first
2) If you move the read/write pointer of either file, both will move
3) The handle in BX must be open
47h Get Current Directory
places full pathname of current directory/drive into a buffer
entry AH 47h
DL drive (0=default, 1=A:, etc.)
DS:SI points to 64-byte buffer area
return CF clear DS:DI pointer to ASCIIZ pathname of current directory
set AX error code (0Fh)
note String does not begin with a drive identifier or a backslash
48h Allocate Memory
allocates requested number of 16-byte paragraphs of memory
entry AH 48h
BX number of 16-byte paragraphs desired
return CF clear AX segment address of allocated space
BX maximum number paragraphs available
set AX error code (7, 8)
note BX indicates maximum memory availible only if allocation fails
49h Free Allocated Memory
frees specified memory blocks
entry AH 49h
ES segment address of area to be freed
return CF clear successful
set AX error code (7, 9)
note 1) This call is only valid when freeing memory obtained by function 48h.
2) A program should not try to release memory not belonging to it.
4Ah Modify Allocated Memory Blocks (SETBLOCK)
expand or shrink memory for a program
entry AH 4AH
BX new size in 16 byte paragraphs
ES segment address of block to change
return CF clear nothing
set AX error code (7, 8, 9)
or BX max number paragraphs available
note 1) Max number paragraphs availible is returned only if the call fails
2) Memory can be expanded only if there is memory availible
4Bh Load or Execute a Program
(EXEC)
entry AH 4Bh
AL 00h load and execute program. A PSP is built for the program
the ctrl-break and terminate addresses are set to the
new PSP.
*01h load but don't execute (note 1)
*02h load (internal) but do not execute
03h load overlay (do not create PSP, do not begin execution)
DS:DX points to the ASCIIZ string with the drive, path, and filename
to be loaded
ES:BX points to a parameter block for the load
(AL=00h) word segment address of environment string to be
passed
dword pointer to the command line to be placed at
PSP+80h
dword pointer to default FCB to be passed at PSP+5Ch
dword pointer to default FCB to be passed at PSP+5Ch
(AL=03h) word segment address where file will be loaded
word relocation factor to be applied to the image
return CF clear successful
set error
AX error code (1, 2, 8, 0Ah, 0Bh)
note 1) If you make this call with AL=1 the program will be loaded as if you
made the call with AL=0 except that the program will not be executed.
Additionally, with AL=1 the stack segment and pointer along with the
program's CS:IP entry point are returned to the program which made the
4B01h call. These values are put in the four words at ES:BX+0eh. On
entry to the call ES:BX points to the environment address, the command
line and the two default FCBs. This form of EXEC is used by DEBUG.COM.
2) Application programs may invoke a secondary copy of the command
processor (normally COMMAND.COM) by using the EXEC function. Your
program may pass a DOS command as a parameter that the secondary
command processor will execute as though it had been entered from the
standard input device.
The procedure is:
A. Assure that adequate free memory (17k for 2.x and 3.0, 23k for 3.1
up) exists to contain the second copy of the command processor and
the command it is to execute. This is accomplished by executing
function call 4Ah to shrink memory allocated to that of your current
requirements. Next, execute function call 48h with BX=0FFFFh. This
returns the amount of memory availible.
B. Build a parameter string for the secondary command processor in the
form:
1 byte length of parameter string
xx bytes parameter string
1 byte 0Dh (carriage return)
For example, the assembly language statement below would build the
string to cause execution of the command FOO.EXE:
DB 19, "/C C:FOO" , 13
C. Use the EXEC function call (4Bh), function value 0 to cause execution
of the secondary copy of the command processor. (The drive,
directory, and name of the command processor can be gotten from the
COMSPEC variable in the DOS environment passed to you at PSP+2Ch.)
D. Remember to set offset 2 of the EXEC control block to point to the
string built above.
3) All open files of a process are duplicated in the newly created
process after an EXEC, except for files originally opened with the
inheritance bit set to 1.
4) The environment is a copy of the original command processor's
environment. Changes to the EXECed environment are not passed back to
the original. The environment is followed by a copy of the DS:DX
filename passed to the child process. A zero value will cause the
child process to inherit the environment of the calling process. The
segment address of the environment is placed at offset 2Ch of the
PSP of the program being invoked.
5) This function uses the same resident part of COMMAND.COM, but makes a
duplicate of the transient part.
6) How EXEC knows where to return to: Basically the vector for int 22h
holds the terminate address for the current process. When a process
gets started, the previous contents of int 22h get tucked away in the
PSP for that process, then int 22h gets modified. So if Process A
EXECs process B, while Process B is running, the vector for int 22h
holds the address to return to in Process A, while the save location in
Process B's PSP holds the address that process A will return to when
*it* terminates. When Process B terminates by one of the usual legal
means, the contents of int 22h are (surmising) shoved onto the stack,
the old terminate vector contents are copied back to int 22h vector from
Process B's PSP, then a RETF or equivalent is executed to return control
to process A.
4Ch Terminate a Process (EXIT)
Quit with exit code
entry AH 4Ch
AL exit code in AL when called, if any, is passed to next process
return none
note 1) Control passes to DOS or calling program
2) return code from AL can be retrieved by ERRORLEVEL or function 4Dh
3) all files opened by this process are closed, buffers are flushed, and
the disk directory is updated
4) Restores Terminate vector from PSP:000Ah
Ctrl-C vector from PSP:000Eh
Critical Error vector from PSP:0012h
4Dh Get Return Code of a Subprocess (WAIT)
gets return code from functions 31h and 4Dh (ERRORLEVEL)
entry AH 4Dh
return AL exit code of subprogram (functions 31h or 4Ch)
AH circumstance which caused termination
00h normal termination
01h control-break
02h critical device error
03h terminate and stay resident (function 31h)
note The exit code is only returned once
4Eh Find First Matching File (FIND FIRST)
Find first ASCIIZ
entry AH 4Eh
CX search attributes
DS:DX pointer to ASCIIZ filename (with attributes)
return CF set AX error code (2, 12h)
clear data block written at current DTA
format of block is:
documented by Micro- |00h 1 byte attribute byte of search
soft as "reserved for |01h 1 byte drive used in search
DOS' use on subsquent |02h 11 bytes the search name used
Find Next calls" |0Ch 2 bytes word value of last entry
function 4Fh |0Fh 4 bytes dword pointer to this DTA
|13h 2 bytes word directory start
15h 1 byte file attribute
16h 2 bytes file time
18h 2 bytes file date
1Ah 2 bytes low word of file size
1Ch 2 bytes high word of file size
1Eh 13 bytes name and extension of file found, plus
1 byte of 0s. All blanks are removed
from the name and extension, and if an
extension is present it is preceded by a
period.
note 1) Will not find volume label
2) This function does not support network operations
3) Wildcards are allowed in the filespec
4) If the attribute is zero, only ordinary files are found. If the volume
label bit is set, only volume labels will be found. Any other attribute
will return that attribute and all normal files together.
5) To look for everything except the volume label, set the hidden, system,
and subdirectory bits all to 1
4Fh Find Next Matching File (FIND NEXT)
Find next ASCIIZ
entry AH 4Fh
return CF clear data block written at current DTA
set AX error code (2, 12h)
note 1) If file found, DTA is formatted as in call 4Eh
2) Volume label searches using 4Eh/4Fh reportedly aren't 100% reliable
under DOS 2.x. The calls sometime report there's a volume label and
point to a garbage DTA, and if the volume label is the only item they
often won't find it
3) This function does not support network operations
4) Use of this call assumes that the original filespec contained wildcards
50h Set PSP
* Set new Program Segment Prefix; current process ID
entry AH 50h
BX segment address of new PSP
return none - swaps PSP's regarded as current by DOS
note 1) By putting the PSP segment value into BX and issuing call 50h DOS stores
that value into a variable and uses that value whenever a file call is
made.
2) Note that in the PSP (or PDB) is a table of 20 (decimal) open file
handles. The table starts at offset 18h into the PSP. If there is an
0FFh in a byte then that handle is not in use. A number in one of the
bytes is an index into an internal FB table for that handle. For
instance the byte at offset 18h is for handle 0, at offset 19h handle
1, etc. up to 13h. If the high bit is set then the file associated by
the handle is not shared by child processes EXEC'd with call 4Bh.
3) Function 50h is dangerous in background operations prior to DOS 3.x as
it uses the wrong stack for saving registers. (same as functions
0..0Ch in DOS 2.x)
4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler
without setting the Critical Error flag
5) Open File information, etc. is stored in the PSP DOS views as current.
If a program (eg. a resident program) creates a need for a second PSP,
then the second PSP should be set as current to make sure DOS closes
that as opposed to the first when the second application finishes.
6) See PC Mag Vol.5, No 9, p.314 for discussion.
51h Get Program Segment Prefix
* Returns the PSP address of currently executing program
entry AH 51h
return BX address of currently executing program
note 1) Used in DOS 2.x, 3.x uses 62h
2) Function 51h is dangerous in background operations prior to DOS 3.x as
it uses the wrong stack for saving registers. (same as functions
0..0Ch in DOS 2.x)
3) 50h and 51h might be used if you have more than one process in a PC.
For instance if you have a resident program that needs to open a file
you could first call 51h to save the current id and then call 50h to set
the ID to your PSP.
4) Under DOS 2.x, this function cannot be invoked inside an int 28h handler
52h IN-VARS
* returns a pointer to a set of DOS data variables MCB chain, pointer to
first device driver and a pointer to disk parameter blocks (first one)
entry AH 52h
return ES:BX pointer to the DOS list of lists, for disk information. Does not
access the disk, so information in tables might be incorrect if
disk has been changed. Returns a pointer to the following array
of longword pointers:
Bytes Value
-2h,-1h segment of first memory control block
0h-3h pointer to first DOS disk block (see function 36h)
4h-7h partially unknown. Pointer to a device driver. Maybe
first resident driver?
8h-0Bh pointer to CLOCK$ device driver, whether installable or
resident
0Ch-0Fh pointer to actual CON: device driver, whether
installable or resident
(DOS 2.x)
10 Number of logical drives in system
11-12 Maximum bytes/block of any block device
13-16 unknown
17 Beginning (not a pointer. The real beginning!) of NUL
device driver. This is the first device on DOS's linked
list of device drivers.
(DOS 3.x)
10h-11h maximum bytes/block of any block device (0200h)
12h-15h unknown. Pointer to current directory block?
16h-19h partially undefined: Pointer to array of drive info:
51h bytes per drive, starting with A: ...
00h-3Fh current path as ASCIIZ, starting with 'x:\'
40h-43h unknown zeros always
44h unknown flags? Usually 40h, except for
entry after last valid entry = 00h
45h-48h pointer to DOS disk block for this drive
49h-4Ah unknown. Current track or block?
-1 if never accessed
4Bh-4Eh unknown -1 always
4Fh-52h unknown 2 always
1Ah-1Dh unknown. Pointer to data area, maybe including cluster
allocation table?
1Eh-1Fh unknown. Zero always
20h Number of block devices
21h value of LASTDRIVE command in CONFIG.SYS (default 5)
22h Beginning (not a pointer. The real beginning!) of NUL
device driver. This is the first device on DOS's linked
list of device drivers.
note This call is not supported in OS/2 1.0's DOS Compatibility Box
53h Translate BPB
* Translates BPB (BIOS Parameter Block, see below) into a DOS Disk Block (see
function call 32h).
entry AH 53h
DS:SI pointer to BPB
ES:BP pointer to area for DOS Disk Block.
Layout of Disk Block:
bytes value
00h-01h bytes per sector, get from DDB bytes 02h-03h.
02h sectors per cluster, get from (DDB byte 4) + 1
03h-04h reserved sectors, get from DDB bytes 06h-07h
05h number of FATs, get from DDB byte 08h
06h-07h number of root dir entries, get from DDB bytes 09h-0Ah
08h-09h total number of sectors, get from:
((DDB bytes 0Dh-0Eh) - 1) * (sectors per cluster (BPB
byte 2)) + (DDB bytes 0Bh-0Ch)
0Ah media descriptor byte, get from DDB byte 16h
0Bh-0Ch number of sectors per FAT, get from DDB byte 0Fh
return unknown
54h Get Verify Setting
Get verify flag status
entry AH 54h
return AL 00h if flag off
01h if flag on
note Flag can be set with function 2Eh
55h Create "Child" PSP
* Create PSP: similar to function 26h (which creates a new Program Segment
Prefix at segment in DX) except creates a "child" PSP rather than copying
the existing one.
entry AH 55h
DX segment number at which to create new PSP.
return unknown
note 1) This call is similar to call 26h which creates a PSP except that unlike
call 26h the segment address of the parent process is obtained from the
current process ID rather than from the CS value on the stack (from the
INT 21h call). DX has the new PSP value and SI contains the value to be
placed into PSP:2 (top of memory).
2) Function 55 is merely a substitute for function 26h. It will copy the
current PSP to the segment address DX with the addition that SI is
assumed to hold the new memory top segment. This means that function
26h sets SI to the segment found in the current PSP and then calls
function 55h.
56h Rename a File
if the first file exists, it is renamed to the name in ES:DI
entry AH 56h
DS:DX pointer to ASCIIZ old pathname
ES:DI pointer to ASCIIZ new pathname
return CF clear successful rename
set AX error code (2, 3, 5, 11h)
note 1) Works with files in same drive only
2) Global characters not allowed in filename
3) The name of a file is its full pathname. The file's full pathname can
be changed, while leaving the actual FILENAME.EXT unchanged. Changing
the pathname allows the file to be "moved" from subdirectory to
subdirectory on a logical drive without actually copying the file.
4) DOS 3.x allows renaming of directories
57h Get/Set a File's Date and Time
read or modify time and date stamp on a file's directory entry
entry AH 57h
AL function code
00h get date and time
01h set date and time
CX time to be set
DX date to be set
BX file handle
return CF clear CX time of last write (if AL = 0)
DX date of last write (if AL = 0)
set AX error code (1, 6)
note Date/time formats are:
CX bits 0Bh-0Fh hours (0-23) DX bits 09h-0Fh year (relative to 1980)
05h-0Ah minutes (0-59) 05h-08h month (0-12)
00h-04h #2 sec. incr. (0-29) 00h-04h day of the month (0-31)
58h Get/Set Allocation Strategy
DOS 3.x
entry AH 58h
AL 0 set current strategy
1 set new current strategy
BX new strategy if AH=1
0 first fit - chooses the lowest block in memory which
will fit (this is the default). (use first memory block
large enough)
1 best fit - chooses the smallest block which will fill
the request.
2 last fit - chooses the highest block which will fit.
return CF clear (0) successful
set (1) error (1)
AX error code
AX strategy code (CF=0)
note 1) Documented in Zenith DOS version 3.1, some in Advanced MSDOS
2) The set subfunction accepts any value in BL; 2 or greater means last
fit. The get subfunction returns the last value set, so programs should
check whether the value is greater than or equal to 2.
59h Get Extended Error Code (DOS 3.x)
returns additional error information when requested
The Get Extended Error function call (59h) is intended to provide a common
set of error codes and to supply more extensive information about the error
to the application. The information returned from function call 59h, in
addition to the error code, is the error class, the locus, and the
recommended action. The error class provides information about the error
type (hardware, internal, system, etc.). The locus provides information
about the area involved in the failure (serial device, block device,
network, or memory). The recommended action provides a default action for
programs that do not understand the specific error code.
Newly written programs should use the extended error support both from
interrupt 24h hard error handlers and after any int 21h function calls. FCB
function calls report an error by returning 0FFh in AL. Handle function
calls report an error by setting the carry flag and returning the error
code in AX. Int 21h handle function calls for DOS 2.x continue to return
error codes 0-18. Int 24h handle function calls continue to return error
codes 0-12. But the application can obtain any of the error codes used in
the extended error codes table by issuing function call 59h. Handle
function calls for DOS 3.x can return any of the error codes. However, it
is recommended that the function call be followed by function call 59h to
obtain the error class, the locus, and the recommended action.
The Get Extended Error function (59h) can always be called, regardless of
whether the previous DOS call was old style (error code in AL) or new style
(carry bit). It can also be used inside an int 24h handler.
You can either check AL or the carry bit to see if there was no error,
and call function 59h only if there was an error, or take the simple
approach of always calling 59h and letting it tell you if there was an
error or not. When you call function 59h it will return with AX=0 if the
previous DOS call was successful.
entry AH 59h
BX version code (0000 for DOS 3.0 and 3.1)
return AX extended error code:
01h Invalid function number
02h File not found
03h Path not found
04h Too many open files, no file handles left
05h Access denied
06h Invalid handle
07h Memory control blocks destroyed
08h Insufficient memory
09h Invalid memory block address
0Ah Invalid environment
0Bh Invalid format
0Ch Invalid access code
0Dh Invalid data
0Eh Reserved
0Fh Invalid drive was specified
10h Attempt to remove the current directory
11h Not same device
12h No more files
13h Attempt to write on write-protected diskette
14h Unknown unit
15h Drive not ready
16h Unknown command
17h Bad CRC check
18h Bad request structure length
19h Seek error
1Ah Unknown media type
1Bh Sector not found
1Ch Printer out of paper
1Dh Write fault
1Eh Read fault
1Fh General Failure
20h Sharing violation
21h Lock violation
22h Invalid disk change
23h FCB unavailible
24h Sharing buffer overflow
25h Reserved
26h "
27h "
28h "
29h "
2Ah "
2Bh "
2Ch "
2Dh "
2Eh "
2Fh "
30h "
31h Reserved
32h Network: request not supported (DOS 3.1 + MS Networks)
33h Remote computer not listening
34h Duplicate name on network
35h Network: name not found
36h Network: busy
37h Network: device no longer exists
38h NETBIOS command limit exceeded
39h Network: adapter hardware error
3Ah Incorrect response from network
3Bh Unexpected network error
3Ch Incompatible remote adapter
3Dh Print queue full
3Eh Not enough space for print file
3Fh Print file was deleted
40h Network: name was deleted
41h Network: Access denied
42h Network: device type incorrect
43h Network: name not found
44h Network: name limit exceeded
45h NETBIOS session limit exceeded
46h Temporarily paused
47h Network: request not accepted
48h Print or disk redirection paused (DOS 3.1 + MS Networks)
49h Reserved
4Ah "
4Bh "
4Ch "
4Dh "
4Eh "
4Fh Reserved
50h File exists
51h Reserved
52h Cannot make directory entry
53h Fail on interrupt 24h
54h Too many redirections
55h Duplicate redirection
56h Invalid password
57h Invalid parameter
58h Network: device fault
BH class of error:
01h Out of resource
02h Temporary situation
03h Authorization (denied access)
04h Internal
05h Hardware failure
06h System failure
07h Application program error
08h Not found
09h Bad format
0Ah Locked
0Bh Media error (wrong volume ID, disk failure)
0Ch Already exists
0Dh Unknown
BL suggested action code:
01h Retry
02h Delayed retry
03h Prompt user
04h Abort after cleanup
05h Immediate abort
06h Ignore
07h Retry after user intervention
CH locus (where error occurred):
01h Unknown or not appropriate
02h Block device
03h Network related
04h Serial device
05h Memory related
note 1) Not all DOS functions use the carry flag to indicate an error. Carry
should be tested only on those functions which are documented to use it.
2) None of the DOS functions which existed before 2.0 use the carry
indicator. Many of them use register AL as an error indication instead,
usually by putting 0FFh in AL on an error. Most, but not all, the "new"
(2.x, 3.x) functions do use carry, and most, but not all, of the "old"
(1.x) functions use AL.
3) On return, CL, DI, DS, DX, ES, BP, and SI are destroyed - save before
calling this function if required.
4) DOS 2.x Error Codes: If you are using function calls 38h-57h with DOS
2.x, to check if an error has occurred, check for the following error
codes in the AX register:
call error code call error code call error code
38h 2 41h 2,3,5 4Ah 7,8,9
39h 3,5 42h 1,6 4Bh 1,2,3,5,8,10,11
3Ah 3,5,15 43h 1,2,3,5 4Eh 2,3,18
3Bh 3 44h 1,3,5,6 4Fh 18
3Ch 3,4,5 45h 4,6 56h 2,3,5,17
3Dh 2,3,4,5,12 46h 4,6 57h 1,6
3Eh 6 47h 15
3Fh 5,6 48h 7,8
40h 5,6 49h 7,9
5) note that extended error codes 13h through 1Fh correspond to error
codes 00h through 0Ch returned by int 24h
5Ah Create Temporary File
Create unique filename (for temporary use) (DOS 3.x)
entry AH 5Ah
DS:DX pointer to ASCIIZ directory pathname ending with a backslash (\)
CX file attribute
return CF clear DS:DX new ASCIIZ path name
AX handle
set AX error code (3 or 5)
note 1) The file created is not truly "temporary". It must be removed by the
user.
2) If the filename created already exists in the current directory, this
function will call itself again with another unique filename until
a unique filename is found
5Bh Create a New File
(DOS 3.x)
entry AH 5Bh
DS:DX pointer to directory ASCIIZ path name
CX file attribute
return CF clear AX file handle
DS:DX new ASCIIZ path name
set AX error code (3, 4, 5, 50h)
note 1) Unlike function 3Ch, function 5Bh will fail if the file already exists.
2) The new file is opened in read/write mode
5Ch Lock/Unlock File Access
(DOS 3.x)
entry AH 5Ch
AL 00h to lock file
01h to unlock file
BX file handle
CX:DX starting offset of region to lock
SI:DI size of region to lock
return CF clear successful
set AX error code (1, 6, 21h)
note 1) Close all files before exiting or undefined results may occur
2) Programs spawned with EXEC inherit all the parent's file handles but
not the file locks
5Dh Set Extended Error Information
* DOS Internal - partial (DOS 3.x)
entry AH 5dh
AL subfunction
06h get address of critical error flag
return DS:SI pointer to critical error flag
08h (unknown - used by command.com)
09h (unknown - used by command.com)
0Ah set error info (Error, Class, Action, and Locus)
DS:DX address of 11-word error information
words 0 to 7: values of AX,BX,CX,DX,SI,DI,DS,
ES that function 59h will return
words 8 to 10: zero (reserved)
return: CX unknown
DX unknown
DS:SI (for 06h) pointer to critical error flag
note 1) This call seems to have many different functions
2) Function 0Ah; DOS 3.1+
3) Function 06h; setting CritErr flag allows use of functions 50h/51h from
int 28h under DOS 2.x by forcing use of correct stack
5Eh Network Printer (Partially documented by Microsoft)
DOS 3.1+ with Networks software
entry AH 5Eh
AL 00 Get Machine Name
DS:DX pointer to buffer for ASCIIZ name
return CH 0 if name not defined
CL NETBIOS name number if CH <> 0
DS:DX pointer to identifier if CH <> 0
note the ASCIIZ name is a 15 byte string padded
to length with zeroes
01 Set Machine Name
DS:DX pointer to ASCIIZ name
CH unknown
CL name number
02 Set Printer Control String
BX redirection list index
CX length of setup string (max 64 bytes)
DS:SI pointer to string buffer
03 Get Printer Control String
BX redirection list index
ES:DI pointer to string buffer
return CX length of setup string (max 64 bytes)
return CF clear successful
set error
AX error code (1 for all listed subfunctions)
note 1) Used in IBM's & Microsoft's Network programs
2) Partial documentation in Fall 1985 Byte
3) These services require that the network software be installed
4) Partial documentation in Advanced MS-DOS
5) SHARE must be loaded or results can be unpredictable on 00h, or fail
with 02h or 03h
5Fh Network Redirection
(DOS 3.1 + Microsoft Networks)
entry AH 5Fh
AL *00h Unknown
*01h Unknown
02h Get Redirection List Entry
BX redirection list index
DS:SI pointer to 16 byte buffer for local device name
ES:DI pointer to 128 byte buffer for network name
return BH device status flag (bit 0 = 0 if valid)
(bit 0 = 1 if invalid)
BL device type
03 printer device
04 drive device
CX stored parameter value
DS:SI pointer to 16 byte local device name
ES:DI pointer to 128 byte network name
note DX and BP are destroyed by this call!
03h Redirect Device
BL device type
03 printer device
04 file device
CX stored parameter value
DS:SI pointer to source device name
ES:DI pointer to destination ASCIIZ network path +
ASCIIZ password
04h Cancel Redirection
DS:SI pointer to ASCIIZ device name or network path
return CF clear successful
set if error
AX error code
return as above
note 1) Used in IBM's Network program
2) Partial documentation in Fall 1985 Byte
3) These services require that the network software be installed
4) Partial documentation in Advanced MS-DOS
5) SHARE must be loaded or the call will fail
6) The network device name requires a password
60h Parse pathname (DOS 3.x)
* Translate - perform name processing on a string (internal to DOS)
entry AH 60h
DS:SI pointer to source string (null terminated)
ES:DI pointer to destination string buffer.
return ES:DI buffer filled with qualified name
CF 0 no error
1 error
AX error code
note 1) Documented in Zenith 3.05 Tech Ref
2) All name processing is performed on the input string: string
substitution is performed on the components, current drive/directories
are prepended, . and .. are removed.
3) Example: If current drive/directory is c:\test, myfile.x is translated
to c:\test\myfile.x; ..\source\sample.asm is tranlated to c:\source\
sample.asm
4) It is the caller's responsibility to make sure DS:SI does not point to
a null string. If it does, SI is incremented, a null byte is stored at
ES:DI, and the routine returns.
61h No Information Availible (DOS 3.x)
* internal to DOS - parameters not known
entry AH 61h
return AL 0
note Supposedly documented in Zenith DOS 3.05 Tech Ref
62h Get Program Segment Prefix (PSP)
Get PSP address (DOS 3.x)
entry AH 62h
return BX segment address of PSP
63h Get Lead Byte Table (MS-DOS 2.25 only)
added in MS-DOS version 2.25 for additional foreign character set support.
entry AH 63h
AL subfunction
00h get system lead byte table address
01h set/clear interim console flag
DL 0 to clear interim console flag
1 to set interim console flag
02h get interim console flag
return DS:SI pointer to lead byte table (AL = 00h)
DL interim console flag (AL = 02h)
note Function 63h destroys all registers on return.
64h Internal
unknown (DOS 3.3+)
entry AH 64h
65h Get Extended Country Information (DOS 3.3+)
returns information about the selected country formats, code pages, and
conversion tables
entry AH 65h
AL info ID (1 - 6)
BX code page (-1 = global code page)
CX size of buffer
DX country ID (-1 = current country)
ES:DI pointer to country information buffer
return AX error code if carry set, otherwise
CX size of country information returned
CF set on error
ES:DI pointer to country information:
1 byte info ID
if info ID <> 1
dword pointer to information
if info ID = 1
word size
word country ID
word code page
34 bytes (see function 38h)
66h Get/Set Global Code Page Table (DOS 3.3+)
query/reset code page defaults
entry AH 66h
AL 00h Get Global Code Page
01h Set Global Page
BX active code page
DX system code page (active page at boot time)
return CF clear successful
set AX error code
if 00h BX active code page
DX system code page (active page at boot time)
note BX = active code page: 437 = US, 860 = Portugal, 863 = Canada (French)
865 = Norway/Denmark
67h Set Handle Count (DOS 3.3+)
supports more than 20 open files per process
entry AH 67h
BX desired number of handles (max 255)
return CF clear if OK
CF set if error
AX error code
68h Commit File (DOS 3.3+)
Write all buffered data to disk
entry AH 68h
BX file handle
return CF set AX error code
clear successful
note Faster and more secure method of closing a file in a network than
current close commands
69h Disk Serial Number DOS 4.0 (US)
Places and reads "Volume Serial Number" on disks formatted with 4.0+
entry unknown
return unknown
note A call for DOS function 69h (AL=0, possibly a subfunction) uses DS:DX
as a pointer to a table. On return, the table is filled in as follows:
word unknown (zeroes on my system)
dword disk serial number (binary)
char[11] volume label or "NO NAME " if none
char[8] FAT type
The FAT type field returns "FAT16 " on hard disk formatted with DOS
3.3 and "FAT12 " on a 360K floppy.
6Ah unknown (DOS 4.0?)
6Bh unknown (DOS 4.0?)
6Ch Extended Open/Create DOS 4.0 (US)
Combines functions available with Open, Create, Create New, and Commit
entry AH 6Ch
AL 00h reserved [which means there might be other subfunctions?]
BX mode format 0WF0 0000 ISSS 0AAA
AAA is access code (read, write, read/write)
SSS is sharing mode
I 0 pass handle to child
1 no inherit [interesting!]
F 0 use int 24h for errors
1 disable int 24h for all
I/O on this handle; use own
error routine
W 0 no commit
1 auto commit on all writes
CX create attribute
DL action if file exists/does not exists
bits 7-4 action if file does not exist
0000 fail
0001 create
bits 3-0 action if file exists
0000 fail
0001 open
0010 replace/open
DH 0
DS:SI pointer to ASCIIZ file name
return CF set on error
AX error code
clear
AX file handle
CX action taken
01h file opened
02h created/opened
03h replaced/opened
89h DOS_Sleep
* not documented by Microsoft
entry AH 89h
return unknown
note 1) Function included in Microsoft C 4.0 startup code MSDOS.INC
2) Debugging shows that the first instruction on entry to DOS compares AH
with 64h (at least in DOS 3.2) and aborts the call if AH > 64.
3) Possibly used in European MSDOS 4.0?
Aftermarket Application Installed Function Calls:
0B6h, 0B8h, 0BBh, 0BCh, B0Eh, 0BFh, 0C0h, 0C1h, 0C2h, 0C3h, 0C4h, 0C5h, 0C6h,
0C7h, 0C8h, 0C9h, 0CAh, 0CBh, 0CCh, 0CDh, 0CEh, 0CFh, 0D0h, 0D1h, 0D2h, 0D3h,
0D4h, 0D5h, 0D6h, 0D7h, 0DAh, 0DBh
Used by Novell NetWare
0DCh Novell NetWare
Get Station Number
entry AH 0DCh
return AL station number
00h if NetWare not loaded or this machine is a non-
dedicated server
0DDh Novell NetWare
0DEh Novell NetWare
Set Broadcast Mode
0DFh Novell NetWare
0E0h Novell NetWare
0E1h Novell NetWare
Broadcast Messages
entry AH E1h
AL 00h send broadcast message
01h get broadcase message
02h-09h unknown
0E2h Novell NetWare
0E3h Novell NetWare
Connection Control
entry AH E3h
AL 00h-14h unknown
15h get object connection numbers
16h get connection information
32h-47h unknown
E4h DoubleDOS
check status
entry AX 00h
return AL <> 0 if DoubleDOS is active
0E4h Novell NetWare
0E5h, 0E6h, 0E7h, 0E8h, 0E9h
Novell NetWare
0EAh DoubleDOS
turn off task switching
entry AX EAh
return task switching turned off
0EAh Novell NetWare
0EBh DoubleDOS
turn on task switching
entry AH EBh
return Task switching turned on
0EBh Novell NetWare
0ECh DoubleDOS
get virtual screen address
entry AH ECh
return ES segment of virtual screen
note Screen address can change if task switching is on!
0ECh Novell NetWare
0EDh Novell NetWare
0EEh DoubleDOS
give away time to other tasks
entry AH EEh
AL number of 55ms time slices to give away
return Returns after giving away time slices
0EEh Novell NetWare
Get Node Address
entry AH EEh
return CX:BX:AX = six-byte address
0EFh, 0F0h, 0F1h, 0F2h, 0F3h Reportedly used by Novell NetWare.
No parameters known
0FFh CED (CJ Dunford's DOS macro and command-line editor)
CED installable commands
entry AH 0FFh
AL 00h add installable command
01h remove installable command
02h reserved, may be used to test for CED installation
BL mode bit 0 = 1 callable from DOS prompt
bit 1 = 1 callable from application
DS:SI pointer to cr-terminated command name
ES:DI pointer to far routine entry point
return CF set on error
AX 01h invalid function
02h command not found (subfunction 1 only)
08h insufficient memory (subfunction 0 only)
0Eh bad data (subfunction 0 only)
AH 0FFh if CED not installed
Chapter 5
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
Interrupts 22h Through 86h
Interrupt 22h Terminate Address
(0:0088h)
This interrupt transfers control to the far (dword) address at this interrupt
location when an application program terminates. The default address for this
interrupt is 0:0088h through 0:008Bh. This address is copied into the program's
Program Segment Prefix at bytes 0Ah through 0Dh at the time the segment is
created and is restored from the PSP when the program terminates. The calling
program is normally COMMAND.COM or an application. Do not issue this interrupt
directly, as the EXEC function call does this for you. If an application
spawns a child process, it must set the Terminate Address prior to issuing the
EXEC function call, otherwise when the second program terminated it would
return to the calling program's Terminate Address rather than its own. This
address may be set with int 21, function 25h.
Interrupt 23h Ctrl-Break Exit Address
(0:008Ch)
If the user enters a Ctrl-Break during STDIN, STDOUT, STDPRN, or STDAUX, int
23h is executed. If BREAK is on, int 23h is checked on MOST function calls
(notably 06h). If the user written Ctrl-Break routine saves all registers, it
may end with a return-from-interrupt instruction (IRET) to continue program
execution. If the user-written interrupt program returns with a long return, the
carry flag is used to determine whether the program will be aborted. If the
carry flag is set, the program is aborted, otherwise execution continues (as
with a return by IRET). If the user-written Ctrl-Break interrupt uses function
calls 09h or 0Ah, then ctrl-C/CR/LF are output. If execution is continued with
an IRET, I/O continues from the start of the line. When the interrupt occurs,
all registers are set to the value they had when the original function call to
DOS was made. There are no restrictions on what the Ctrl-Break handler is
allowed to do, including DOS function calls, as long as the registers are
unchanged if an IRET is used. If the program creates a new segment and loads a
second program which itself changes the Ctrl-Break address, the termination of
the second program and return to the first causes the Ctrl-Break address to
be restored from the PSP to the value it had before execution of the second
program.
Interrupt 24h Critical Error Handler
(0:0090h)
When a critical error occurs within DOS, control is transferred to an error
handler with an int 24h. This may be the standard DOS error handler (ABORT,
RETRY, IGNORE) or a user-written routine.
On entry to the error handler, AH will have its bit 7=0 (high order bit)
if the error was a disk error (probably the most common error), bit 7=1 if
not.
BP:SI contains the address of a Device Header Control Block from which
additional information can be retrieved (see below).
The register is set up for a retry operation and an error code is in the
lower half of the DI register with the upper half undefined. These are the
error codes:
The user stack is in effect and contains the following from top to bottom:
IP DOS registers from issuing int 24h
CS int 24h
flags
AX user registers at time of signal
BX int 21h request
CX
DX
SI
DI
BP
DS
ES
IP from original int 21h
CS
flags
To reroute the critical error handler to a user-writen critical error handler,
the following should be done:
Before an int 24h occurs:
1) The user application initialization code should save the int 24h vector and
replace the vector with one pointing to the user error routine.
When the int 24h occurs:
2) When the user error routine received control it should push the flag
registers onto the stack and execute a far call to the original int 24h
vector saved in step 1.
3) DOS gives the appropriate prompt, and waits for user input (Abort, Retry,
Ignore, Fail). After the user input, DOS returns control to the user error
routine instruction following the far call.
4) The user error routine can now do any tasks nescessary. To return to the
original application at the point the error occurred, the error routine needs
to execute an IRET instruction. Otherwise, the user error routine should
remove the IP, CS, and flag registers from the stack. Control can then be
passed to the desired point.
Int 24h provides the following values in registers on entry to interrupt
handler:
entry AH status byte
bit 7 0 disk I/O hard error
1 other error - if block device, bad FAT
- if char device, code in DI
6 unused
5 0 if IGNORE is not allowed
1 if IGNORE is allowed
4 0 if RETRY is not allowed
1 if RETRY is allowed
3 0 if FAIL is not allowed
1 if FAIL is allowed
2 \ disk area of error 00 = DOS area 01 = FAT
1 / 10 = root dir 11 = data area
0 0 if read operation
1 if write operation
AL drive number if AH bit 7 = 1, otherwise undefined
If it is as hard error on disk (AH bit 7=0), register AL
contains the failing drive number (0=A:, 1=B:, etc.).
BP:SI address of a Device Header Control Block for which error
occurred block device if high bit of BP:SI+4 = 1
low byte of DI: error code (note: high byte is undefined)
error code description
00h attempt to write on write-protected diskette
01h unknown unit
02h drive not ready
03h unknown command
04h data error (bad CRC)
05h bad request structure length
06h seek error
07h unknown media type
08h sector not found
09h printer out of paper
0Ah write fault
0Bh read fault
0Ch general failure
0Fh invalid disk change (DOS 3.x)
handler must return
The registers are set such that if an IRET is executed, DOS responds according
to (AL) as follows:
AL 00h ignore the error
01h retry the operation
02h terminate via int 22h
03h fail the system call that is in progress (DOS 3.2+)
note 1) Be careful when choosing to ignore a response because this causes DOS to
beleive that an operation has completed successfully when it may not
have.
2) If the error was a character device, the contents of AL are invalid.
OTHER ERRORS
If AH bit 7=1, the error occurred on a character device, or was the result of
a bad memory image of the FAT. The device header passed in BP:SI can be examined
to determine which case exists. If the attribute byte high-order bit indicates
a block device, then the error was a bad FAT. Otherwise, the error is on a
character device.
If a character device is involved, the contents of AL are unpredictable, the
error code is in DI as above.
Notes:
1. Before giving this routine control for disk errors, DOS performs several
retries. The number of retries varies according to the DOS version.
2. For disk errors, this exit is taken only for errors occurring during an
int 21h function call. It is not used for errors during an int 25h or 26h.
3. This routine is entered in a disabled state.
4. All registers must be preserved.
5. This interrupt handler should refrain from using DOS function calls. If
necessary, it may use calls 01h through 12h. Use of any other call destroys
the DOS stack and leaves DOS in an unpredictable state.
6. The interrupt handler must not change the contents of the device header.
7. If the interrupt handler handles errors itself rather than returning to DOS,
it should restore the application program's registers from the stack,
remove all but the last three words on the stack, then issue an IRET. This
will return to the program immediately after the int 21h that experienced
the error. Note that if this is done DOS will be in an unstable state until
a function call higher than 12h is issued, therefore not recommended.
8. For DOS 3.x, IGNORE requests (AL=0) are converted to FAIL for critical
errors that occur on FAT or DIR sectors.
9. For DOS 3.10 up, IGNORE requests (AL=0) are converted to FAIL requests
for network critical errors (50-79).
The device header pointed to by BP:SI is as follows:
DWORD Pointer to next device (0FFFFh if last device)
WORD Attributes:
Bit 15 1 if character device.
If bit 15 is 1:
Bit 0 = 1 if current standard input
Bit 1 = 1 if current standard output
Bit 2 = 1 if current NULL device
Bit 3 = 1 if current CLOCK device
0 if block device
Bit 14 is the IOCTL bit
WORD pointer to device driver strategy entry point
WORD pointer to device driver interrupt entry point
8-BYTE character device named field for block devices. The first byte is the
number of units.
To tell if the error occurred on a block or character device, look at bit 15
in the attribute field (WORD at BP:SI+4).
If the name of the character device is desired, look at the eight bytes
starting at BP:SI+10.
HANDLING OF INVALID RESPONSES (DOS 3.x)
A) If IGNORE (AL=0) is specified by the user and IGNORE is not allowed
(bit 5=0), make the response FAIL (AL=3).
B) If RETRY (AL=1) is specified by the user and RETRY is not allowed
(bit 4=0), make the response FAIL (AL=3).
C) If FAIL (AL=3) is specified by the user and FAIL is not allowed (bit
3=0), make the response ABORT. (AL=2)
Interrupt 25h Absolute Disk Read
Interrupt 26h Absolute Disk Write
(0:0094h, 0:0098h)
These transfer control directly to the device driver. On return, the original
flags are still on the stack (put there by the INT instruction). This is
necessary because return information is passed back in the current flags.
The number of sectors specified is transferred between the given drive and the
transfer address. Logical sector numbers are obtained by numbering each sector
sequentially starting from track 0, head 0, sector 1 (logical sector 0) and
continuing along the same head, then to the next head until the last sector on
the last head of the track is counted. Thus, logical sector 1 is track 0, head
0, sector 2; logical sector 2 is track 0, head 0, sector 3; and so on. Numbering
then continues wih sector 1 on head 0 of the next track. Note that although the
sectors are sequentially numbered (for example, sectors 2 and 3 on track 0 in
the example above), they may not be physically adjacent on disk, due to
interleaving. Note that the mapping is different from that used by DOS 1.10 for
dual-sided diskettes.
The request is as follows:
int 25 for Absolute Disk Read,
int 26 for Absolute Disk Write
entry AL drive number (0=A:, 1=B:, etc)
CX number of sectors to read
DS:BX disk transfer address (buffer)
DX first relative sector to read - beginning logical sector number
return CF set if error
AL error code issued to int 24h in low half of DI
AH 01h bad command
02h bad address mark
03h write-protected disk
04h requested sector not found
08h DMA failure
10h data error (bad CRC)
20h controller failed
40h seek operation failed
80h attachment failed to respond
note 1) Original flags on stack! Be sure to pop the stack to prevent
uncontrolled growth
2) Ints 25 and 26 will try rereading a disk if they get an error the first
time.
3) All registers except the segment registers are destroyed by these calls
Interrupt 27h Terminate And Stay Resident
(0:009Ch) (obsolete)
This vector is used by programs that are to remain resident when COMMAND.COM
regains control.
After initializing itself, the program must set DX to its last address plus
one relative to the program's initial DS or ES value (the offset at which other
programs can be loaded), then execute interrupt 27h. DOS then considers the
program as an extension of itself, so the program is not overlaid when other
programs are executed. This is useful for loading programs such as utilities
and interrupt handlers that must remain resident.
entry CS current program segment
DX last program byte + 1
return none
note 1) This interrupt must not be used by .EXE programs that are loaded into
the high end of memory.
2) This interrupt restores the interrupt 22h, 23h, and 24h vectors in the
same manner as interrupt 20h. Therefore, it cannot be used to install
permanently resident Ctrl-Break or critical error handler routines.
3) The maximum size of memory that can be made resident by this method is
64K.
4) Memory can be more efficiently used if the block containing a copy of
the environment is deallocated before terminating. This can be done by
loading ES with the segment contained in 2Ch of the PSP, and issuing
function call 49h (Free Allocated Memory).
5) DOS function call 4Ch allows a program to pass a completion code to DOS,
which can be interpreted with processing (see function call 31h).
6) Terminate and stay resident programs do not close files.
7) Int 21, function 31h is the preferred method to cause a program to
remain resident because this allows return information to be passed and
allows a program larger than 64K to remain resident.
Interrupt 28h (not documented by Microsoft)
* DOS Idle Interrupt
This interrupt is continuously called by DOS itself whenever it is in a wait
state (i.e., when it is waiting for keyboard input) during a function call of
01h through 0Ch. DOS uses 3 separate internal stacks: one for calls 01h through
0Ch; another for calls 0Dh and above; and a third for calls 01h through 0Ch when
a Critical Error is in progress. When int 28h is called, any calls above 0Ch can
be executed without destroying the internal stack used by DOS at the time.
It is used primarily by the PRINT.COM routines, but any number of other
routines can be chained to it by saving the original vector and calling it with
a FAR call (or just JMPing to it) at the end of the new routine.
Int 28h is being issued it is usually safe to do DOS calls. You won't get int
28hs if a program is running that doesn't do its keyboard input through DOS. You
should rely on the timer interrupt for these.
Int 28h is not called at all when any non-trivial foreground task is running.
As soon as a foreground program has a file open, INT28 no longer gets called.
Could make a good driver for for abackground program that really works as long
as there is nothing else going on in the machine.
entry no parameters availible
return none
note 1) The int 28h handler may invoke any int 21h function except functions
00h through 0Ch (and 50h/51h under DOS 2.x).
2) Apparently int 28h is also called during screen writes
3) Until some program installs its own routine, this interrupt vector
simply points to an IRET opcode.
4) Supported in OS/2 1.0's DOS Compatibility Box
Interrupt 29h (not documented by Microsoft)
* Internal - Quick Screen Output
This method is extremely fast (much faster than DOS 21h subfunctions 2 and 9,
for example), and it is portable, even to "non-compatible" MS-DOS computers.
entry AL character to output to screen
return unknown
note 1) Documented by Digital Research's DOS Reference as provided with the
DEC Rainbow
2) If ANSI.SYS is installed, character output is filtered through it.
3) Works on the IBM PC and compatibles, Wang PC, HP-150 and Vectra, DEC
Rainbow, NEC APC, Texas Instruments PC and others
4) This interrupt is called from the DOS's output routines if output is
going to a device rather than a file, and the device driver's attribute
word has bit 3 (04h) set to "1".
5) This call has been tested with MSDOS 2.11, PCDOS 2.1, PCDOS 3.1, PCDOS
3.2, and PCDOS 3.3.
6) Used in IBMBIO.COM as a vector to int 10, function 0Eh (write TTY)
followed by an IRET.
Interrupt 2Ah Microsoft Networks - Session Layer Interrupt
* (not documented by Microsoft)
entry AH 00h check to see if network BIOS installed
return: AH <> 0 if installed
01h execute NETBIOS request
02h set net printer mode
03h get shared-device status (check direct I/O)
AL 00h
DS:SI pointer to ASCIIZ disk device name
return CF 0 if allowed
04h execute NETBIOS
AL 0 for error retry
1 for no retry
ES:BX pointer to ncb
return AX 0 for no error
AH 1 if error
AL error code
05h get network resource information
AL 00h
return AX reserved
BX number of network names
CX number of commands
DX number of sessions
82h unknown
return ??
note called by the int 21h function dispatcher in DOS 3.10
Interrupt 2Bh (not documented by Microsoft)
* Unknown - Internal Routine for DOS (IRET)
Interrupt 2Ch (not documented by Microsoft)
* Unknown - Internal Routine for DOS (IRET)
Interrupt 2Dh (not documented by Microsoft)
* Unknown - Internal Routine for DOS (IRET)
Interrupt 2Eh (undocumented by Microsoft)
* Internal Routine for DOS (Alternate EXEC)
This interrupt passes a command line addressed by DS:SI to COMMAND.COM. The
command line must be formatted just like the unformatted parameter area of a
Program Segment Prefix. That is, the first byte must be a count of characters,
and the second and subsequent bytes must be a command line with parameters,
terminated by a carriage return character.
When executed, int 2Eh will reload the transient part of the command
interpreter if it is not currently in memory. If called from a program that
was called from a batch file, it will abort the batch file. If executed from a
program which has been spawned by the EXEC function, it will abort the whole
chain and probably lock up the computer. Int 2Eh also destroys all registers
including the stack pointer.
Int 2Eh is called from the transient portion of the program to reset the DOS
PSP pointers using the above Functions #81 & #80, and then reenters the
resident program.
When called with a valid command line, the command will be carried out by
COMMAND.COM just as though you had typed it in at the DOS prompt. Note that the
count does not include the carriage return. This is an elegant way to perform a
SET from an application program against the master environment block for
example.
entry DS:SI pointer to an ASCIIZ command line in the form:
count byte
ASCII string
carriage return
null byte
note 1) Destroys all registers including stack pointer
2) Seems to work OK in both DOS 2.x and 3.x
3) It is reportedly not used by DOS.
4) As far as known, int 2Eh is not used by DOS 3.1, although it was called
by COMMAND.COM of PCDOS 3.0, so it appears to be in 3.1 only for the
sake of compatibility.
Interrupt 2Fh Multiplex Interrupt
Interrupt 2Fh is the multiplex interrupt. A general interface is defined
between two processes. It is up to the specific application using interrupt
2Fh to define specific functions and parameters.
Every multiplex interrupt handler is assigned a specific multiplex number.
The multiplex number is specified in the AH register; the AH value tells which
program your request is directed toward. The specific function that the handler
is to perform is placed in the AL register. Other parameters are places in the
other registers as needed. The handlers are chained into the 2Fh interrupt
vector and the multiplex number is checked to see if any other application is
using the same multiplex number. There is no predefined method for assigning a
multiplex number to a handler. You must just pick one. To avoid a conflict if
two applications choose the same multiplex number, the multiplex numbers used by
an application should be patchable. In order to check for a previous
installation of the current application, you can search memory for a unique
string included in your program. If the value you wanted in AH is taken but
you don't find the string, then another application has grabbed that location.
Int 2Fh was not documented under DOS 2.x. There is no reason not to use int 2Fh
as the multiplex interrupt in DOS 2.x. The only problem is that DOS does not
initialize the int 2Fh vector, so when you try to chain to it like you are
supposed to, it will crash. But if your program checks the vector for being zero
and doesn't chain in that case, it will work for you in 2.x just the same as
3.x.
Int 2Fh doesn't require any support from DOS itself for it to be used in
application programs. It's not handled by DOS, but by the programs themselves.
The only support DOS has to provide is to initialize the vector to an IRET. DOS
3.2 does itself contain some int 2Fh handlers - it uses values of 08h, 13h, and
0F8h. There may be more.
entry AH 01h PRINT.COM
AL 00h PRINT Get Installed State
This call must be defined by all int 2Fh handlers. It
is used by the caller of the handler to determine if
the handler is present. On entry, AL=0. On return, AL
contains the installed state as follows:
return AL 0FFh installed
01h not installed, not OK to install
00h not installed, OK to install
01h PRINT Submit File
DS:DX pointer to submit packet
format BYTE level
DWORD pointer to ASCIIZ filename
return CF set if error
AX error code
note 1) A submit packet contains the level (BYTE) and a pointer
to the ASCIIZ string (DWORD in offset:segment form).
The ASCIIZ string must contain the drive, path, and
filename of the file you want to print. The filename
cannot contain global filename characters.
return CF set if error
AX error code
02h PRINT Cancel File
On entry, AL=2 and DS:DX points to the ASCIIZ string for
the print file you want to cancel. Global filename
characters are allowed in the filename.
DS:DX pointer to ASCIIZ file name to cancel (wildcards OK)
return CF set if error
AX error code
03h PRINT remove all files
return CF set if error
AX error code
04h PRINT hold queue/get status
This call holds the jobs in the print queue so that you
can scan the queue. Issuing any other code releases the
jobs. On entry, AL=4. On return, DX contains the error
count. DS:SI points to the print queue. The print queue
consists of a series of filename entries. Each entry is
64 bytes long. The first entry in the queue is the file
currently being printed. The end of the queue is marked
by the entry having a null as the first character.
return DX error count
DS:SI pointer to print queue (null-string terminated
list of 64-byte ASCIIZ filenames)
CF set if error
AX error code
01h function invalid
02h file not found
03h path not found
04h too many open files
05h access denied
08h queue full
09h spooler busy
0Ch name too long
0Fh drive invalid
05h PRINT restart queue
return CF set if error
AX error code
AH 05h DOS 3.x critical error handler
AL 00h installation check
return AL 00h not installed, OK to install
01h not installed, can't install
0FFh installed
note This set of functions allows a user program to
partially or completely override the default
critical error handler in COMMAND.COM
AL 01h handle error - nonzero error code in AL
return CF clear
ES:DI pointer to ASCIIZ error message
CF set use default error handler
AL (?)
AH 06h ASSIGN
00h installation check
return AH <> 0 if installed
01h get memory segment
return ES segment of ASSIGN work area
AH 10h SHARE
00h installation check
return AL 00h not installed, OK to install
01h not installed, not OK to install
0FFh installed
AH 11h multiplex - network redirection
00h installation check
return AL 00h not installed, OK to install
01h not installed, not OK to install
0FFh installed
01h unknown
02h unknown
03h unknown
04h unknown
05h unknown
06h close remote file
07h unknown
08h unknown
09h unknown
0Ah unknown
STACK: WORD (?)
return CF set on error
0Bh unknown
STACK: WORD (?)
return CF set on error(?)
0Ch unknown
0Dh unknown
0Eh unknown
STACK: WORD (?)
return (?)
0Fh unknown
11h unknown
13h unknown
16h unknown
17h unknown
STACK: WORD (?)
return (?)
18h unknown
STACK: WORD (?)
return (?)
19h unknown
1Bh unknown
1Ch unknown
1Dh unknown
1Eh do redirection
STACK: WORD function to execute
return CF set on error
1Fh printer setup
STACK: WORD function(?)
return CF set on error(?)
20h unknown
21h unknown
22h unknown
23h unknown
24h unknown
25h unknown
STACK: WORD (?)
26h unknown
AH 12h multiplex, DOS 3.x internal services
00h installation check
return AL 0FFh for compatibility with other
int 2Fh functions
01h close file (?)
stack word value - unknown
return BX unknown
CX unknown
ES:DI pointer to unknown value
note Can be called only from within DOS
02h get interrupt address
stack: word vector number
return ES:BX pointer to interrupt vector
Stack unchanged
03h get DOS data segment
return DS segment of IBMDOS
04h normalize path separator
stack: word character to normalize
return AL normalized character (forward
slash turned to backslash)
Stack unchanged
05h output character
stack: word character to output
return Stack unchanged
note Can be called only from within DOS
06h invoke critical error
return AL 0-3 for Abort, Retry, Ignore,
Fail
note Can be called only from within DOS
07h move disk buffer (?)
DS:DI pointer to disk buffer
return buffer moved to end of buffer list
note Can be called only from within DOS
08h decrement word
ES:DI pointer to word to decrement
return AX new value of word
note Word pointed to by ES:DI decremented,
skipping zero
09h unknown
DS:DI pointer to disk buffer(?)
return (?)
note Can be called only from within DOS
0Ah unknown
note Can be called only from within DOS
0Bh unknown
ES:DI pointer to system file table entry(?)
return AX (?)
note Can be called only from within DOS
0Ch unknown
note Can be called only from within DOS
0Dh get date and time
return AX current date in packed format
DX current time in packed format
note Can be called only from within DOS
0Eh do something to all disk buffers (?)
return DS:DI pointer to first disk buffer
note can be called only from within DOS
0Fh unknown
DS:DI pointer to (?)
return DS:DI pointer to (?)
note 1) Can be called only from within DOS
2) Calls on function 1207h
10h find dirty/clean(?) buffer
DS:DI pointer to first disk buffer
return DS:DI pointer to first disk buffer
which has (?) flag clear
ZF clear if found
set if not found
11h normalize ASCIIZ filename
DS:SI pointer to ASCIZ filename to normalize
ES:DI ptr to buffer for normalized filename
return destination buffer filled with upper-
case filename, with slashes turned to
backslashes
12h get length of ASCIIZ string
ES:DI pointer to ASCIZ string
return CX length of string
13h uppercase character
stack: word character to convert to uppercase
return AL uppercase character
Stack unchanged
14h compare far pointers
DS:SI first pointer
ES:DI second pointer
return ZF set if pointers are equal
ZF clear if not equal
15h unknown
DS:DI pointer to disk buffer
stack: word (?)
return Stack unchanged
note Can be called only from within DOS
16h get address of system FCB
BX system file table entry number
return ES:DI pointer to system file table entry
17h set default drive (?)
stack: word drive (0 = A:, 1 = B:, etc)
return DS:SI pointer to drive data block for
specified drive
Stack unchanged
note Can be called only from within DOS
18h get something (?)
return DS:SI pointer to (?)
19h unknown
stack: word drive (0 = default, 1 = A:, etc)
return (?)
Stack unchanged
note 1) Can be called only from within DOS
2) Calls function 1217h
1Ah get file's drive
DS:SI pointer to filename
return AL drive
(0=default, 1=A:, etc, 0FFh=invalid)
1Bh set something (?)
CL unknown
return AL (?)
note Can be called only from within DOS
1Ch checksum memory
DS:SI pointer to start of memory to checksum
CX number of bytes
DX initial checksum
return DX checksum
note Can be called only from within DOS
1Dh unknown
DS:SI pointer to (?)
CX (?)
DX (?)
return AX (?)
CX (?)
DX = (?)
1Eh compare filenames
DS:SI pointer to first ASCIIZ filename
ES:DI pointer to second ASCIIZ filename
return ZF set if filenames equivalent
clear if not
1Fh build drive info block
stack: word drive letter
return ES:DI pointer to drive info block
(will be overwritten by next call)
Stack unchanged
note Can be called only from within DOS
20h get system file table number
BX file handle
return CF set on error
AL 6 (invalid file handle)
CF clear if successful
byte ES:[DI] = system file table entry
number for file handle
21h unknown
DS:SI pointer to (?)
return (?)
note Can be called only from within DOS
22h unknown
SS:SI pointer to (?)
return nothing(?)
note Can be called only from within DOS
23h check if character device (?)
return DS:SI pointer to device driver with
same name as (?)
note Can be called only from within DOS
24h delay
return after delay of (?) ms
note Can be called only from within DOS
25h get length of ASCIIZ string
DS:SI pointer to ASCIIZ string
return CX length of string
AH 43h Microsoft Extended Memory Specification (XMS)
AH 64h SCRNSAV2.COM
AL 00h installation check
return AL 00h not installed
0FFh installed
note SCRNSAV2.COM is a screen saver for PS/2's with
VGA by Alan Ballard
AH 7Ah Novell NetWare
AL 00h installation check
note Returns address of entry point for IPX and SPX
AH 0AAh VIDCLOCK.COM
AL 00h installation check
return AL 00h not installed
0FFh installed
note VIDCLOCK.COM is a memory-resident clock by
Thomas G. Hanlin III
AH 0B7h APPEND
AL 00h APPEND installation check
return AH <> 0 if installed
01h APPEND - unknown
02h APPEND - version check
AH 0B8h Microsoft Networks
AL 00h network program installation check
return AH <> 0 if installed
BX installed component flags (test in this
order!)
bit 6 server
bit 2 messenger
bit 7 receiver
bit 3 redirector
01h unknown
02h unknown
03h get current POST address
return ES:BX POST address
04h set new POST address
ES:BX new POST address
09h version check
AH 0BBh Network functions
AL 00h net command installation check
03h get server POST address
04h get server POST address
AH 0F7h AUTOPARK.COM (PD TSR hard disk parking utility)
AL 00h installation check
return AL 00h not installed
0FFh installed
note AUTOPARK is a TSR HD parker by Alan D. Jones
01h set parking delay
BX:CX 32 bit count of 55ms timer ticks
return AX Error
Codes Description
01h invalid function number
02h file not found
03h path not found
04h too many open files
05h access denied
06h invalid handle
08h queue full
09h busy
0Ch name too long
0Fh invalid drive was specified
CF clear (0) if OK
set (1) if error - error returned in AX
note 1) The multiplex numbers AH=0h through AH=7Fh are reserved for DOS.
Applications should use multiplex numbers 80h through 0FFh.
2) When in the chain for int 2Fh, if your code calls DOS or if you execute
with interrupts enabled, your code must be reentrant/recursive.
Interrupt 30h (not a vector!) far jump instruction for CP/M-style calls
Interrupt 31h Unknown
note The CALL 5 entry point does a FAR jump to here
Interrupt 32h Unknown
Interrupt 33h Used by Microsoft Mouse Driver
Function Calls
00h Reset Driver and Read Status
entry AH 00h
return AH status
0 hardware/driver not installed
-1 hardware/driver installed
BX number of buttons
-1 two buttons
0 other than two
3 Mouse Systems mouse
01h Show Mouse Cursor
entry AH 01h
return unknown
02h Hide Mouse Cursor
entry AH 02h
return unknown
note multiple calls to hide the cursor will require multiple calls
to function 01h to unhide it.
03h Return Position and Button Status
entry AH 03h
return BX button status
bit 0 left button pressed if 1
bit 1 right button pressed if 1
bit 2 middle button pressed if 1 (Mouse Systems mouse)
CX column
DX row
04h Position Mouse Cursor
entry AH 04h
CX column
DX row
return unknown
05h Return Button Press Data
entry AH 05h
BX button
0 left
1 right
2 middle (Mouse Systems mouse)
return AH button states
bit 0 left button pressed if 1
bit 1 right button pressed if 1
bit 2 middle button pressed if 1 (Mouse Systems mouse)
BX no. of times specified button pressed since last call
CX column at time specified button was last pressed
DX row at time specified button was last pressed
06h Return Button Release Data
entry AH 06h
BX button
0 left
1 right
2 middle (Mouse Systems mouse)
return AH button states
bit 0 left button pressed if 1
bit 1 right button pressed if 1
bit 2 middle button pressed if 1 (Mouse Systems mouse)
BX no. of times specified button released since last call
CX column at time specified button was last released
DX row at time specified button was last released
07h Define Horizontal Curos Range
entry AH 0007h
CX minimum column
DX maximum column
return unknown
08h Define Vertical Cursor Range
entry AH 08h
CX minimum row
DX maximum row
return unknown
09h Define Graphics Cursor
entry AH 09h
BX column of cursor hot spot in bitmap (-16 to 16)
CX row of cursor hot spot (-16 to 16)
ES:DX pointer to bitmap
16 words screen mask
16 words cursor mask
return unknown
note Each word defines the sixteen pixels of a row, low bit
rightmost
0Ah Define Text Cursor
entry AH 0Ah
BX hardware/software text cursor
00h software
CX screen mask
DX cursor mask
01h hardware
CX start scan line
DX end scan line
return unknown
note When the software cursor is selected, the char/attribute data
at the current screen position is ANDed with the screen mask
and the with the cursor mask
0BH Read Motion Counters
entry AH 0Bh
return CX number of mickeys mouse moved horiz. since last call
DX number of mickeys mouse moved vertically
note 1) A mickey is the smallest increment the mouse can sense.
Positive values indicate up/right
0Ch Define Interrupt Subroutine Parameters
entry AH 0Ch
CX call mask bit
bit 0 call if mouse moves
bit 1 call if left button pressed
bit 2 call if left button released
bit 3 call if right button pressed
bit 4 call if right button released
bit 5 call if middle button pressed (Mouse Systems)
bit 6 call if middle button released (Mouse Systems)
ES:DX address of FAR routine
return unknown
note when the subroutine is called, it is passed these values:
AH condition mask (same bit assignments as call mask)
BX button state
CX cursor column
DX cursor row
DI horizontal mickey count
SI vertical mickey count
0Dh Light Pen Emulation On
entry AH 0Dh
return unknown
0Eh Light Pen Emulation Off
entry AH 0Eh
return unknown
0Fh Define Mickey/Pixel Ratio
entry AH 0Fh
CX number of mickeys per 8 pixels horizontally
DX number of mickeys per 8 pixels vertically
return unknown
10h Define Screen Region for Updating
entry AH 10h
CX,DX X,Y coordinates of upper left corner
SI,DI X,Y coordinates of lower right corner
return unknown
note Mouse cursor is hidden during updating, and needs to be
explicitly turned on again
11h not documented by Microsoft
12h Set Large Graphics Cursor Block
AH 12h
BH cursor width in words
CH rows in cursor
BL horizontal hot spot (-16 to 16)
CL vertical hot spot (-16 to 16)
ES:DX pointer to bit map of screen and cursor maps
return AH -1 if successful
note PC Mouse. Not dodcumented by Microsoft
13h Define Double-Speed Threshold
entry AH 13h
DX threshold speed in mickeys/second,
0 = default of 64/second
return unknown
note If speed exceeds threshold, the cursor's on-screen motion
is doubled
14h Exchange Interrupt Subroutines
entry AH 14h
return unknown
15h Return Drive Storage Requirements
entry AH 15h
return BX size of buffer needed to store driver state
16h Save Driver State
entry AH 16h
ES:DX pointer to buffer
return unknown
17h Restore Driver State
entry AH 17h
ES:DX pointer to buffer containing saved state
return unknown
18h not documented by Microsoft
19h not documented by Microsoft
1Ah not documented by Microsoft
1Bh not documented by Microsoft
1Ch not documented by Microsoft
1Dh Define Display Page Number
entry AH 1Dh
1Eh Return Display Page Number
entry AH 1Eh
return unknown
42h PCMouse - Get MSmouse Storage Requirements
AH 42h
return AX 0FFFFh successful
BX buffer size in bytes for functions 50h and 52h
00h MSmouse not installed
42h functions 42h, 50h, and 52h not supported
52h PCMouse - Save MSmouse State
entry AH 50h
BX buffer size
ES:DX pointer to buffer
return AX 0FFFFh if successful
52h PCMouse - restore MSmouse state
entry AH 52h
BX buffer size
ES:DX pointer to buffer
return AX 0FFFFh if successful
Int 33: In addition, the following functions are appended to BIOS int 10h and
implemented as the EGA Register Interface Library:
0F0h read one register
0F1h write one register
0F2h read consecutive register range
0F3h write consecutive register range
0F4h read non-consecutive register set
0F5h write non-consecutive register set
0F6h revert to default register values
0F7h define default register values
0FAh get driver status
Interrupt 34h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0D8h
Interrupt 35h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0D9h
Interrupt 36h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DAh
Interrupt 37h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DBh
Interrupt 38h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DCh
Interrupt 39h Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DDh
Interrupt 3Ah Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DEh
Interrupt 3Bh Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates opcode 0DFh
Interrupt 3Ch Turbo C/Microsoft languages - Floating Point emulation
This int emulates instructions with an ES segment override
Interrupt 3Dh Turbo C/Microsoft languages - Floating Point emulation
This interrupt emulates a standalone FWAIT instruction
Interrupt 3Eh Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Fh Overlay manager interrupt (Microsoft LINK.EXE)
Interrupt 40h Hard Disk BIOS
Pointer to disk BIOS entry when a hard disk controller is
installed. The BIOS routines use int 30h to revector the
diskette handler (original int 13h) here so int 40 may be used
for hard disk control
Interrupt 41h Hard Disk Parameters (XT,AT,XT2,XT286,PS except ESDI disks)
Pointer to first Hard Disk Parameter Block, normally located
in the controller card's ROM. This table may be copied to RAM
and changed, and this pointer revectored to the new table.
note 1) format of parameter table is:
dw cylinders
db heads
dw starting reduced write current cylinder (XT only, 0 for others)
db maximum ECC burst length
db control byte
bits 0-2 drive option (XT only, 0 for others)
bit 3 set if more than 8 heads
bit 4 always 0
bit 5 set if manufacturer's defect map on max cylinder+1
bit 6 disable ECC retries
bit 7 disable access retries
db standard timeout (XT only, 0 for others)
db formatting timeout (XT only, 0 for others)
db timeout for checking drive (XT only, 0 for others)
dw landing zone (AT, PS/2)
db sectors/track (AT, PS/2)
db 0
2) normally vectored to ROM table when system is initialized.
Interrupt 42h Pointer to screen BIOS entry (EGA, VGA, PS/2)
Relocated (by EGA, etc.) video handler (original int 10h).
Revectors int 10 calls to EGA BIOS.
Interrupt 43h Pointer to EGA initialization parameter table. The POST
initializes this vector pointing to the default table located
in the EGA ROM BIOS. (PC-2 and up). Not initialized if EGA not
present.
Interrupt 44h Pointer to EGA graphics character table (also PCjr). This
(0:0110h) table contains the dot patterns for the first 128 characters
in video modes 4,5, and 6, and all 256 characters in all
additional graphics modes. Not initialized if EGA not present.
2) EGA/VGA/CONV/PS - EGA/PCjr fonts, characters 00h to 7Fh
3) Novell NetWare - High-Level Language API
Interrupt 45h Reserved by IBM (not initialized)
Interrupt 46h Pointer to second hard disk, parameter block (AT, XT/286, PS/2)
(see int 41h) (except ESDI hard disks) (not initialized unless
specific user software calls for it)
Interrupt 47h Reserved by IBM (not initialized)
Interrupt 48h Cordless Keyboard Translation (PCjr, XT [never delivered])
(0:0120h) This vector points to code to translate the cordless keyboard
scancodes into normal 83-key values. The translated scancodes
are then passed to int 9. (not initialized on PC or AT)
Interrupt 49h Non-keyboard Scan Code Translation Table Address (PCjr)
(0:0124h) This interrupt has the address of a table used to translate
non-keyboard scancodes (greater than 85 excepting 255). This
interrupt can be revectored by a user application. IBM
recommends that the default table be stored at the beginning
of an application that required revectoring this interrupt,
and that the default table be restored when the application
terminates. (not initialized on PC or AT)
Interrupt 4Ah Real-Time Clock Alarm (Convertible, PS/2)
(not initialized on PC or AT)
Invoked by BIOS when real-time clock alarm occurs
Interrupt 4Bh Reserved by IBM (not initialized)
Interrupt 4Ch Reserved by IBM (not initialized)
Interrupt 4Dh Reserved by IBM (not initialized)
Interrupt 4Eh Reserved by IBM (not initialized)
Interrupt 4Fh Reserved by IBM (not initialized)
Interrupt 50-57 IRQ0-IRQ7 relocated by DesQview
(normally not initialized)
Interrupt 58h Reserved by IBM (not initialized)
Interrupt 59h Reserved by IBM (not initialized)
GSS Computer Graphics Interface (GSS*CGI)
DS:DX Pointer to block of 5 array pointers
return CF 0
AX return code
CF 1
AX error code
note 1) Int 59 is the means by which GSS*CGI language bindings
communicate with GSS*CGI device drivers and the GSS*CGI
device driver controller.
2) Also used by the IBM Graphic Development Toolkit
Interrupt 5Ah Reserved by IBM (not initialized)
Interrupt 5Bh Reserved by IBM (not initialized)
Interrupt 5Ah Cluster Adapter BIOS entry address
(normally not initialized)
Interrupt 5Bh Reserved by IBM (not initialized) (cluster adapter?)
Interrupt 5Ch NETBIOS interface entry port
ES:BX pointer to network control block
note 1) When the NETBIOS is installed, interrupts 13 and 17 are interrupted by
the NETBIOS; interrupt 18 is moved to int 86 and one of int 2 or 3 is
used by NETBIOS. Also, NETBIOS extends the int 15 function 90 and 91h
functions (scheduler functions)
2) Normally not initialized.
3) TOPS network card uses DMA 1, 3 or none.
Interrupt 5Dh Reserved by IBM (not initialized)
Interrupt 5Eh Reserved by IBM (not initialized)
Interrupt 5Fh Reserved by IBM (not initialized)
Interrupt 60h-67h User Program Interrupts (availible for general use)
Interrupt 67h Used by Lotus-Intel-Microsoft Expanded Memory Specification
user and Ashton-Tate/Quadram/AST Enhanced Expanded Memory
specification (See Chapter 10)
Interrupt 68h Not Used (not initialized)
Interrupt 69h Not Used (not initialized)
Interrupt 6Ah Not Used (not initialized)
Interrupt 6Bh Not Used (not initialized)
Interrupt 6Ch System Resume Vector (Convertible) (not initialized on PC)
Interrupt 6Dh Not Used (not initialized)
Interrupt 6Fh Not Used (not initialized)
Interrupt 70h IRQ 8, Real Time Clock Interrupt (AT, XT/286, PS/2)
Interrupt 71h IRQ 9, Redirected to IRQ 8 (AT, XT/286, PS/2)
LAN Adapter 1 (rerouted to int 0Ah by BIOS)
Interrupt 72h IRQ 10 (AT, XT/286, PS/2) Reserved
Interrupt 73h IRQ 11 (AT, XT/286, PS/2) Reserved
Interrupt 74h IRQ 12 Mouse Interrupt (AT, XT/286, PS/2)
Interrupt 75h IRQ 13, Coprocessor Error, BIOS Redirect to int 2 (NMI) (AT)
Interrupt 76h IRQ 14, Hard Disk Controller (AT, XT/286, PS/2)
Interrupt 77h IRQ 15 (AT, XT/286, PS/2) Reserved
Interrupt 78h Not Used
Interrupt 79h Not Used
Interrupt 7Ah Novell NetWare - LOW-LEVEL API
Interrupt 7Bh-7Fh Not Used
Interrupt 80h-85h Reserved by BASIC
note interrupts 80h through ECh are apparently unused and not initialized.
Interrupt 86h Relocated by NETBIOS int 18
Interrupt 86h-0F0h Used by BASIC when BASIC interpreter is running
Intrerrupt 0E0h CP/M-86 function calls
Interrupt 0E4h Logitech Modula-2 v2.0 MONITOR
entry AX 05h monitor entry
06h monitor exit
BX priority
Interrupts 0F1h-0FFh (absolute addresses 3C4-3FF)
Location of Interprocess Communications Area
Interrupt 0F8h Set Shell Interrupt (OEM)
Set OEM handler for int 21h calls from 0F9h through 0FFh
entry AH 0F8h
DS:DX pointer to handler for Functions 0F9h thru 0FFh
note 1) To reset these calls, pass DS and DX with 0FFFFh. DOS is set up to
allow ONE handler for all 7 of these calls. Any call to these handlers
will result in the carry bit being set and AX will contain 1 if they are
not initialized. The handling routine is passed all registers just as
the user set them. The OEM handler routine should be exited through an
IRET.
2) 10 ms interval timer (Tandy?)
Interrupt 0F9h First of 8 SHELL service codes, reserved for OEM shell (WINDOW);
use like HP Vectra user interface?
Interrupt 0FAh USART ready (RS-232C)
Interrupt 0FBh USART RS ready (keyboard)
Interrupt 0FCh Unknown
Interrupt 0FDh reserved for user interrupt
Interrupt 0FEh AT/XT286/PS50+ - destroyed by return from protected mode
Interrupt 0FFh AT/XT286/PS50+ - destroyed by return from protected mode
CHAPTER 6
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
DOS CONTROL BLOCKS AND WORK AREAS
When DOS loads a program, it first sets aside a section of memory for the
program called the program segment, or code segment. Then it constructs a
control block called the program segment prefix, or PSP, in the first 256
(100h) bytes. Usually, the program is loaded directly after the PSP at
100h.
The PSP contains various information used by DOS to help run the program.
The PSP is always located at offset 0 within the code segment. When a program
recieves control certain registers are set to point to the PSP. For a COM
file, all registers are set to point to the beginning of the PSP and the
program begins at 100h. For the more complex EXE file structures, only DS and
ES registers are set to point to the PSP. The linker passes the settings for
the DS, IP, SS, and SP registers and may set the starting location in CS:IP to
a location other than 100h.
IBMBIO provides an IRET instruction at absolute address 847h for use as a
dummy routine for interrupts that are not used by DOS. This lets the
interrupts do nothing until their vectors are rerouted to ttheir appropriate
handlers.
A storage block is used by DOS to record the amount and location of allocated
memory withion the machine's address spacd.
A storage block, a Program Segment Prefix, and an environment area are built
by FDOS for each program currently resident in the address space. The storage
block is used by DOS to record the address range of memory allocated to a
program. IOt us used by DOs to find th enext availible area to load a program
and to determine if there is a\enough memory to run that porogram. When a
memory area is in use, it is said to be allocated. Then the program ends, or
releases memory, it is said to bne deallocated.
A storage block contains a pointer ro rhe Program Segment Prefix assoiciated
with each program. This control block is constructed by IBMDOS for the purpose
opf providing stanfdardized areas for DOS/program communication., Within ghr
PSP are arsas which are used to save interrupt vectors, pass parameters to
the program, record disk directory information, and to buffer disk reads and
writes. This control block is 100h bytes in lengrth and is followed by the
program mopdule loaded by DOS.
The PSP contains a pointer to the environment area for that program. This
area contains a copy of the current DOS SET, PROMPT, COMSPEC, and PATH values
as well as any user-set variables. The program may examine and modify this
information as desired.
Each storage block is 10h bytes long, although only 5 bytes are currently
used by DOS. The first byte contains 4Dh (a capital M) to indicate that it
contains a pointer to the next storage block. A 5Ah (a capital Z) in the
first byte of a storage block indicatres there are no more storage blocks
following this one (it is the end of the chain). The identifier byte is
followeed by a 2 byte segment number for the associated PSP for that program.
The next 2 bytes contain the number of segments what are allocated to the
program. If this is not the last storage block, then another storage block
follows the allocated memory area.
When thge storage block contains zero for the nuymber of allocated segments,
then no storage is allocated to thius block and the next storage block
immediately follows this one. This can ha-p[en whjen memory is allocated and
then deallocated repeatedly.
IBMDOS constructs a storage block and PSP before loading the command
interpreter (default is COMMAND.COM).
If the copy of COMMAND.COM is a secondary copy, it will lack an environment
address as PSP+2Ch.
The Disk Transfer Area (DTA)
DOS uses an area in memory to contain the data for all file reads and writes
that are performed with FCB function calls. This are is known as the disk
transfer area. This disk transfer area (DTA) is sometimes called a buffer.
It can be located anywhere in the data area of your application program and
should be set by your program.
Only one DTA can be in effect at a time, so your program must tell DOS what
memory location to use before using any disk read or write functions. Use
function call 1Ah (Set Disk Transfer Address) to set the disk transfer address.
Use function call 2Fh (Get Disk Transfer Address) to get the disk transfer
address. Once set, DOS continues to use that area for all disk operations until
another function call 1Ah is issued to define a new DTA. When a program is given
control by COMMAND.COM, a default DTA large enough to hold 128 bytes is
established at 80h into the program's Program Segment Prefix.
For file reads and writes that are performed with the extended function calls,
there is no need to set a DTA address. Instead, specify a buffer address when
you issue the read or write call.
DOS Program Segment
When you enter an external command or call a program through the EXEC function
call, DOS determines the lowest availible address space to use as the start of
available memory for the program being started. This area is called the Program
Segment.
At offset 0 within the program segment, DOS builds the Program Segment Prefix
control block. EXEC loads the program after the Program Segment Prefix (at
offset 100h) and gives it control.
The program returns from EXEC by a jump to offset 0 in the Program Segment
Prefix, by issuing an int 20h, or by issuing an int 21h with register AH=00h or
4Ch, or by calling location 50h in the PSP with AH=00h or 4Ch.
It is the responsibility of all programs to ensure that the CS register
contains the segment address of the Program Segment Prefix when terminating by
any of these methods except call 4Ch.
All of these methods result in returning to the program that issued the EXEC.
During this returning process, interrupt vectors 22h, 23h, and 24h (Terminate,
Ctrl-Break, and Critical Error Exit addresses) are restored from the values
saved in the PSP of the terminating program. Control is then given to the
terminate address.
When a program receives control, the following conditions are in effect:
For all programs:
1) The segment address of the passed environment is contained at offset 2Ch in
the Program Segment Prefix.
2) The environment is a series of ASCII strings totalling less than 32k bytes
in the form: NAME=parameter The default environment is 160 bytes.
Each string is terminated by a byte of zeroes, and the entire set of strings
is terminated by abother byte of zeroes. Following the byte of zeroes that
terminates the set of environment string is a set of initial arguments passed
to a program that contains a word count followed by an ASCIIZ string. The
ASCIIZ string contains the drive, path, and filename.ext of the executable
program. Programs may use this area to determine where the program was loaded
from. The environment built by the command processor (and passed to all
programs it invokes) contains a COMSPEC=string at a minimum (the parameter on
COMSPEC is the path used by DOS to locate COMMAND.COM on disk). The last PATH
and PROMPT commands issued will also be in the environment, along with any
environment strings entered through the SET command.
The environment that you are passed is actually a copy of the invoking
process's environment. If your application terminates and stays resident
through int 27h, you should be aware that the copy of the environment passed
to you is static. That is, it will not change even if subsequent PATH,
PROMPT, or SET commands are issued.
The environment can be used to transfer information between processes or to
store strings for later use by application programs. The environment is
always located on a paragraph boundary. This is its format:
byte ASCIIZ string 1
byte ASCIIZ string 2
....
byte ASCIIZ string n
byte of zeros (0)
Typically the environment strings have the form:
parameter = value
Following the byte of zeros in the environment, a WORD indicates the number
of other strings following.
If the environment is part of an EXECed command interpreter, it is followed
by a copy of the DS:DX filename passed to the child process. A zero value
causes the newly created process to inherit the parent's environment.
3) Offset 80h in the PSP contains code to invoke the DOS function dispatcher.
Thus, by placing the desired function number in AH, a program can issue a
long call to PSP+50h to invoke a DOS function rather than issuing an int 21h.
4) The disk transfer address (DTA) is set to 80h (default DTA in PSP).
5) File Control Blocks 5Ch and 6Ch are formatted from the first two parameters
entered when the command was invoked. Note that if either parameter contained
a path name, then the corresponding FCB will contain only a valid drive
number. The filename field will not be valid.
6) An unformatted parameter area at 81h contains all the characters entered
after the command name (including leading and imbedded delimiters), with 80h
set to the number of characters. If the <, >, or | parameters were entered
on the command line, they (and the filenames associated with them) will not
appear in this area, because redirection of standard input and output is
transparent to applications.
(For EXE files only)
7) DS and ES registers are set to point to the PSP.
8) CS, IP, SS, and SP registers are set to the values passed by the linker.
(For COM files only)
9) For COM files, offset 6 (one word) contains the number of bytes availible in
the segment.
10) Register AX reflects the validity of drive specifiers entered with the
first two parameters as follows:
AL=0FFh is the first parameter contained an invalid drive specifier,
otherwise AL=00h.
AL=0FFh if the second parameter contained an invalid drive specifier,
otherwise AL=00h.
11) All four segment registers contain the segment address of the inital
allocation block, that starts within the PSP control block. All of user
memory is allocated to the program. If the program needs to invoke another
program through the EXEC function call (4Bh), it must first free some memory
through the SETBLOCK function call to provide space for the program being
invoked.
12) The Instruction Pointer (IP) is set to 100h.
13) The SP register is set to the end of the program's segment. The segment size
at offset 6 is rounded down to the paragraph size.
14) A word of zeroes is placed on top of the stack.
The PSP (with offsets in hexadecimal) is formatted as follows:
????????????????????????????????????????????????????????????????????????????????
? P R O G R A M S E G M E N T P R E F I X ?
????????????????????????????????????????????????????????????????????????????????
? offset? size ? C O N T E N T S ?
????????????????????????????????????????????????????????????????????????????????
? 0000h ? 2 bytes ? int 20h ?
????????????????????????????????????????????????????????????????????????????????
? 0002h ? 2 bytes ? segment address, end of allocation block ?
????????????????????????????????????????????????????????????????????????????????
? 0004h ? 1 byte ? reserved, normally 0 ?
????????????????????????????????????????????????????????????????????????????????
? 0005h ? 5 bytes ? long call to MSDOS function dispatcher ?
????????????????????????????????????????????????????????????????????????????????
? 000Ah ? 4 bytes ? previous termination handler interrupt vector (int 22h) ?
????????????????????????????????????????????????????????????????????????????????
? 000Eh ? 4 bytes ? previous contents of ctrl-C interrupt vector (int 23h) ?
????????????????????????????????????????????????????????????????????????????????
? 0012h ? 4 bytes ? prev. critical error handler interrupt vector (int 24h) ?
????????????????????????????????????????????????????????????????????????????????
? 0016h ? 22 bytes ? reserved for DOS ?
????????????????????????????????????????????????????????????????????????????????
? 002Ch ? 2 bytes ? segment address of environment block ?
????????????????????????????????????????????????????????????????????????????????
? 002Eh ? 34 bytes ? reserved, DOS work area ?
????????????????????????????????????????????????????????????????????????????????
? 4 bytes ? stores the calling process's stack pointer when ?
? ? switching to DOS's internal stack. ?
????????????????????????????????????????????????????????????????????????????????
? 0050h ? 3 bytes ? int 21h, RETF instructions ?
????????????????????????????????????????????????????????????????????????????????
? 0053h ? 2 bytes ? reserved ?
????????????????????????????????????????????????????????????????????????????????
? 0055h ? 7 bytes ? reserved, or FCB#1 extension ?
????????????????????????????????????????????????????????????????????????????????
? 005Ch ? ? default File Control Block #1 ?
????????????????????????????????????????????????????????????????????????????????
? 006Ch ? ? default File Control Block #2 (overlaid if FCB #1 opened) ?
????????????????????????????????????????????????????????????????????????????????
? 0080h ? 1 byte ? parameter length ?
????????????????????????????????????????????????????????????????????????????????
? 0081h ? ? parameters ?
????????????????????????????????????????????????????????????????????????????????
? 00FFh ? 128 bytes? command tail and default Disk Transfer Area (DTA) ?
????????????????????????????????????????????????????????????????????????????????
1. The first segment of availible memory is in segment (paragraph) form. For
example, 1000h would respresent 64k.
2. The word at offset 6 contains the number of bytes availible in the segment.
3. Offset 2Ch contains the segment address of the environment.
4. Programs must not alter any part of the PSP below offset 5Ch.
offset 0 contains hex bytes CD 20, the int 20h opcode. A program can end
by making a jump to this location when the CS points to the PSP.
For normal cases, int 21, function 4Ch should be used.
offset 2 contains the segment-paragraph address of the end of memory as
reported by DOS. (which may not be the same as the real end of RAM).
Multiply this number by 10h or 16 to get the amount of memory availible.
offset 4 reserved
offset 05 contains a long call to the DOS function dispatcher. Programs may
jump to this address instead of calling int 21 if they wish.
offsets 10, 14, 18 vectors
offset 2C is the segment:offset address of the environment for the program
using this particular PSP.
offset 2E The DWORD at PSP+2EH is used by DOS to store the calling process's
stack pointer when switching to DOS's own private stack - at the end of
a DOS function call, SS:SP is restored from this address.
offset 50h contains a long call to the DOS function dispatcher.
offsets 5C, 65, 6C contain FCB information for use with FCB function calls.
The first FCB may overlay the second if it is an extended call; your
program should revector these areas to a safe place if you intend to
use them.
offset 80h and 81h contain th elength and value of parameters passed on the
command line.
offset FF contains the DTA
STANDRD FILE CONTROL BLOCK
The standard file control block is defined as follows, with the offsets in
decimal:
????????????????????????????????????????????????????????????????????????????????
? F I L E C O N T R O L B L O C K ?
????????????????????????????????????????????????????????????????????????????????
? Bytes ? Function ?
????????????????????????????????????????????????????????????????????????????????
? 0 ? Drive number. For example: ?
? ? Before open: 00h = default drive ?
? ? 01h = drive A: ?
? ? 02h = drive B: etc. ?
? ? After open: 00h = drive C: ?
? ? 01h = drive A: ?
? ? 02h = drive B: etc. ?
? ? 0 is replaced by the actual drive number during open. ?
????????????????????????????????????????????????????????????????????????????????
? 1-8 ? Filename, left justified with trailing blanks. If a reserved device ?
? ? name is placed here (such as PRN) do not include the optional colon. ?
????????????????????????????????????????????????????????????????????????????????
? 9-11 ? Filename extension, left justified with trailing blanks. ?
????????????????????????????????????????????????????????????????????????????????
? 12-13 ? Current block number relative to beginning of file, starting with 0 ?
? ? (set to 0 by the open function call). A block consists of 128 ?
? ? records, each of the size specified in the logical record size field.?
? ? The current block number is used with the current record field ?
? ? (below) for sequential reads and writes. ?
????????????????????????????????????????????????????????????????????????????????
? 14-15 ? Logical record size in bytes. Set to 80h by the OPEN function call. ?
? ? If this is not correct, you must set the value because DOS uses it ?
? ? to determine the proper locations in the file for all disk reads and ?
? ? writes. ?
????????????????????????????????????????????????????????????????????????????????
? 16-19 ? File size in bytes. In this 2 word field, the first word is the ?
? ? low-order part of the size. ?
????????????????????????????????????????????????????????????????????????????????
? 20-21 ? Date the file was created or last updated. The mm/dd/yy are mapped ?
? ? as follows: ?
? ? 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 ?
? ? y y y y y y y m m m m d d d d d ?
? ? where: mm is 1-12 ?
? ? dd is 1-31 ?
? ? yy is 0-119 (1980-2099) ?
????????????????????????????????????????????????????????????????????????????????
? 22-31 ? Reserved for system use. ?
????????????????????????????????????????????????????????????????????????????????
? 32 ? Current relative record number (0-127) within the current block. ?
? ? (See above). You must set this field before doing sequential ?
? ? read/write operations to the diskette. This field is not initialized ?
? ? by the open function call. ?
? ? If the record size is less than 64 bytes, both words are used. ?
? ? Otherwise, only the first 3 bytes are used. Note that if you use the ?
? ? File Control Block at 5Ch in the program segment, the last byte of ?
? ? the FCB overlaps the first byte of the unformatted parameter area. ?
????????????????????????????????????????????????????????????????????????????????
note 1) An unopened FCB consists of the FCB prefix (if used), drive number, and
filename.ext properly filled in. An open FCB is one in which the
remaining fields have been filled in by the CREAT or OPEN function
calls.
2) Bytes 0-5 and 32-36 must be set by the user program. Bytes 16-31 are set
by DOS and must not be changed by user programs.
3) All word fields are stored with the least significant byte first. For
example, a record length of 128 is stored as 80h at offset 14, and 00h
at offset 15.
EXTENDED FILE CONTROL BLOCK
The extended file control block is used to create or search for files in the
disk directory that have special attributes.
It adds a 7 byte prefix to the FCB, formatted as follows:
????????????????????????????????????????????????????????????????????????????????
? E X T E N D E D F I L E C O N T R O L B L O C K ?
????????????????????????????????????????????????????????????????????????????????
? Bytes ? Function ?
????????????????????????????????????????????????????????????????????????????????
? 0 ? Flag byte containing 0FFh to indicate an extended FCB. ?
????????????????????????????????????????????????????????????????????????????????
? 1-6 ? Reserved ?
????????????????????????????????????????????????????????????????????????????????
? 6-7 ? Attribute byte. Refer to function call 11h (search first) for ?
? ? details on using the attribute bits during directory searches. This ?
? ? function is present to allow applications to define their own files ?
? ? as hidden (and thereby excluded from normal directory searches) and ?
? ? to allow selective directory searches. ?
????????????????????????????????????????????????????????????????????????????????
Any reference in the DOS function calls to an FCB, whether opened or unopened,
may use either a normal or extended FCB. If you are using an extended FCB, the
appropriate register should be set to the first byte of the prefix, rather than
the drive-number field.
MEMORY CONTROL BLOCKS
DOS keeps track of allocated and availible memory blocks, and provides three
function calls for application programs to communicate their memory needs to
DOS. These calls are 48h to allocate a memory block, 49h to free a previously
allocated memory block, and 4Ah (SETBLOCK) to change the size of an allocated
memory block.
CONTROL BLOCK
DOS manages memory as follows:
DOS build a control block for each block of memory, whether free or allocated.
For example, if a program issues an "allocate" (48h), DOS locates a block of
free memory that satisfies the request, and then "carves" the requested memory
out of that block. The requesting program is passed the location of the first
byte of the block that was allocated for it - a memory management control block,
describing the allocated block, has been built for the allocated block and a
second memory management control block describes the amount of space left in the
original free block of memory. When you do a setblock to shrink an allocated
block, DOS builds a memory management control block for the area being freed and
adds it to the chain of control blocks. Thus, any program that changed memory
that is not allocated to it stands a chance of destroying a DOS memory
management control block. This causes unpredictable results that don't show up
until an activity is performed where DOS uses its chain of control blocks. The
normal result is a memory allocation error, which means a system reset will be
required.
When a program (command or application program) is to be loaded, DOS uses the
EXEC function call 4Bh to perform the loading.
This is the same function call that is availible to applications programs for
loading other programs. This function call has two options:
Function 00h, to load and execute a program (this is what the command
processor uses to load and execute external commands)
Function 03h, to load an overlay (program) without executing it.
Although both functions perform their loading in the same way (relocation is
performed for EXE files) their handling of memory management is different.
FUNCTION 0: For function 0 to load and execute a program, EXEC first allocates
the largest availible block of memory (the new program's PSP will be at offset
0 in that block). Then EXEC loads the program. Thus, in most cases, the new
program owns all the memory from its PSP to the end of memory, including memory
occupied by the transient parent of COMMAND.COM. If the program were to issue
its own EXEC function call to load and execute another program, the request
would fail because no availible memory exists to load the new program into.
NOTE: For EXE programs, the amount of memory allocated is the size of the
program's memory image plus the value in the MAX_ALLOC field of the file's
header (offset 0Ch, if that much memory is availible. If not, EXEC
allocates the size of the program's memory image plus the value in the
MIN_ALLOC field in the header (offset 0Ah). These fields are set by the
Linker).
A well-behaved program uses the SETBLOCK function call when it receives
control, to shrink its allocated memory block down to the size it really needs.
A COM program should remember to set up its own stack before doing the SETBLOCK,
since it is likely that the default stack supplied by DOS lies in the area of
memory being used. This frees unneeded memory, which can be used for loading
other programs.
If the program requires additional memory during processing, it can obtain
the memory using the allocate function call and later free it using the free
memory function call.
When a program is loaded using EXEC function call 00h exits, its initial
allocation block (the block beginning with its PSP) is automatically freed
before the calling program regains control. It is the responsibility of all
programs to free any memory they allocate before exiting to the calling
program.
FUNCTION 3: For function 3, to load an overlay, no PSP is built and EXEC
assumes the calling program has already allocated memory to load the new program
into - it will NOT allocate memory for it. Thus the calling program should
either allow for the loading of overlays when it determines the amount of memory
to keep when issuing the SETBLOCK call, or should initially free as much memory
as possible. The calling program should then allocate a block (based on the size
of the program to be loaded) to hold the program that will be loaded using the
"load overlay" call. Note that "load overlay" does not check to see if the
calling program actually owns the memory block it has been instructed to load
into - it assumes the calling program has followed the rules. If the calling
program does not own the memory into which the overlay is being loaded, there is
a chance the program being loaded will overlay one of the control blocks that
DOS uses to keep track of memory blocks.
Programs loaded using function 3 should not issue any SETBLOCK calls since
they don't own the memory they are operating in. (This memory is owned by the
calling program)
Because programs loaded using function 3 are given control directly by (and
return contrrol directly to) the calling program, no memory is automatically
freed when the called program exits. It is up to the calling program to
determine the disposition of the memory that had been occupied by the exiting
program. Note that if the exiting program had itself allocated any memory, it
is responsible for freeing that memory before exiting.
MEMORY CONTROL BLOCKS
Only the first 5 bytes of the memory control block are used. The first byte
will always have the value of 4Dh or 5Ah. The value 5Ah indicates the block is
the last in a chain, all memory above it is unused. 4Dh means that the block is
intermediate in a chain, the memory above it belongs to the next program or to
DOS.
The next two bytes hold the PSP segment address of the program that owns the
corresponding block of memory. A value of 0 means the block is free to be
claimed, any other value represents a segment address. Bytes 3 and 4 indicate
the size in paragraphs of the memory block. If you know the address of the first
block, you can find the next block by adding the length of the memory block plus
1 to the segment address of the control block.
Finding the first block can be difficult, as this varies according to the DOS
version and the configuration.
The remaining 11 bytes are not currently used by DOS, and may contain "trash"
characters left in memory from previous applications.
If DOS determines that the allocation chain of memory control blocks has been
corrupted, it will halt the system and display the message "Memory Allocation
Error", and the system will halt, requiring a reboot.
Each memory block consists of a signature byte (4Dh or 5Ah) then a word which
is the PSP value of the owner of the block (which allocated it), followed by a
word which is the size in paragraphs of the block. The last block has a
signature of 5Ah. All others have 4Dh. If the owner is 0000 then the block is
free.
User memory is allocated from the lowest end of available memory that will
satisfy the request for memory.
CHAPTER 7
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
DOS File Structure
File Management Functions
Use DOS function calls to create, open, close, read, write, rename, find, and
erase files. There are two sets of function calls that DOS provides for support
of file management. They are:
- File Control Block function calls (0Fh-24h)
- Handle function calls (39h-62h)
Handle function calls are easier to use and are more powerful than FCB calls.
Microsoft recommends that the handle function calls be used when writing new
programs. DOS 3.0 up have been curtailing use of FCB function calls; it is
possible that future versions of DOS may not support FCB function calls.
The following table compares the use of FCB calls to Handle function calls:
FCB Calls Handle Calls
Access files in current Access files in ANY directory
directory only.
Requires the application Does not require use of an FCB.
program to maintain a file Requires a string with the drive,
control block to open, path, and filename to open, create,
create, rename or delete rename, or delete a file. For file
a file. For I/O requests, I/O requests, the application program
the application program must maintain a 16 bit file handle
also needs an FCB that is supplied by DOS.
The only reason an application should use FCB function calls is to maintain
the ability to run under DOS 1.x. To to this, the program may use only function
calls 00h-2Eh.
FCB FUNCTION CALLS
FCB function calls require the use of one File Control Block per open file,
which is maintained by the application program and DOS. The application program
supplies a pointer to the FCB and fills in ther appropriate fields required by
the specific function call. An FCB function call can perform file management on
any valid drive, but only in the current logged directory. By using the current
block, current record, and record length fields of the FCB, you can perform
sequential I/O by using the sequential read or write function calls. Random I/O
can be performed by filling in the random record and record length fields.
Several possible uses of FCB type calls are considered programming errors and
should not be done under any circumstances to avoid problems with file sharing
and compatibility with later versions of DOS.
Some errors are:
1) If program uses the same FCB structure to access more than one open file. By
opening a file using an FCB, doing I/O, and then replacing the filename field
in the file control block with a new filename, a program can open a second
file using the same FCB. This is invalid because DOS writes control info-
rmation about the file into the reserved fields of the FCB. If the program
then replaces the filename field with the original filename and then tries to
perform I/O on this file, DOS may become confused because the control info-
rmation has been changed. An FCB should never be used to open a second file
without closing the one that is currently open. If more than one File Control
Block is to be open concurrently, separate FCBs should be used.
2) A program should never try to use the reserved fields in the FCB, as the
function of the fields changes with different versions of DOS.
3) A delete or a rename on a file that is currently open is considered an error
and should not be attempted by an application program.
It is also good programming practice to close all files when I/O is done. This
avoids potential file sharing problems that require a limit on the number of
files concurrently open using FCB function calls.
HANDLE FUNCTION CALLS
The recommended method of file management is by using the extended "handle"
set of function calls. These calls are not restricted to the current directory.
Also, the handle calls allow the application program to define the type of
access that other processes can have concurrently with the same file if the file
is being shared.
To create or open a file, the application supplies a pointer to an ASCIIZ
string giving the name and location of the file. The ASCIIZ string contains an
optional drive letter, optional path, mandatory file specification, and a
terminal byte of 00h. The following is an example of an ASCIIZ string:
format [drive][path] filename.ext,0
DB "A:\path\filename.ext",0
If the file is being created, the application program also supplies the
attribute of the file. This is a set of values that defines the file read
only, hidden, system, directory, or volume label.
If the file is being opened, the program can define the sharing and access
modes that the file is opened in. The access mode informs DOS what operations
your program will perform on this file (read-only, write-only, or read/write)
The sharing mode controls the type of operations other processes may perform
concurrently on the file. A program can also control if a child process inherits
the open files of the parent. The sharing mode has meaning only if file sharing
is loaded when the file is opened.
To rename or delete a file, the appplication program simply needs to provide
a pointer to the ASCIIZ string containing the name and location of the file
and another string with the neew name if the file is being renamed.
The open or create function calls return a 16-bit value referred to as the
file handle. To do any I/O to a file, the program uses the handle to reference
the file. Once a file is opened, a program no longer needs to maintain the
ASCIIZ string pointing to the file, nor is there any need to stay in the same
directory. DOS keeps track of the location of the file regardless of what
directory is current.
Sequential I/O can be performed using the handle read (3Fh) or write (40h)
function calls. The offset in the file that IO is performed to is automatically
moved to the end of what was just read or written. If random I/O is desired, the
LSEEK (42h) function call can be used to set the offset into the file where I/O
is to be performed.
SPECIAL FILE HANDLES
DOS reserves five special file handles for use by itself and applications
programs. They are:
0000h STDIN Standard Input Device
0001h STDOUT Standard Output Device
0002h STDERR Standard Error Output Device
0003h STDAUX Standard Auxiliary Device
0004h STDPRN Standard Printer Device
These handles are predefined by DOS and can be used by an application program.
They do not need to be opened by a program, although a program can close these
handles. STDIN should be treated as a read-only file, and STDOUT and STDERR
should be treated as write-only files. STDIN and STDOUT can be redirected. All
handles inherited by a process can be redirected, but not at the command line.
These handles are very useful for doing I/O to and from the console device.
For example, you could read input from the keyboard using the read (3Fh)
function call and file handle 0000h (STDIN), and write output to the console
screen with the write function call (40h) and file handle 0001h (STDOUT). If
you wanted an output that could not be redirected, you could output it using
file handle 0002h (STDERR). This is very useful for error messages that must
be seen by a user.
File handles 0003h (STDAUX) and 0004h (STDPRN) can be both read from and
written to. STDAUX is typically a serial device and STDPRN is usually a parallel
device.
ASCII and BINARY MODE
I/O to files is done in binary mode. This means that the data is read or
written without modification. However, DOS can also read or write to devices in
ASCII mode. In ASCII mode, DOS does some string processing and modification to
the characters read and written. The predefined handles are in ASCII mode when
initialized by DOS. All other file handles that don't refer to devices are in
binary mode. A program, can use the IOCTL (44h) function call to set the mode
that I/O is to a device. The predefined file handles are all devices, so the
mode can be changed from ASCII to binary via IOCTL. Regular file handles that
are not devices are always in binary mode and cannot be changed to ASCII mode.
The ASCII/BINARY bit was called "raw" in DOS 2.x, but it is called ASCII/BINARY
in DOS 3.x.
The predefined file handles STDIN (0000h) and STDOUT (0001h) and STDERR
(0002h) are all duplicate handles. If the IOCTL function call is used to change
the mode of any of these three handles, the mode of all three handles is
changed. For example, if IOCTL was used to change STDOUT to binary mode, then
STDIN and STDERR would also be changed to binary mode.
FILE I/O IN BINARY (RAW) MODE
The following is true when a file is read in binary mode:
1) The characters ^S (scroll lock), ^P (print screen), ^C (control break) are
not checked for during the read. Therefore, no printer echo occurs if ^S or
^P are read.
2) There is no echo to STDOUT (0001h).
3) Read the number of specified bytes and returns immediately when the last
byte is received or the end of file reached.
4) Allows no editing of the ine input using the function keys if the input is
from STDIN (0000h).
The following is true when a file is written to in binary mode:
1) The characters ^S (scroll lock), ^P (print screen), ^C (control break) are
not checked for during the write. Therefore, no printer echo occurs.
2) There is no echo to STDOUT (0001h).
3) The exact number of bytes specified are written.
4) Does not caret (^) control characters. For example, ctrl-D is sent out as
byte 04h instead of the two bytes ^ and D.
5) Does not expand tabs into spaces.
FILE I/O IN ASCII (COOKED) MODE
The following is true when a file is read in ASCII mode:
1) Checks for the characters ^C,^S, and ^P.
2) Returns as many characters as there are in the device input buffer, or the
number of characters requested, whichever is less. If the number of
characters requested was less than the number of characters in the device
buffer, then the next read will address the remaining characters in the
buffer.
3) If there are no more bytes remaining in the device input buffer, read a
line (terminated by ^M) into the buffer. This line may be edited with the
function keys. The characters returned terminated with a sequence of 0Dh,
0Ah (^M,^J) if the number of characters requested is sufficient to include
them. For example, if 5 characters were requested, and only 3 were entered
before the carriage return (0Dh or ^M) was presented to DOS from the console
device, then the 3 characters entered and 0Dh and 0Ah would be returned.
However, if 5 characters were requested and 7 were entered before the
carriage return, only the first 5 characters would be returned. No 0Dh,0Ah
sequence would be returned in this case. If less than the number of
characters requested are entered when the carriage return is received, the
characters received and 0Dh,0Ah would be returned. The reason the 0Ah
(linefeed or ^J) is added to the returned characters is to make the devices
look like text files.
4) If a 1Ah (^Z) is found, the input is terminated at that point. No 0Dh,0Ah
(CR,LF) sequence is added to the string.
5) Echoing is performed.
6) Tabs are expanded.
The following is true when a file is written to in ASCII mode:
1) The characters ^S,^P,and ^C are checked for during the write operation.
2) Expands tabs to 8-character boundaries and fills with spaces (20h).
3) Carets control characters, for example, ^D is written as two bytes, ^ and D.
4) Bytes are output until the number specified is output or a ^Z is
encountered. The number actually output is returned to the user.
NUMBER OF OPEN FILES ALLOWED
The number of files that can be open concurrently is restricted by DOS. This
number is determined by how the file is opened or created (FCB or handle
function call) and the number specified by the FCBS and FILES commands in the
CONFIG.SYS file. The number of files allowed open by FCB function calls and the
number of files that can be opened by handle type calls are independent of one
another.
RESTRICTIONS ON FCB USAGE
If file sharing is not loaded using the SHARE command, there are no
restrictions on the nuumber of files concurrently open using FCB function calls.
However, when file sharing is loaded, the maximum number of FCBs open is set
by the the FCBS command in the CONFIG.SYS file.
The FCBS command has two values you can specify, 'm' and 'n'. The value for
'm' specifies the number of files that can be opened by FCBs, and the value 'n'
specifies the number of FCBs that are protected from being closed.
When the maximum number of FCB opens is exceeded, DOS automatically closes the
least recently used file. Any attempt to access this file results in an int 24h
critical error message "FCB not availible". If this occurs while an application
program is running, the value specified for 'm' in the FCBS command should be
increased.
When DOS determines the least recently used file to close, it does not include
the first 'n' files opened, therefore the first 'n' files are protected from
being closed.
RESTRICTIONS ON HANDLE USAGE
The number of files that can be open simultaneously by all processes is
determined by the FILES command in the CONFIG.SYS file. The number of files a
single process can open depends on the value specified for the FILES command. If
FILES is greater than or equal to 20, a single process can open 20 files. If
FILES is less than 20, the process can open less than 20 files. This value
includes three predefined handles: STDIN, STDOUT, and STDERR. This means only
17 additional handles can be added. DOS 3.3 includes a function to use more than
20 files per application.
ALLOCATING SPACE TO A FILE
Files are not nescessarily written sequentially on a disk. Space is allocated
as needed and the next location availible on the disk is allocated as space for
the next file being written. Therefore, if considerable file generation has
taken place, newly created files will not be written in sequential sectors.
However, due to the mapping (chaining) of file space via the File Allocation
Table (FAT) and the function calls availible, any file may be used in either a
sequential or random manner.
Space is allocated in increments called clusters. Cluster size varies
according to the media type. An application program should not concern itself
with the way that DOS allocates space to a file. The size of a cluster is only
important in that it determines the smallest amount of space that can be
allocated to a file. A disk is considered full when all clusters have been
allocated to files.
MSDOS / PCDOS DIFFERENCES
There is a problem of compatibility between MS-DOS and IBM PC-DOS having to
do with FCB Open and Create. The IBM 1.0, 1.1, and 2.0 documentation of OPEN
(call 0Fh) contains the following statement:
"The current block field (FCB bytes C-D) is set to zero [when an FCB is
opened]."
This statement is NOT true of MS-DOS 1.25 or MS-DOS 2.00. The difference is
intentional, and the reason is CP/M 1.4 compatibility. Zeroing that field is
not CP/M compatible. Some CP/M programs will not run when machine translated if
that field is zeroed. The reason it is zeroed in the IBM versions is that IBM
specifically requested that it be zeroed. This is the reason for the complaints
from some vendors about the fact that IBM MultiPlan will not run under MS-DOS.
It is probably the reason that some other IBM programs don't run under MS-DOS.
NOTE: Do what all MS/PC-DOS Systems programs do: Set every single FCB field you
want to use regardless of what the documentation says is initialized.
.EXE FILE STRUCTURE
The EXE files produced by the Linker program consist of two parts, control and
relocation information and the load module itself.
The control and relocation information, which is described below, is at the
beginning of the file in an area known as the header. The load module
immediately follows the header. The load module begins in the memory image of
the module contructed by the Linker.
When you are loading a file with the name *.EXE, DOS does NOT assume that it
is an EXE format file. It looks at the first two bytes for a signature telling
it that it is an EXE file. If it has the proper signature, then the load
proceeds. Otherwise, it presumes the file to be a .COM format file.
If the file has the EXE signature, then the internal consistency is checked.
Pre-2.0 versions of MSDOS did not check the signature byte for EXE files.
The .EXE format can support programs larger than 64K. It does this by
allowing separate segments to be defined for code, data, and the stack, each
of which can be up to 64K long. Programs in EXE format may contain explicit
references to segment addresses. A header in the EXE file has information for
DOS to resolve these references.
The .EXE header is formatted as follows:
???????????????????????????????????????????????????????????????????????????????
? Offset ? C O N T E N T S ?
???????????????????????????????????????????????????????????????????????????????
? 00h ? 4Dh ? This is the Linker's signature to mark the file as a valid ?
????????????????? .EXE file (The ASCII letters M and Z, for Mark Zbikowski, ?
? 01h ? 5Ah ? one of the major designers of DOS at Microsoft) ?
???????????????????????????????????????????????????????????????????????????????
? 02h-03h ? Length of the image mod 512 (remainder after dividing the load ?
? ? module image by 512) ?
???????????????????????????????????????????????????????????????????????????????
? 04h-05h ? Size of the file in 512 byte pages including the header. ?
???????????????????????????????????????????????????????????????????????????????
? 06h-07h ? Number of relocation table items. ?
???????????????????????????????????????????????????????????????????????????????
? 08h-09h ? Size of the header in 16 byte increments (paragraphs). This is ?
? ? used to locate the beginning of the load module in the file. ?
???????????????????????????????????????????????????????????????????????????????
? 0Ah-0Bh ? Minimum number of 16 byte paragraphs required above the end of ?
? ? the loaded program. ?
???????????????????????????????????????????????????????????????????????????????
? 0Ch-0Dh ? Maximum number of 16 byte paragraphs required above the end of ?
? ? the loaded program. ?
???????????????????????????????????????????????????????????????????????????????
? 0Eh-0Fh ? Displacement in paragraphs of stack segment within load module. ?
???????????????????????????????????????????????????????????????????????????????
? 10h-11h ? Offset to be in SP register when the module is given control. ?
???????????????????????????????????????????????????????????????????????????????
? 12h-13h ? Word Checksum - negative sum of all the words in the file, ?
? ? ignoring overflow. ?
???????????????????????????????????????????????????????????????????????????????
? 14h-15h ? Offset to be in the IP register when the module is given control. ?
???????????????????????????????????????????????????????????????????????????????
? 16h-17h ? Displacement in paragraphs of code segment within load module. ?
???????????????????????????????????????????????????????????????????????????????
? 18h-19h ? Displacement in bytes of the first relocation item in the file. ?
???????????????????????????????????????????????????????????????????????????????
? 1Ah-1Bh ? Overlay number (0 for the resident part of the program) ?
???????????????????????????????????????????????????????????????????????????????
THE RELOCATION TABLE
The word at 18h locates the first entry in the relocation table. The
relocation table is made up of a variable number of relocation items. The number
of items is contained at offset 06-07. The relocation item contains two fields
- a 2 byte offset value, followed by a 2 byte segment value. These two fields
represent the displacement into the load module before the module is given
control. The process is called relocation and is accomplished as follows:
1. A Program Segment Prefix is built following the resident portion of the
program that is performing the load operation.
2. The formatted part of the header is read into memory (its size is at
offset 08-09)
3. The load module size is determined by subtracting the header size from the
file size. Offsets 04-05 and 08-09 can be used for this calculation. The
actual size is downward adjusted based on the contents of offsets 02-03.
Note that all files created by the Linker programs prior to version 1.10
always placed a value of 4 at this location, regardless of the actual
program size. Therefore, Microsoft recommends that this field be ignored if
it contains a value of 4. Based on the setting of the high/low loader switch,
an appropriate segment is determined for loading the load module. This
segment is called the start segment.
4. The load module is read into memory beginning at the start segment. The
relocation table is an ordered list of relocation items. The first relocation
item is the one that has the lowest offset in the file.
5. The relocation table items are read into a work area one or more at a time.
6. Each relocation table item segment value is added to the start segment value.
The calculated segment, in conjunction with the relocation item offset value,
points to a word in the load module to which is added the start segment
value. The result is placed back into the word in the load module.
7. Once all the relocation items have been processed, the SS and SP registers
are set from the values in the header and the start segment value is added
to SS. The ES and DS registers are set to the segment address of the program
segment prefix. The start segment value is added to the header CS register
value. The result, along with the header IP value, is used to give the
module control.
"NEW" .EXE FORMAT (Microsoft Windows and OS/2)
The "old" EXE format is documented here. The "new" EXE format puts more
information into the header section and is currently used in applications that
run under Microsoft Windows. The linker that creates these files comes with the
Microsoft Windows Software Development Kit and is called LINK4. If you try to
run a Windows-linked program under DOS, you will get the error message "This
program requires Microsoft Windows".
CHAPTER 8
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
DOS DISK INFORMATION
THE DOS AREA
All disks and diskettes formatted by DOS are created with a sector size of 512
bytes. The DOS area (entire area for a diskette, DOS partition for hard disks)
is formatted as follows:
Boot record - 1 sector
First copy of the FAT - variable size
Second copy of the FAT - same size as first copy
Root directory - variable size
Data area
The following sections describe each of the allocated areas:
THE BOOT RECORD
The boot record resides on track 0, sector 1, side 0 of every diskette
formatted by the DOS FORMAT program. It is put on all disks to provide an error
message is you try to start up with a nonsystem disk in drive A:. For hard disks
the boot record resides on the first sector of the DOS partition.
THE DOS FILE ALLOCATION TABLE (FAT)
This section explains how DOS uses the FAT to convert the clusters of a file
into logical sector numbers. We recommend that system utilities use the DOS
handle calls rather than interpreting the FAT.
The FAT is used by DOS to allocate disk space for files, one cluster at a time.
The FAT consists of a 12 bit entry (1.5 bytes) for each cluster on the disk or
a 16 bit (2 bytes) entry when a hard disk has more than 20740 sectors as is the
case with fixed disks larger than 10Mb.
The first two FAT entries map a portion of the directory; these FAT entries
contain indicators of the size and format of the disk. The FAT can be in a 12
or 16 bit format. DOS determines whether a disk has a 12 or 16 bit FAT by
looking at the total number of allocation units on a disk. For all diskettes
and hard disks with DOS partitions less than 20,740 sectors, the FAT uses a 12
bit value to map a cluster. For larger partitions, DOS uses a 16 bit value.
The second, third, and fourth bit applicable for 16 bit FAT bytes always
contains 0FFFFh. The first byte is used as follows:
hex value meaning normally used
0F8h hard disk bootable hard disk at C:800
double sided 18 sector diskette PS/2 1.44 meg DSQD
0F9h double sided 15 sector diskette AT 1.2 meg DSQD
double sided 9 sector diskette Convertible 720k DSHD
0FCh single sided 9 sector diskette DOS 2.0, 180k SSDD
0FDh double sided 9 sector diskette DOS 2.0, 360k DSDD
0FEh single sided 8 sector diskette DOS 1.0, 160k SSDD
0FFh double sided 8 sector diskette DOS 1.1, 320k SSDD
The third FAT entry begins mapping the data area (cluster 002).
NOTE: These values are provided as a reference. Therefore, programs should not
make use of these values.
Each entry contains a hexadecimal character (or 4 for 16 bit FATs). ()
indicates the high order four bit value in the case of 16 bit FAT entries.
They can be:
(0)000h if the cluster is unused and availible
(0F)FF8h - (0F)FFFh to indicate the last cluster of a file
(X)XXXh any other hexadecimal numbers that are the cluster number
of the next cluster in the file. The cluster number is the
first cluster in the file that is kept in the file's
directory entry.
The values (0F)FF0h - (0F)FF7h are used to indicate reserved clusters.
(0F)FF7h indicates a bad cluster if it is not part of the allocation chain.
(0F)FF8h - (0F)FFFh are used as end of file markers.
The file allocation table always occupies the sector or sectors immediately
following the boot record. If the FAT is larger than 1 sector, the sectors
occupy consecutive sector numbers. Two copies of the FAT are written, one
following the other, for integrity. The FAT is read into one of the DOS buffers
whenever needed (open, allocate more space, etc).
USE OF THE 12 BIT FILE ALLOCATION TABLE
Obtain the starting cluster of the file from the directory entry.
Now, to locate each subsequent sector of the file:
1. Multiply the cluster number just used by 1.5 (each FAT entry is 1.5
bytes long).
2. The whole part of the product is offset into the FAT, pointing to the entry
that maps the cluster just used. That entry contains the cluster number of
the next cluster in the file.
3. Use a MOV instruction to move the word at the calculated FAT into a register.
4. If the last cluster used was an even number, keep the low order 12 bits of
the register, otherwise, keep the high order 12 bits.
5. If the resultant 12 bits are (0FF8h-0FFFh) no more clusters are in the file.
Otherwise, the next 12 bits contain the cluster number of the next cluster in
the file.
To convert the cluster to a logical sector number (relative sector, such as
that used by int 25h and 26h and DEBUG):
1. Subtract 2 from the cluster number
2. Multiply the result by the number of sectors per cluster.
3. Add the logical sector number of the beginning of the data area.
USE OF THE 16 BIT FILE ALLOCATION TABLE
Obtain the starting cluster of the file from the directory entry. Now to
locate each subsequent cluster of the file:
1. Multiply the cluster number used by 2 (each FAT entry is 2 bytes long).
2. Use the MOV word instruction to move the word at the calculated FAT offset
into a register.
3. If the resultant 16 bits are (0FF8h-0FFFFh) no more clusters are in the
file. Otherwise, the 16 bits contain the cluster number of the next cluster
in the file.
Compaq DOS makes availible a new disk type (6) with 32 bit partition values,
allowing 512 megabytes per hard disk (Compaq DOS 3.3.1)
DOS DISK DIRECTORY
The FORMAT command initially builds the root directory for all disks. Its
location (logical sector number) and the maximum number of entries are
availible through the device driver interfaces.
DIRECTORY ENTRIES
Since directories other than the root directory are actually files, there is
no limit to the number of entries that they may contain.
All directory entries are 32 bytes long, and are in the following format (byte
and offset are decimal). The following paragraphs describe the directory entry
bytes:
Bytes 0-7 represent the filename. The first byte of the filename indicates the
status of the filename. The status of a filename can contain the following
values:
00h Filename never used. This is used to limit the length of directory
searches, for performance reasons.
05h Indicates that the first character of the filename actually has an 0Edh
character.
0E5h Filename has been used but the file has been erased.
2Eh This entry is for a directory. If the second byte is also 2Eh, the
cluster field contains the cluster number of this directory's parent
directory. (0000h if the parent directory is the root directory).
Any other character is the first character of a filename.
These bytes indicate the filename extension.
This byte indicates the file's attribute. The attribute byte is mapped as
follows (values are in hexadecimal):
NOTE: Attributes 08h and 10h cannot be changed using function call 43h (CHMOD).
The system files IBMBIO.COM and IBMDOS.COM (or customized equivalent) are
marked as read-only, hidden, and system files. Files can be marked hidden when
they are created. Also, the read-only, hidden, and system and archive attributes
may be changed through the CHMOD function call.
01h Indicates that the file is marked read-only. An attempt to open the file
for output using function call 3Dh results in an error code being returned.
This value can be used with other values below.
02h Indicates a hidden file. The file is excluded from normal directory
searches.
04h Indicates a system file. This file is excluded from normal directory
searches.
08h Indicates that the entry contains the volume label in the first 11 bytes.
The entry contains no other usable information and may exist only in the
root directory.
20h Indicates an archive bit. This bit is set on whenever the file is written
to and closed. It is used by BACKUP and RESTORE.
All other bits are reserved, and must be 0.
reserved by DOS
These bytes contain the time when the file was created or last updated. The
time is mapped in the bits as follows:
?????????????????????????????????????????????????????????????????
? B Y T E 23 ? B Y T E 22 ?
?????????????????????????????????????????????????????????????????
? F E D C B A 9 8 ? 7 6 5 4 3 2 1 0 ?
?????????????????????????????????????????????????????????????????
? H H H H H ? M M M M M M ? D D D D D ?
?????????????????????????????????????????????????????????????????
? binary # hrs 0-23 ? binary # minutes 0-59 ? bin. # 2-sec incr ?
?????????????????????????????????????????????????????????????????
NOTE: The time is stored with the least significant byte first.
This area contains the date when the file was created or last updated. The
mm/dd/yy are mapped in the bits as follows:
?????????????????????????????????????????????????????????????????
? B Y T E 25 ? B Y T E 24 ?
?????????????????????????????????????????????????????????????????
? F E D C B A 9 8 ? 7 6 5 4 3 2 1 0 ?
?????????????????????????????????????????????????????????????????
? Y Y Y Y Y Y Y ? M M M M ? D D D D D ?
?????????????????????????????????????????????????????????????????
? 0-119 (1980-2099) ? 1-12 ? 1-31 ?
?????????????????????????????????????????????????????????????????
NOTE: The date is stored with the least significant byte first.
This area contains the starting cluster number of the first cluster in the
file. The first cluster for data space on all fixed disks and floppy disks is
always cluster 002. The cluster number is stored with the least significant
byte first.
This area contains the file size in bytes. The first word contains the low
order part of the size. Both words are stored with the least significant byte
first.
File Allocation Table
offset size description
3 8 bytes system id (such as IBM 3.3)
11 2 bytes number of bytes per sector, ie 512 bytes is 200h
13 1 byte sectors per cluster (1 or 2)
14 2 bytes number of reserved sectors at beginning, 1 for floppies
16 1 byte number of copies of FAT, 2 for floppies
17 2 bytes number of root directory entries (64, 112, 256,etc)
19 2 bytes total sectors per disk
21 1 byte format ID (F8, F9, FC, FF, etc)
22 2 bytes number of sectors per FAT (1 or 2)
24 2 bytes number of sectors per track (8 or 9, 17)
26 2 bytes number of sides, heads, or cylinders (1 or 2 for floppy)
28 2 bytes number of special reserved sectors
THE DATA AREA
Allocation of space for a file (in the data area) is done only when needed
(it is not preallocated). The space is allocated one cluser (unit allocation)
at a time. A cluster is always one or more consecutive sector numbers, and all
of the clusters in a file are "chained" together in the FAT.
The clusters are arranged on disk to minimize head movement for multisided
media. All of the space on a track (or cylinder) is allocated before moving
on to the next track. This is accomplished by using the sequential sector
numbers on the lowest-numbered head, then all the sector numbers on the next
head, and so on until all sectors of all heads of the track are used. Then the
next sector used will be sector 1 of head 0 on the next track.
An interesting innovation that was introduced in MS-DOS 3.0: disk space that
is freed by erasing a file is not re-used immediately, unlike earlier versions
of DOS. Instead, free space is obtained from the area not yet used during the
current session, until all of it is used up. Only then will space that is freed
during the current session be re-used.
This feature minimizes fragmentation of files, since never-before-used space
is always contiguous. However, once any space has been freed by deleting a file,
that advantage vanishes at the next system boot. The feature also greatly
simplifies un-erasing files, provided that the need to do an un-erase is found
during the same session and also provided that the file occupies contiguous
clusters.
However, when one is using programs which make extensive use of temporary
files, each of which may be created and erased many times during a session,
the feature becomes a nuisance; it forces the permanent files to move farther
and farther into the inner tracks of the disk, thus increasing rather than
decreasing the amount of fragmentation which occurs.
The feature is implemented in DOS by means of a single 16-bit "last cluster
used" (LCU) pointer for each physical disk drive; this pointer is a part of
the physical drive table maintained by DOS. At boot time, the LCU pointer is
zeroed. Each time another cluster is obtained from the free-space pool (the
FAT), its number is written into the LCU pointer. Each time a fresh cluster
is required, the FAT is searched to locate a free one; in older versions of
DOS this search always began at Cluster 0000, but in 3.x it begins at the
cluster pointed to by the LCU pointer.
For hard disks, the size of the file allocation table and directory are
determined when FORMAT initializes it and are based on the size of the DOS
partition.
The following table gives the specifications for floppy disk formats:
# of sectors FAT size DIR DIR sectors total
disk DOS ver sides /track (sectors) (sectors) (entries) /cluster sectors
(5-1/4 inch)
160k (DOS 1.0) 1 8 (40) 1 4 64 1 320
320k (DOS 1.1) 2 8 (40) 1 7 112 2 360
180k (DOS 2.0) 1 9 (40) 2 4 64 1 640
360k (DOS 2.0) 2 9 (40) 2 7 112 2 720
1.2M (DOS 3.0) 2 15 (80) 7 14 224 1 2400
(3-1/2 inch)
720k (DOS 3.2) 2 9 (80) 3 7 112 2 1440
1.44M(DOS 3.3) 2 18 (80) 9 14 224 1 2880
Files in the data area are not nescessarily written sequentially on the first.
The data area space is allocated one cluster at a time, skipping over clusters
already allocated. The first free cluster found is the next cluster allocated,
regardless of its physical location on the disk. This permits the most efficient
utilization of disk space because clusters freed by erasing files can be
allocated for new files. Refer back to the description of the DOS FAT in this
chapter for more information.
Hard Disk Layout
The DOS hard disk routines perform the following services:
1) Allow multiple operating systems to utilize the hard disk without the need
to backup and restore files when changing operating systems.
2) Allow a user-selected operating system to be started from the hard disk.
I) In order to share the hard disk among operating systems, the disk may be
logically divided into 1 to 4 partitions. The space within a given
partition is contiguous, and can be dedicated to a specific operating
system. Each operating system may "own" only one partition in DOS versions
2.0 through 3.2. PCDOS 3.3 introduced the "Extended DOS Partition" which
allows multiple DOS partitions on the same hard disk. The FDISK.COM (or
similar program from other DOS vendors) utility allows the user to select
the number, type, and size of each partition. The partition information is
kept in a partition table that is embedded in the master fixed disk boot
record on the first sector of the disk. The format of this table varies
from version to version of DOS.
II) An operating system must consider its partition to be the entire disk,
and must ensure that its functions and utilities do not access other
partitions on the disk.
III) Each partition may contain a boot record on its first sector, and any
other programs or data that you choose - including a copy of an operating
system. For example, the DOS FORMAT command may be used to format and
place a copy of DOS in the DOS partition in the same manner that a
diskette is formatted. With the FDISK utility, you may designate a
partition as "active" (bootable). The master hard disk boot record causes
that partition's boot record to receive control when the system is started
or reset. Additional disk partitions could be FORTH, UNIX, Pick, CP/M-86,
or the UCSD p-System.
SYSTEM INITIALIZATION
The boot sequence is as follows:
1. System initialization first attempts to load an operating system from
diskette drive A. If the drive is not ready or a read error occurs, it then
attempts to read a master hard disk boot record on the first sector of the
first hard disk in the system. If unsuccessful, or if no hard disk is
present, it invokes ROM BASIC in an IBM PC or displays a disk error
message on most compatibles.
2. If initialization is successful, the master hard disk boot record is given
control and it examines the partition table embedded within it. If one of
the entries indicates an active (bootable) partition, its boot record is
read from the partition's first sector and given control.
3. If none of the partitions is bootable, ROM BASIC is invoked on an IBM PC or
a disk error on most compatibles.
4. If any of the boot indicators are invalid, or if more than one indicator is
marked as bootable, the message INVALID PARTITION TABLE is displayed and the
system stops.
5. If the partition's boot record cannot be successfully read within five
retries due to read errors, the message ERROR LOADING OPERATING SYSTEM
appears and the system stops.
6. If the partition's boot record does not contain a valid "signature", the
message MISSING OPERATING SYSTEM appears, and the system stops.
NOTE: When changing the size or location of any partition, you must ensure that
all existing data on the disk has been backed up. The partitioning program
will destroy the data on the disk.
BOOT RECORD/PARTITION TABLE
A boot record must be written on the first sector of all hard disks, and
must contain the following:
1. Code to load and give control to the boot record for one of four possible
operating systems.
2. A partition table at the end of the boot record. Each table entry is 16
bytes long, and contains the starting and ending cylinder, sector, and head
for each of four possible partitions, as well as the number of sectors
preceding the partition and the number of sectors occupied by the partition.
The "boot indicator" byte is used by the boot record to determine if one of
the partitions contains a loadable operating system. FDISK initialization
utilities mark a user-selected partition as "bootable" by placing a value
of 80h in the corresponding partition's boot indicator (setting all other
partitions' indicators to 0 at the same time). The presence of the 80h tells
the standard boot routine to load the sector whose location is contained in
the following three bytes. That sector is the actual boot record for the
selected operating system, and it is responsible for the remainder of the
system's loading process (as it is from the diskette). All boot records are
loaded at absolute address 0:7C00.
The partition table with its offsets into the boot record is:
(except for Wyse DOS 3.2 with 32 bit allocation table, and DOS 3.3-up)
????????????????????????????????????????????????????????????????????????????????
? Offset / Purpose ? ? Head ? Sector ? Cylinder ?
????????????????????????????????????????????????????????????????????????????????
? 1BEh partition 1 begin ? boot ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1C2h partition 1 end ? syst ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1C6h partition 1 relative sector ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1CAh partition 1 # sectors ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1CEh partition 2 begin ? boot ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1D2h partition 2 end ? syst ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1D6h partition 2 relative sector ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1DAh partition 2 # sectors ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1DEh partition 3 begin ? boot ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1E2h partition 3 end ? syst ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1E6h partition 3 relative sector ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1EAh partition 3 # sectors ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1EEh partition 4 begin ? boot ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1F2h partition 4 end ? syst ind ? H ? S ? cyl ?
????????????????????????????????????????????????????????????????????????????????
? 1F6h partition 4 relative sector ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1FAh partition 4 # sectors ? low word ? high word ?
????????????????????????????????????????????????????????????????????????????????
? 1FEh signature ? hex 55 ? hex AA ?
??????????????????????????????????????????????????????????
HARD DISK TECHNICAL INFORMATION
Boot indicator (boot ind): The boot indicator byte must contain 0 for a non-
bootable partition or 80h for a bootable partition. Only one partition can be
marked as bootable at a time.
System Indicator (sys ind): The sys ind field contains an indicator of the
operating system that "owns" the partition.
The system indicators are:
00h unknown (unspecified or non-DOS)
01h DOS 12 bit FAT
02h DOS 16 bit FAT
Cylinder (CYL) and Sector (S): The 1 byte fields labelled CYL contain the low
order 8 bits of the cylinder number - the high order 2 bits are in the high
order 2 bits of the sector (S) field. This corresponds with the ROM BIOS
interrupt 13h (disk I/O) requirements, to allow for a 10 bit cylinder number.
The fields are ordered in such a manner that only two MOV instructions are
required to properly set up the DX and CX registers for a ROM BIOS call to
load the appropriate boot record (hard disk booting is only possible from the
first hard disk in the system, where a BIOS drive number of 80h corresponds
to the boot indicator byte).
All partitions are allocated in cylinder multiples and begin on sector 1,
head 0.
EXCEPTION: The partition that is allocated at the beginning of the disk starts
at sector 2, to account for the hard disk's master boot record.
Relative Sector (rel sect): The number of sectors preceding each partition
on the disk is kept in the 4 byte field labelled "rel sect". This value is
obtained by counting the sectors beginning with cylinder 0, sector 1, head 0
of the disk, and incrementing the sector, head, and then track values up to
the beginning of the partition. This, if the disk has 17 sectors per track and
4 heads, and the second partition begins at cylinder 1, sector 1, head 0,and
the partition's starting relative sector is 68 (decimal) - there were 17
sectors on each of 4 heads on 1 track allocated ahead of it. The field is stored
with the least significant word first.
Number of sectors (#sects): The number of sectors allocated to the partition
is kept in the "# of sects" field. This is a 4 byte field stored least
significant word first.
Signature: The last 2 bytes of the boot record (55AAh) are used as a signature
to identify a valid boot record. Both this record and the partition boot record
are required to contain the signature at offset 1FEh.
The master disk boot record invokes ROM BASIC if no indicator byte reflects a
bootable system.
When a partition's boot record is given control. It is passed its partition
table entry address in the DS:SI registers.
DETERMINING FIXED DISK ALLOCATION
DOS determines disk allocation using the following formula:
D * BPD
TS - RS - ???????????
BPS
SPF = ??????????????????????????????
BPS * SPC
CF + ??????????????
BPC
where:
TS total sectors on disk
RS the number of sectrs at the beginning of the disk that are reserved
for the boot record. DOS reserves 1 sector.
D The number of directory entries in the root directory.
BPD the number of bytes per directory entry. This is always 32.
BPS the number of bytes per logical sector. Typically 512, but you can
specify a different number with VDISK.
CF The number of FATS per disk. Usually 2. VDISK is 1.
SPF the number of sectors per FAT. Maximum 64.
SPC The number of sectors per allocation unit.
BPC the number of bytes per FAT entry. BPC is 1.5 for 12 bit FATs.
2 for 16 bit FATS.
To calculate the minimum partition size that will force a 16-bit FAT:
CYL = (max clusters * 8)/(HEADS * SPT)
where:
CYL number of cylinders on the disk
max clusters 4092 (maximum number of clusters for a 12 bit FAT)
HEADS number of heads on the hard disk
SPT sectors per track (normally 17 on MFM)
note: DOS 2.0 uses a "first fit" algorithm when allocating file space on the
hard disk. Each time an application requests disk space, it will scan from the
beginning of the FAT until it finds a contiguous peice of storage large enough
for the file.
DOS 3.0 keeps a pointer into the disk space, and begins its search from the
point it last left off. This pointer is lost when the system is rebooted.
This is called the "next fit" algorithm. It is faster than the first fit and
helps minimize fragmentation.
In either case, if the FCB function calls are used instead of the handle
function calls, the file will be broken into peices starting with the first
availible space on the disk.
Comment to 826. Comment(s).
----------
Better late than never...
A partition table entry for the IBM AT is set up as follows:
DB drive ; 0 or 80H, 80H marks a bootable, active partition
DB head1 ; starting heads
DW trksec1 ; starting track/sector (CX value for INT 13)
DB system ; see below
DB head2 ; ending head
DW trksec2 ; ending track/sector
DD sector1 ; absolute # of starting sector
DD sector2 ; absolute # of last sector
The system byte is different for different O/S entries:
1 DOS, 12-bit FAT entries
4 DOS, 16-bit FAT entries
DB Concurrent DOS
F2 2nd partition for Sperry machines with large disks
And so on. There are bytes for XENIX, Prologue and lots of other O/S.
Many manufacturers diddle with these system bytes to implement more than 1 DOS
partition per disk. The only one I know about who violates the rule that only
one DOS partition (1 or 4) per disk may exist is Tandon.
CHAPTER 9
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
INSTALLABLE DEVICE DRIVERS
DEVICE DRIVER FORMAT
A device driver is a COM or EXE file that contains all of the code needed to
control an add-in device. It has a special header to identify it as a device,
define the strategy and interrupt entry points, and define its various
attributes.
NOTE: For device drivers the COM file must not use the ORG 100h. Since the
driver does not use the program segment prefix, it is simply loaded
without offset. Therefore the memory image file must have an origin of 0
(ORG 0 or no ORG statement).
TYPES OF DEVICES
There are two types of devices: Character devices and Block devices. Their
attributes are as follows:
Character devices are designed to do character I/O in a serial manner like
CON, AUX, and PRN. These devices have names like CON, AUX, CLOCK$, and you can
open channels (handles or FCBs) to do input and output with them. Because
character devices have only one name, they can only support one device.
Block devices are the fixed disk or diskette drives on a system. They can do
random I/O in peices called blocks, which are usually the physical sector
size of the disk. These devices are not named as character devices are, and
cannot be opened directly. Instead they are mapped by using the drive letters
A,B,C etc. Block devices can have units within them. In this way, a single block
driver can be responsible for one or more disk drives. For example, the first
block device driver can be responsible for drives A,B,C,and D. This means it has
four units defined and therefore takes up four drive letters. The position of
the driver in the chain of all drives determines the way in which the drive
letters correspond. For example, if the device driver is the first block driver
in the device chain, and it defines four units, then these devices are called
A,B,C, and D. If the second device driver defines three units, then those units
are E,F,and G. DOS 1.x allows 16 devices. DOS 2.x allows 63, and DOS 3.x allows
26. It is recommended that drivers limit themselves to 26 devices for
compatibility with DOS 3.x.
DOS doesn't care about the position of installed character devices versus
block devices. The installed character devices get put into the chain ahead of
resident character devices so that you can override the system's default driver
for CON etc.
Although it is sometimes beleived that installed block devices get linked into
the chain BEHIND the resident block devices, if you look at the actual device
chain, this is not true (though it is true in the sense that installed block
devices get assigned drive letters in sequence, starting with the next letter
after the last one assigned to a resident block device).
DEVICE HEADER
A device driver requires a device header at the beginning of the file. This
is the format of the device header:
Field Length
Pointer to next device header field dword
Attribute word
Pointer to device strategy routine word
Pointer to device interrupt routine word
Name/Unit field 8 bytes
POINTER TO NEXT DEVICE HEADER FIELD
The device header field is a pointer to the device header of the next device
driver. It is a doubleword field that is set by DOS at the time the device
driver is loaded. The first word is an offset and the second word is the
segment.
If you are loading only one device driver, set the device header field to -1
before loading the device. If you are loading more than one device driver, set
the first word of the device driver header to the offset of the next device
driver's header. Set the device driver header field of the last device driver
to -1.
ATTRIBUTE FIELD
The attribute field is a word field that describes the attributes of the
device driver to the system. The attributes are:
word bits (decimal)
15 1 character device
0 block device
14 1 supports IOCTL
0 doesn't support IOCTL
13 1 non-IBM format (block only)
0 IBM format
12 not documented - unknown
11 1 supports removeable media
0 doesn't support removeable media
10 reserved for DOS
through
4 reserved for DOS
3 1 current block device
0 not current block device
2 1 current NUL device
0 not current NUL device
1 1 current standard output device
0 not current standard output device
BIT 15 is the device type bit. Use it to tell the system the that driver is a
block or character device.
BIT 14 is the IOCTL bit. It is used for both character and block devices. Use
it to tell DOS whether the device driver can handle control strings
through the IOCTL function call 44h.
If a device driver cannot process control strings, it should set bit
14 to 0. This way DOS can return an error is an attempt is made through
the IOCTL function call to send or receive control strings to the
device. If a device can process control strings, it should set bit 14
to 1. This way, DOS makes calls to the IOCTL input and output device
function to send and receive IOCTL strings.
The IOCTL functions allow data to be sent to and from the device
without actually doing a normal read or write. In this way, the device
driver can use the data for its own use, (for example, setting a baud
rate or stop bits, changing form lengths, etc.) It is up to the device
to interpret the information that is passed to it, but the information
must not be treated as a normal I/O request.
BIT 13 is the non-IBM format bit. It is used for block devices only. It affects
the operation of the Get BPB (BIOS parameter block) device call.
BIT 11 is the open/close removeable media bit. Use it to tell DOS if the
device driver can handle removeable media. (DOS 3.x only)
BIT 3 is the clock device bit. It is used for character devices only. Use it
to tell DOS if your character device driver is the new CLOCK$ device.
BIT 2 is the NUL attribute bit. It is used for character devices only. Use it
to tell DOS if your character device driver is a NUL device. Although
there is a NUL device attribute bit, you cannot reassign the NUL device.
This is an attribute that exists for DOS so that DOS can tell if the NUL
device is being used.
BIT 0 are the standard input and output bits. They are used for character
& devices only. Use these bits to tell DOS if your character device
BIT 1 driver is the new standard input device or standard output device.
POINTER TO STRATEGY AND INTERRUPT ROUTINES
These two fields are pointers to the entry points of the strategy and input
routines. They are word values, so they must be in the same segment as the
device header.
NAME/UNIT FIELD
This is an 8-byte field that contains the name of a character device or the
unit of a block device. For the character names, the name is left-justified and
the space is filled to 8 bytes. For block devices, the number of units can be
placed in the first byte. This is optional because DOS fills in this location
with the value returned by the driver's INIT code.
CREATING A DEVICE DRIVER
To create a device driver that DOS can install, perform the following:
1) Create a memory image file or an EXE file with a device header at the start
of the file.
2) Originate the code (including the device header) at 0, not 100h.
3) Set the next device header field. Refer to "Pointer to Next Device Header
Attribute Field" for more information.
4) Set the attribute field of the device header. Refer to "Attribute Field" for
more information.
5) Set the entry points for the interrupt and strategy routines.
6) Fill in the name/unit field with the name of the character device or the unit
number of the block device.
DOS always processes installable character device drivers before handling the
default devices. So to install a new CON device, simply name the device CON.
Be sure to set the standard input device and standard output device bits in
the attribute field of a new CON device. The scan of the device list stops on
the first match so the installable device driver takes precedence.
NOTE: Because DOS can install the device driver anywhere in memory, care
must be taken in any FAR memory references. You should not expect that
your driver will be loaded in the same place every time.
INSTALLING DEVICE DRIVERS
DOS installs new device drivers dynamically at boot time by reading and
processing the DEVICE command in the config.sys file. For example, if you have
written a device driver called DRIVER1, to install it put this command in the
CONFIG.SYS file:
DEVICE=DRIVER1
DOS calls a device driver at its strategy entry point first, passing in a
request header the information describing what DOS wants the device driver
to do.
This strategy routine does not perform the request but rather queues the
request or saves a pointer to the request header. The second entry point is
the interrupt routine and is called by DOS immediately after the strategy
routine returns. The interrupt routine is called with no parameters. Its
function is to perform the operation based on the queued request and set up
any return infromation.
DOS passes the pointer to the request header in ES:BX. This structure consists
of a fixed length header (Request Header) followed by data pertinent to the
operation to be performed.
NOTE: It is the responsibility of the device driver to preserve the machine
state. For example, save all registers on entry and restore them on exit.
The stack used by DOS has enough room on it to save all the registers. If more
stack space is needed, it is the device driver's responsibility to allocate and
maintain another stack.
All calls to execute device drivers are FAR calls. FAR returns should be
executed to return to DOS.
INSTALLING CHARACTER DEVICES
One of the functions defined for each device is INIT. This routine is called
only once when the device is installed and never again. The INIT routine returns
the following:
A) A location to the first free byte of memory after the device driver, like a
TSR that is stored in the terminating address field. This way, the
initialization code can be used once and then thrown away to save space.
B) After setting the address field, a character device driver can set the status
word and return.
INSTALLING BLOCK DEVICES
Block devices are installed in the same way as character devices. The
difference is that block devices return additional information. Block devices
must also return:
A) The number of units in the block device. This number determines the logical
names the devices will have. For example, if the current logical device
letter is F at the time of the install call, and the block device driver INIT
routine returns three logical units, the letters G, H, and I are assigned to
the units. The mapping is determined by the position of the driver in the
device list and the number of units in the device. The number of units
returned by INIT overrides the value in the name/unit field of the device
header.
B) A pointer to a BPB (BIOS parameter block) pointer array. This is a pointer
to an array of *n* word pointers there *n* is the number of units defined.
These word pointers point to BPBs. This way, if all of the units are the
same, the entire array can point to the same BPB to save space.
The BPB contains information pertinent to the devices such as the sector
size, number of sectors per allocation unit, and so forth. The sector size of
the BPB cannot be greater than the maximum allotted size set at DOS
initialization time.
NOTE: This array must be protected below the free pointer set by the return.
C) The media descriptor byte. This byte is passed to devices so that they know
what parameters DOS is currently using for a particular drive unit.
Block devices can take several approaches. They can be *dumb* or *smart*. A
dumb device would define a unit (and therefore a BPB) for each possible media
drive combination. Unit 0=drive 0;single side, unit 1=drive 0;double side, etc.
For this approach, the media descriptor bytes would mean nothing. A smart
device would allow multiple media per unit. In this case, the BPB table
returned at INIT must define space large enough to acommodate the largest
possible medias supported (sector size in BPB must be as large as maximum
sector size DOS is currently using). Smart drivers will use the media byte to
pass information about what media is currently in a unit.
REQUEST HEADER
The request header passes the information describing what DOS wants the
device driver to do.
????????????????????????????????????????????????????????????????????????????????
? Length ? F i e l d ?
????????????????????????????????????????????????????????????????????????????????
? BYTE ? Length in bytes of the request header plus any data at end ?
????????????????????????????????????????????????????????????????????????????????
? BYTE ? Unit code. The subunit the operation is for (minor device) ?
? ? Has no meaning for character devices. ?
????????????????????????????????????????????????????????????????????????????????
? WORD ? Command code ?
????????????????????????????????????????????????????????????????????????????????
? 8 BYTES ? Deserved for DOS ?
????????????????????????????????????????????????????????????????????????????????
?(variable)? Data appropriate for the operation ?
????????????????????????????????????????????????????????????????????????????????
UNIT CODE FIELD
The unit code field identifies which unit in a block device driver the request
is for. For example, if a block device driver has three units defined, then the
possible values of the unit code field would be 0,1,and 2.
COMMAND CODE FIELD
The command code field in the request header can have the following values:
CODE FUNCTION
0 INIT
1 MEDIA CHECK (block only,NOP for character)
2 BUILD BPB (block only, NOP for character)
3 IOCTL input (called only if IOCTL bit is 1)
4 INPUT (read)
5 NONDESTRUCTIVE INPUT NO WAIT (character devices only)
6 INPUT STATUS (character devices only)
7 INPUT FLUSH (character devices only)
8 OUTPUT (write)
9 OUTPUT (write with verify)
10 OUTPUT STATUS (character devices only)
11 OUTPUT FLUSH (character devices only)
12 IOCTL OUTPUT (called only if IOCTL bit is 1)
13 DEVICE OPEN (called only if OPEN/CLOSE/RM bit is set)
14 DEVICE CLOSE (called only if OPEN/CLOSE/RM bit is set)
15 REMOVEABLE MEDIA (called only if OPEN/CLOSE/RM bit is set and
device is block)
NOTE: Command codes 13,14,and 15 are for use with DOS versions 3.x.
STATUS FIELD
The status field in the request header contains:
????????????????????????????????????????????????????????????????????????????????
? D E V I C E D R I V E R S T A T U S F I E L D ?
????????????????????????????????????????????????????????????????????????????????
? B ? 0 ? ?
? ? 1 ? ?
? Y ? 2 ? ?
? ? 3 ? Error message return code ?
? T ? 4 ? (with bit 15=1) ?
? ? 5 ? ?
? E ? 6 ? ?
? ? 7 ? ?
????????????????????????????????????????????????????????????????????????????????
? bit ? 8 ? DONE ?
????????????????????????????????????????????????????????????????????????????????
? bit ? 9 ? BUSY ?
????????????????????????????????????????????????????????????????????????????????
? bits ? 10 - 14 ? Reserved ?
????????????????????????????????????????????????????????????????????????????????
? bit ? 15 ? Error ?
????????????????????????????????????????????????????????????????????????????????
The status word field is zero on entry and is set by the driver interrupt
routine on return.
BIT 15 is the error bit. If this bit is set, the low 8 bits of the status word
(7-0) indicate the error code.
BITS 14-10 are reserved.
BIT 9 is the busy bit. It is only set by status calls and the removable media
call. See "STATUS" and "REMOVABLE MEDIA" in this chapter for more
information about the calls.
BIT 8 is the done bit. If it is set, it means the operation is complete. The
driver sets the bit to 1 when it exits.
The low 8 bits of the status word define an error message if bit 15 is set.
These errors are:
00h Write protect violation 01h Unknown unit
02h Device not ready 03h Unknown command
04h CRC error 05h Bad drive request structure length
06h seek error 07h unknown media
08h sector not found 09h printer out of paper
0Ah write fault 0Bh read fault
0Ch general failure 0Dh reserved
0Eh reserved 0Fh invalid disk change
DEVICE DRIVER FUNCTIONS
All strategy routines are called with ES:BX pointing to the request header.
The interrupt routines get the pointers to the request header from the queue
the strategy routines store them in. The command code in the request header
tells the driver which function to perform.
NOTE: all DWORD pointers are stored offset first, then segment.
The following function call parameters are described:
INIT
MEDIA CHECK
BUILD BPB (BIOS PARAMETER BLOCK)
MEDIA DESCRIPTOR BYTE
INPUT OR OUTPUT
NONDESTRUCTIVE INPUT NO WAIT
STATUS
FLUSH
OPEN OR CLOSE
REMOVABLE MEDIA
INIT
Command code=0
ES:BX pointer to request header. Format of header:
length field
13 bytes request header
dword number of units (not set by character devices)
dword Ending address of resident program code
dword Pointer to BPB array (not set by character devices)
/pointer to remainder of arguments
byte Drive number (3x only)
The driver must do the following:
A) set the number of units (block devices only)
B) set up the pointer to the BPB array (block devices only)
C) perform any initialization code (to modems, printers, etc)
D) Set the ending address of the resident program code
E) set the status word in the request header.
To obtain information obtained from CONFIG.SYS to a device driver at INIT
time, the BPB pointer field points to a buffer containing the information
passed from CONFIG.SYS following the =. The buffer that DOS passes to the
driver at INIT after the file specification contains an ASCII string for the
file OPEN. The ASCII string (ending in 0h) is terminated by a carriage return
(0Dh) and linefeed (0Ah). If there is no parameter information after the file
specification, the file specification is immediately followed by a linefeed
(0Ah). This information is read-only and only system calls 01h-0Ch and 30h can
be issued by the INIT code of the driver.
The last byte parameter contains the drive letter for the first unit of a
block driver. For example, 0=A, 1=B etc.
If an INIT routine determines that it cannot set up the device and wants to
abort without using any memory, follow this procedure:
A) set the number of units to 0
B) set the ending offset address at 0
C) set the ending offsret segment address to the code segment (CS)
NOTE: If there are multiple device drivers in a single memory image file, the
ending address returned by the last INIT called is the one DOS uses. It is
recommended that all device drivers in a single memory image file return
the same ending address.
MEDIA CHECK
command code=1
ES:BX pointer to request header. Format of header:
length field
13 bytes request header
byte media descriptor from DOS
byte return
dword returns a pointer to the previous volume ID (if bit
11=1 and disk change is returned) (DOS 3.x)
When the command code field is 1, DOS calls MEDIA CHECK for a drive unit and
passes its current media descriptor byte. See "Media Descriptor Byte" later in
this chapter for more information about the byte. MEDIA CHECK returns one of
the following:
A) media not changed C) not sure
B) media changed D) error code
The driver must perform the following:
A) set the status word in the request header
B) set the return byte
-1 media has been changed
0 don't know if media has been changed
1 media has not been changed
DOS 3.x: If the driver has set the removable media bit 11 of the device header
attribute word to 1 and the driver returns -1 (media changed), the driver must
set the DWORD pointer to the previous volume identification field. If DOS
determines that the media changed is an error, DOS generates an error 0Fh
(invalid disk change) on behalf of the device. If the driver does not implement
volume identification support, but has bit 11 set to 1, the driver should set a
pointer to the string "NO NAME",0.
MEDIA DESCRIPTOR
Currently the media descriptor byte has been defined for a few media types.
This byte should be idetnical to the media byte if the device has the non-IBM
format bit off. These predetermined values are:
media descriptor byte => 1 1 1 1 1 0 0 0
(numerical order) 7 6 5 4 3 2 1 0
BIT MEANING
0 1=2 sided 0=not 2 sided
1 1=8 sector 0=not 8 sector
2 1=removeable 0=nonremoveable
3-7 must be set to 1
Examples of current DOS media descriptor bytes:
media sides sectors ID byte
hard disk * * 0F8h
5-1/4 floppy 2 15 0F9h
5-1/4 floppy 1 9 0FCh
5-1/4 floppy 2 9 0FDh
5-1/4 floppy 2 8 0FFh
5-1/4 floppy 1 8 0FEh
8" floppy 1 26 0FEh *
8" floppy 2 26 0FDh
8" floppy 2 8 0FEh *
- NOTE: The two Media Descriptor Bytes that are the same for 8" diskettes (0FEh)
are not a misprint. To determine whether you are using a single sided or double
sided diskette, attempt to read the second side, and if an error occurs you can
assume the diskette is single sided.
BUILD BPB (BIOS Parameter Block)
command code =2
ES:BX pointer to request header. Format:
length field
13 bytes request header
byte media descriptor from DOS
dword transfer address (buffer address)
dword pointer to BPB table
DOS calls BUILD BPB under the following two conditions:
A) If "media changed" is returned
B) If "not sure" is returned, there are no used buffers. Used buffers are
buffers with changed data that has not yet been written to the disk.
The driver must do the following:
A) set the pointer to the BPB
B) set the status word in the request header.
The driver must determine the correct media type currently in the unit to
return the pointer to the BPB table. The way the buffer is used (pointer
passed by DOS) is determined by the non-IBM format bit in the attribute field
of the device header. If bit 13=0 (device is IBM compatible), the buffer
contains the first sector of the FAT (most importantly the FAT ID byte). The
driver must not alter this buffer in this case. If bit 13=1 the buffer is a
one sector scratch area which can be used for anything.
For drivers that support volume identification and disk change, the call
should cause a new volume identification to be read off the disk. This call
indicates that the disk has been legally changed.
If the device is IBM compatible, it must be true that the first sector of the
first FAT is located at the same sector for all possible media. This is
because the FAT sector is read before the media is actually determined.
The information relating to the BPB for a particular media is kept in the boot
sector for the media. In particular, the format of the boot sector is:
????????????????????????????????????????????????????????????????????????????????
? For DOS 2.x, 3 byte near jump (0E9h) For DOS 3.x, 2 byte near jump (0EBh) ?
? followed by a NOP (90h) ?
????????????????????????????????????????????????????????????????????????????????
? 8 bytes ? OEM name and version ?
????????????????????????????????????????????????????????????????????????????????
? BYTE ? ? sectors per allocation unit (must be a power of 2) ?
???????????? ???????????????????????????????????????????????????????????????
? WORD ? B ? reserved sectors (strarting at logical sector 0) ?
???????????? ???????????????????????????????????????????????????????????????
? BYTE ? ? number of FATs ?
???????????? ???????????????????????????????????????????????????????????????
? WORD ? P ? max number of root directory entries ?
???????????? ???????????????????????????????????????????????????????????????
? WORD ? ? number of sectors in logical image (total number of ?
? ? ? sectors in media, including boot sector directories, etc.) ?
???????????? B ???????????????????????????????????????????????????????????????
? BYTE ? ? media descriptor ?
???????????? ???????????????????????????????????????????????????????????????
? WORD ? ? number of sectors occupied by a single FAT ?
????????????????????????????????????????????????????????????????????????????????
? WORD ? sectors per track ?
????????????????????????????????????????????????????????????????????????????????
? WORD ? number of heads ?
????????????????????????????????????????????????????????????????????????????????
? WORD ? number of hidden sectors ?
????????????????????????????????????????????????????????????????????????????????
The three words at the end return information about the media. The number of
heads is useful for supporting different multihead drives that have the same
storage capacity but a different number of surfaces. The number of hidden
sectors is useful for drive partitioning schemes.
INPUT / OUTPUT
command codes=3,4,8,9,and 12
ES:BX pointer to request header. Format:
length field
13 bytes request header
byte media descriptor byte
dword transfer address (buffer address)
word byte/sector count
dword (DOS 3.x) pointer to the volume ID if error code 0Fh
is returned
The driver must perform the following:
A) set the status word in the request header
B) perform the requested function
C) set the actual number of sectors or bytes tranferred
NOTE: No error checking is performed on an IOCTL I/O call. However the driver
must set the return sector or byte count to the actual number of bytes
transferred.
The following applies to block device drivers:
Under certain circumstances the device driver may be asked to do a write
operation of 64k bytes that seems to be a *wrap around* of the transfer address
in the device driver request packet. This arises due to an optimization added to
write code in DOS. It will only happen in writes that are within a sector size
of 64k on files that are being exetended past the current end of file. It is
allowable for the device driver to ignore the balance of the write that wraps
around, if it so chooses. For example, a write of 10000h bytes worth of sectors
with a transfer address of XXXX:1 ignores the last two bytes.
Remember: A program that uses DOS function calls can never request an input or
output function of more than 0FFFFh bytes, therefore, a wrap around
in the transfer (buffer) segment can never occur. It is for this
reason you can ignore bytes that would have wrapped around in the
tranfer segment.
If the driver returns an error code of 0Fh (invalid disk change) it must put
a DWORD pointer to an ASCIIZ string which is the correct volume ID to ask the
user to reinsert the disk.
DOS 3.x:
The reference count of open files on the field (maintained by the OPEN and
CLOSE calls) allows the driver to determine when to return error 0Fh. If there
are no open files (reference count=0) and the disk has been changed, the I/O
is all right, and error 0Fh is not returned. If there are open files
(reference count > 0) and the disk has been changed, an error 0Fh condition
may exist.
NONDESTRUCTIVE INPUT NO WAIT
command code=5
ES:BX pointer to request header. Format:
length field
13 bytes request header
byte read from device
The driver must do the following:
A) return a byte from the device
B) set the status word in the request header.
If the character device returns busy bit=0 (characters in the buffer), then
the next character that would be read is returned. This character is not removed
form the buffer (hence the term nondestructive input). This call allows DOS to
look ahead one character.
STATUS
command codes=8 and 10
ES:BX pointer to a request header. Format:
length field
13 bytes request header
This driver must perform the following:
A) perform the requested function
B) set the busy bit
C) set the status word in the request header.
The busy bit is set as follows:
For input on character devices: if the busy bit is 1 on return, a write
request would wait for completion of a current request. If the busy bit is 0,
there is no current request. Therefore, a write request would start immediately.
For input on character devices with a buffer: if the busy bit is 1 on return,
a read request does to the physical device. If the busy bit is 0, there are
characters in the device buffer and a read returns quickly. It also indicates
that a user has typed something. DOS assumes all character devices have a type-
ahead input buffer. Devices that do not have this buffer should always return
busy=0 so that DOS does not hang waiting for information to be put in a buffer
that does not exist.
FLUSH
command codes=7 and 11
ES:BX pointer
length field
13 bytes request header
This call tells the driver to flush (terminate) all pending requests that it
has knowledge of. Its primary use is to flush the input queue on character
devices.
The driver must set the status word in the request header upon return.
OPEN or CLOSE (3.x)
command codes=13 and 14
ES:BX pointer
length field
13 bytes static request header
These calls are designed to give the device information about the current file
activity on the device if bit 11 of the attribute word is set. On block
devices, these calls can be used to manage local buffering. The device can keep
a reference count. Every OPEN causes the device to increment the reference
count. Every CLOSE causes the device to decrement the reference count. When the
reference count is 0, if means there are no open files in the device. Therefore,
the device should flush buffers inside the device it has written to because now
the user can change the media on a removeable media drive. If the media had been
changed, it is advisable to reset the reference count to 0 without flushing the
buffers. This can be thought of as "last close causes flush". These calls are
more useful on character devices. The OPEN call can be used to send a device
initialization string. On a printer, this could cause a string to be sent to set
the font, page size, etc. so that the printer would always be in a known state
in the I/O stream. Similarly, a CLOSE call can be used to send a post string
(like a form feed) at the end of an I/O stream. Using IOCTL to set these pre and
post strings provides a flexible mechanism of serial I/O device stream control.
NOTE: Since all processes have access to STDIN,STDOUT,STDERR,STDAUX, and STDPRN
(handles 0,1,2,3,and 4) the CON, AUX, and PRN devices are always open.
REMOVABLE MEDIA (DOS 3.x)
command code=15
ES:BX pointer
length field
13 bytes status request header
To use this call, set bit 11 of the attribute field to 1. Block devices can
only use this call through a subfunction of the IOCTL function call (44h).
This call is useful because it allows a utility to know whether it is dealing
with a nonremovable media drive or with a removable media drive. For example,
the FORMAT utility needs to know whether a drive is removable or nonremovable
because it prints different versions of some prompts.
The information is returned in the BUSY bit of the status word. If the busy
bit is 1, the media is nonremovable.
NOTE: No error checking is performed. It is assumed that this call always
succeeds.
THE CLOCK$ DEVICE
To allow a clock board to be integrated into the system for TIME and DATE,
the CLOCK$ device is used. This device defines and performs functions like any
other character device (most functions will be reset done bit, reset error bit,
and return). When a read or write to this device occurs, 6 bytes are
transferred. The first 2 bytes are a word, which is the count of days since
01-01-80. The third byte is minutes, the fourth is hours, the fifth is
hundredths of a second, and the sixth is seconds.
Reading the CLOCK$ device gets the date and time, writing to it sets the date
and time.
CHAPTER 10
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
LOTUS-INTEL-MICROSOFT EXPANDED MEMORY SPECIFICATION
The Expanded Memory Manager ............................................ 10-
History ........................................................ 10-
Page Frames .................................................... 10-
Expanded Memory Services ............................................... 10-
AST/Quadram/Ashton-Tate Enhanced EMM ................................... 10-
Calling the Manager ............................................ 10-
Common EMS Functions (hex calls)
1 (40h) Get Manager Status ............................ 10-
2 (41h) Get Page Frame Segment ........................ 10-
3 (42h) Get Number of Pages ........................... 10-
4 (43h) Get Handle and Allocate Memory ................ 10-
5 (44h) Map Memory .................................... 10-
6 (45h) Release Handle and Memory ..................... 10-
7 (46h) Get EMM Version ............................... 10-
8 (47h) Save Mapping Context .......................... 10-
9 (48h) Restore Mapping Context ....................... 10-
10 (49h) Reserved ...................................... 10-
11 (4Ah) Reserved ...................................... 10-
12 (4Bh) Get Number of EMM Handles ..................... 10-
12 (4Ch) Get Pages Owned By Handle ..................... 10-
14 (4Dh) Get Pages for All Handles ..................... 10-
15 (4Eh) Get Or Set Page Map ........................... 10-
new LIM 4.0 specification:
16 (4Fh) Get/Set Partial Page Map ...................... 10-
17 (50h) Map/Unmap Multiple Pages ...................... 10-
18 (51h) Reallocate Pages .............................. 10-
19 (52h) Handle Attribute Functions .................... 10-
20 (53h) Get Handle Name ............................... 10-
21 (54h) Get Handle Directory .......................... 10-
22 (55h) Alter Page Map & Jump ......................... 10-
23 (56h) Alter Page Map & Call ......................... 10-
24 (57h) Move Memory Region ............................ 10-
25 (58h) Get Mappable Physical Address Array ........... 10-
26 (59h) Get Expanded Memory Hardware .................. 10-
27 (5Ah) Allocate Raw Pages ............................ 10-
28 (5Bh) Get Alternate Map Register Set ................ 10-
29 (5Ch) Prepare Expanded Memory Hardware .............. 10-
30 (5Dh) Enable OS/E Function Set ...................... 10-
31 (5Eh) Unknown ....................................... 10-
32 (5Fh) Unknown ....................................... 10-
33 (60h) Unknown ....................................... 10-
34 (61h) AST Generic Accelerator Card Support .......... 10-
Expanded Memory Manager Error Codes .................................... 10-
THE EXPANDED MEMORY MANAGER
History
The Lotus/Intel/Microsoft Expanded Memory Manager was originally a Lotus and
Intel project and was announced as version 3.0 in the second quarter of 1985
primarily as a means of running larger Lotus worksheets by transparently
paging unused sections to bank-switched memory. Shortly afterward Microsoft
announced support of the standard and version 3.2 was subsequently released
with support for Microsoft Windows. LIM 3.2 supported up to 8 megabytes of
paged memory. The LIM 4.0 supports up to 32 megabytes of paged memory.
AST/QUADRAM/ASHTON-TATE ENHANCED EXPANDED MEMORY SPECIFICATION
The AQA EEMS maintains upward compatibility with the LIM, but is a superset
of functions.
The AQA EEMS permits its pages to be scattered throughout the unused portion
of the machine's address space.
On August 19, 1987, the new version of the Expanded Memory Specification (EMS)
was announced by Lotus, Intel and Microsoft. This new version of the
specification includes many features of the Enhanced Expanded Memory
Specification (EEMS) originally developed by AST Reserach, Quadram and Ashton-
Tate, although the three original sponsoring companies elected not to make the
new specification upward compatible with EEMS. AST Research says that they will
endorse EMS 4.0 without reservation.
The definitive document for the LIM-EMS is Intel part number 300275-004,
August, 1987.
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256K ???????????????? ? ?
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The page frame is located above the 640k system RAM area, anywhere from
0A000h to 0FFFFh. This area is used by the video adapters, network cards, and
add-on ROMs (as in hard disk controllers). The page frames are mapped around
areas that are in use.
WRITING PROGRAMS THAT USE EXPANDED MEMORY
In order to use expanded memory, applications must perform these steps in the
following order:
1. Determine if EMM is installed.
2. Determine if enough expanded memory pages exist for your application.
(Function 3)
3. Allocate expanded memory pages. (Function 4 or 18)
4. Get the page frame base address. (Function 2)
5. Map in expanded memory pages. (Function 5 or 17)
6. Read/write/execute data in expanded memory, just as if it were conventional
memory.
7. Return expanded memory pages to expanded memory pool before exiting. Function
6 or 18)
Programming Guidelines
The following section contains guidelines for programmers writing applications
that use EMM.
A) Do not put a program's stack in expanded memory.
B) Do not replace interrupt 67h. This is the interrupt vector the EMM uses.
Replacing interrupt 67h could result in disabling the Expanded Memory
Manager.
C) Do not map into conventional memory address space your application doesn't
own. Applications that use the EMM to swap into conventional memory space,
must first allocate this space from the operating system. If the operating
system is not aware that a region of memory it manages is in use, it will
think it is available. This could have disastrous results. EMM should not be
used to "allocate" conventional memory. DOS is the proper manager of
conventional memory space. EMM should only be used to swap data in
conventional memory space previously allocated from DOS.
D) Applications that plan on using data aliasing in expanded memory must check
for the presence of expanded memory hardware. Data aliasing occurs when
mapping one logical page into two or more mappable segments. This makes one
16K-byte expanded memory page appear to be in more than one 16K-byte memory
address space. Data aliasing is legal and sometimes useful for applications.
Software-only expanded memory emulators cannot perform data aliasing. A
simple way to distinguish software emulators from actual expanded memory
hardware is to attempt data aliasing and check the results. For example, map
one logical page into four physical pages. Write to physical page 0. Read
physical pages 1-3 to see if the data is there as well. If the data appears
in all four physical pages, then expanded memory hardware is installed in the
system, and data aliasing is supported.
E) Applications should always return expanded memory pages to the expanded
memory manager upon termination. These pages will be made available for other
applications. If unneeded pages are not returned to the expanded memory
manager, the system could run out of expanded memory pages or expanded
memory handles.
F) Terminate and stay resident programs (TSRs) should always save the state of
the map registers before changing them. Since TSRs may interrupt other
programs which may be using expanded memory, they must not change the state
of the page mapping registers without first saving them. Before exiting, TSRs
must restore the state of the map registers.
The following sections describe the three ways to save and restore the state
of the map registers.
1) Save Page Map and Restore Page Map (Functions 8 and 9). This is the
simplest of the three methods. The EMM saves the map register contents in
its own data structures -- the application does not need to provide extra
storage locations for the mapping context. The last mapping context to be
saved, under a particular handle, will be restored when a call to Restore
Page Map is issued with the same handle. This method is limited to one
mapping context for each handle and saves the context for only LIM
standard 64K-byte page frames.
2) Get/Set Page Map (Function 15). This method requires the application to
allocate space for the storage array. The EMM saves the mapping context in
an array whose address is passed to the EMM. When restoring the mapping
context with this method, an application passes the address of an array
which contains a previously stored mapping context. This method is
preferable if an application needs to do more than one save before a
restore. It provides a mechanism for switching between more than one
mapping context.
3) Get/Set Partial Page Map (Function 16). This method provides a way for
saving a partial mapping context. It should be used when the application
does not need to save the context of all mappable memory. This function
also requires that the storage array be part of the application's data.
G) All functions using pointers to data structures must have those data
structures in memory which will not be mapped out. Functions 22 and 23
(Alter Map & Call and Alter Map & Jump) are the only exceptions.
EMS 4.0 SPECIFICATIONS
Page Frames
The bank switched memory chunks are referred to as "page frames". These frame
consist of four 16K memory blocks mapped into some of the normally unused
system ROM address area, 0C0000-0EFFFF. Each 16K page is independent of the
other and they can map to discrete or overlapping areas of the 8 megabyte
expanded memory address area. Most cards allow selection of addresses to prevent
conflict with other cards, such as hard disk controllers and other expanded
memory boards.
Calling the Manager
Applications programs communicate with the EMM device driver directly via user
interrupt 67h. All communication between the application program and the driver
bypasses DOS completely. To call the driver, register AH is loaded with the
number of the EMM service requested; DX is loaded with the file handle; and
interrupt 67h is called. ES:DI is used to pass the address of a buffer or array
if needed.
On return AH contains 0 if the call was successful or an error code from 80h to
8Fh if unsuccessful.
TESTING FOR THE PRESENCE OF THE EXPANDED MEMORY MANAGER
Before an application program can use the Expanded Memory Manager, it must
determine whether the manager is present. The two recommended methods are the
"open handle" technique and the "get interrupt vector" technique.
The majority of application programs can use either the "open handle" or the
"get interrupt vector" method. However, if your program is a device driver or
if it interrupts DOS during file system operations, you must use only the "get
interrupt vector" method.
Device drivers execute from within DOS and can't access the DOS file functions;
programs that interrupt DOS during file operations have a similar restriction.
During their interrupt processing procedures, they can't access the DOS file
functions because another program may be using the system. Since the "get
interrupt vector" method doesn't require the DOS file functions, you must use
it for programs of this type.
The "Open Handle" Method
Most application programs can use the DOS "Open Handle" method to test for
the presence of the EMM. To use this method, follow these steps in order:
1) Issue an "open handle" command (DOS function 3Dh) in "read only" access mode
(register AL = 0). This function requires your program to point to an ASCII
string which contains the path name of the file or device in which you're
interested (register set DS:DX contains the pointer). In this case the file
is actually the reserved name of the expanded memory manager.
you should format the ASCII string as follows:
ASCII_device_name DB 'EMMXXXX0', 0
The ASCII codes for the capital letters EMMXXXX0 are terminated by a byte
containing a value of zero.
2) If DOS returns no error code, skip Steps 3 and 4 and go to Step 5. If DOS
returns a "Too many open files" error code, go to Step 3. If DOS returns a
"File/Path not found" error code, skip Step 3 and go to Step 4.
3) If DOS returns a "Too many open files" (not enough handles) status code, your
program should invoke the "open file" command before it opens any other
files. This will guarantee that at least one file handle will be available to
perform the function without causing this error.
After the program performs the "open file" command, it should perform the
test described in Step 6 and close the "file handle" (DOS function 3Eh).
Don't keep the manager "open" after this status test is performed since
"manager" functions are not available through DOS. Go to Step 6.
4) If DOS returns a "File/Path not found," the memory manager is not installed.
If your application requires the memory manager, the user will have to reboot
the system with a disk containing the memory manager and the appropriate
CONFIG.SYS file before proceeding.
5) If DOS doesn't return an error status code you can assume that either a
device with the name EMMXXXX0 is resident in the system, or a file with this
name is on disk in the current disk drive. Go to Step 6.
6) Issue an "I/O Control for Devices" command (DOS function 44h) with a "get
device information" command (register AL = 0). DOS function 44h determines
whether EMMXXXX0 is a device or a file.
You must use the file handle (register BX) which you obtained in Step 1 to
access the "EMM" device.
This function returns the "device information" in a word (register DX).
Go to Step 7.
7. If DOS returns any error code, you should assume that the memory manager
device driver is not installed. If your application requires the memory
manager, the user will have to reboot the system with a disk containing the
memory manager and the appropriate CONFIG.SYS file before proceeding.
8) If DOS didn't return an error status, test the contents of bit 7 (counting
from 0) of the "device information" word (register DX) the function
returned. Go to Step 9.
9) If bit 7 of the "device information" word contains a zero, then EMMXXXX0 is
a file, and the memory manager device driver is not present. If your
application requires the memory manager, the user will have to reboot the
system with a disk containing the memory manager and the appropriate
CONFIG.SYS file before proceeding.
If bit 7 contains a one, then EMMXXXX0 is a device. Go to Step 10.
10) Issue an "I/O Control for Devices" command (DOS function 44h) with a "get
output status" command (register AL = 7). You must use the file handle you
obtained in Step 1 to access the "EMM" device (register BX). Go to Step 11.
11) If the expanded memory device driver is ready, the memory manager passes
a status value of 0FFh in register AL. The status value is 00h if the device
driver is not ready.
If the memory manager device driver is "not ready" and your application
requires its presence, the user will have to reboot the system with a disk
containing the memory manager and the appropriate CONFIG.SYS file before
proceeding.
If the memory manager device driver is "ready," go to Step 12.
12) Issue a "Close File Handle" command (DOS function 3Eh) to close the expanded
memory device driver. You must use the file handle you obtained in Step 1 to
close the "EMM" device (register BX).
The "Get Interrupt Vector" technique
Any type of program can use this method to test for the presence of the EMM.
Use this method (not the "Open Handle" method) if your program is a device
driver or if it interrupts DOS during file system operations.
Follow these steps in order:
1) Issue a "get vector" command (DOS function 35h) to obtain the contents of
interrupt vector array entry number 67h (addresses 0000:019Ch thru
0000:019Fh).
The memory manager uses this interrupt vector to perform all manager
functions. The offset portion of this interrupt service routine address is
stored in the word located at address 0000:019Ch; the segment portion is
stored in the word located at address 0000:019Eh.
2) Compare the "device name field" with the contents of the ASCII string which
starts at the address specified by the segment portion of the contents of
interrupt vector address 67h and a fixed offset of 000Ah. If DOS loaded the
memory manager at boot time this name field will have the name of the device
in it.
Since the memory manager is implemented as a character device driver, its
program origin is 0000h. Device drivers are required to have a "device
header" located at the program origin. Within the "device header" is an 8
byte "device name field." For a character mode device driver this name field
is always located at offset 000Ah within the device header. The device name
field contains the name of the device which DOS uses when it references the
device.
If the result of the "string compare" in this technique is positive, the
memory manager is present.
Terminate and Stay Resident (TSR) Program Cooperation:
In order for TSR's to cooperate with each other and with other applications,
TSRs must follow this rule: a program may only remap the DOS partition it lives
in. This rule applies at all times, even when no expanded memory is present.
EXPANDED MEMORY SERVICES
FUNCTIONS DEFINED IN EMS 3.2 SPECIFICATION
Interrupt 67h
Function 40h Get Manager Status
LIM Function Call 1
Returns a status code indicating whether the memory manager is
present and the hardware is working correctly.
entry AH 40h
return AH error status: 00h, 80h, 81h, 84h
note 1) upward and downward compatible with both EMS and EEMS 3.2.
this call can be used only after establishing that the EMS driver is in
fact present
2) uses register AX
Function 41h Get Page Frame Segment
LIM Function Call 2
Obtain segment address of the page frame used by the EMM.
entry AH 41h
return AH error status: 00h, 80h, 81h, 84h
BX page frame segment address (error code 0)
note 1) upward and downward compatible with both EMS and EEMS 3.2.
2) uses registers AX & BX
Function 42h Get Unallocated Page Count
LIM Function Call 3
Obtain total number of logical expanded memory pages present in
the system and the number of those pages not already allocated.
entry AH 42h
return AH error status: 00h, 80h, 81h, 84h
BX number of unallocated pages currently availible
DX total number of pages
note 1) upward and downward compatible with both EMS and EEMS 3.2. Note that EMS
and EEMS 3.2 had no mechanism to return the maximum number of handles
that can be allocated by programs. This is handled by the EMS 4.0 new
function 54h/02h.
2) uses registers AX, BX, DX
Function 43h Get Handle and Allocate Memory
LIM Function Call 4
Notifies the EMM that a program will be using extended memory,
obtains a handle, and allocates a certain number of logical pages
of extended memory to be controlled by that handle
entry AH 43h
BX number of 16k logical pages requested (zero OK)
return AH error status: 00h, 80h, 81h, 84h, 85h, 87h, 88h, 89h
DX unique EMM handle (see note 2)
note 1) upward compatible with both EMS and EEMS 3.2; EMS and EEMS 3.2 do not
allow the allocation of zero pages (returns error status 89h). EMS 4.0
does allow zero pages to be requested for a handle, allocating pages
later using function 51h
2) your program must use this EMM handle as a parameter in any function
that requires it. You can use up to 255 handles. The uppermost byte of
the handle will be zero and cannot be used by the application.
3) regs AX & DX are used
Function 44h Map Memory
LIM Function Call 5
Maps one of the logical pages of expanded memory assigned to a
handle onto one of the four physical pages within the EMM's page
frame.
entry AH 44h
AL physical page to be mapped (0-3)
BX the logical page to be mapped (zero through [number of pages
allocated to the EMM handle - 1]). If the logical page number
is 0FFFFh, the physical page specified in AL will be unmapped
(made inaccessible for reading or writing).
DX the EMM handle your program received from Function 4 (Allocate
Pages).
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh
note 1) downward compatible with both EMS and EEMS 3.2; EMS and EEMS 3.2 do not
support unmap (logical page 0FFFFh) capability. Also, EEMS 3.2
specified there were precisely four physical pages; EMS 4.0 uses the
subfunctions of function 58h to return the permitted number of physical
pages. This incorporates the functionality of function 69h ("function
42") of EEMS.
2) uses register AX
Function 45h Release Handle and Memory
LIM Function Call 6
Deallocates the logical pages of expanded memory currently
assigned to a handle and then releases the handle itself.
entry AH 45h
DX handle
return AH error status: 00h, 80h, 81h, 83h, 84h, 86h
note 1) upward and downward compatible with both EMS and EEMS 3.2.
2) uses register AX
3) when a handle is deallocated, its name is set to all ASCII nulls
(binary zeros).
4) a program must perform this function before it exits to DOS or no other
programs can use these pages or the EMM handle.
Function 46h Get EMM Version
LIM Function Call 7
Returns the version number of the Expanded Memory Manager software.
entry AH 46h
return AH error status: 00h, 80h, 81h, 84h
AL version number byte (if AL=00h)
binary coded decimal (BCD) format if version byte:
high nibble: integer digit of the version number
low nibble : fractional digit of version number
i.e., version 4.0 is represented like this:
0100 0000
/ \
4 . 0
note 1) upward and downward compatible with both EMS and EEMS 3.2. It appears
that the intended use for this function is to return the version of the
vendor implementation of the expanded memory manager instead of the
specification version.
2) uses register AX
Function 47h Save Mapping Context
LIM Function Call 8
Save the contents of the expanded memory page-mapping registers on
the expanded memory boards, associating those contents with a
specific EMM handle.
entry AH 47h
DX caller's EMM handle (NOT current EMM handle)
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ch, 8Dh
note 1) upward and downward compatible with both EMS and EEMS 3.2.
2) This only saves the context saved in EMS 3.2 specification; if a driver,
interrupt routine or TSR needs to do more, functions 4Eh (Page Map
functions) or 4Fh (Partial Page Map functions) should be used.
3) no mention is made about the number of save contexts to provide. AST
recommends in their Rampage AT manual one save context for each handle
plus one per possible interrupt (5 + <handles>).
4) uses register AX
5) this function saves the state of the map registers for only the 64K page
frame defined in versions 3.x of the LIM. Since all applications written
to LIM versions 3.x require saving the map register state of only this
64K page frame, saving the entire mapping state for a large number of
mappable pages would be inefficient use of memory. Applications that use
a mappable memory region outside the LIM 3.x page frame should use
functions 15 or 16 to save and restore the state of the map registers.
Function 48h Restore Page Map
LIM Function Call 9
Restores the contents of all expanded memory hardwere page-mapping
registers to the values associated with the given handle by a
previous function 08h (Save Mapping Context).
entry AH 48h
DX caller's EMM handle (NOT current EMM handle)
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Eh
note 1) upward and downward compatible with both EMS and EEMS 3.2.
2) This only restores the context saved in EMS 3.2 specification; if a
driver, interrupt routine or TSR needs to do more, functions 4Eh (Page
Map functions) or 4Fh (Partial Page Map functions) should be used.
3) uses register AX
4) this function saves the state of the map registers for only the 64K page
frame defined in versions 3.x of the LIM. Since all applications written
to LIM versions 3.x require saving the map register state of only this
64K page frame, saving the entire mapping state for a large number of
mappable pages would be inefficient use of memory. Applications that use
a mappable memory region outside the LIM 3.x page frame should use
functions 15 or 16 to save and restore the state of the map registers.
Function 49h Reserved
LIM Function Call 10
This function was used in EMS 3.0, but was no longer documented in
EMS 3.2. It formerly returned the page mapping register I/O port
array. Use of this function is discouraged, and in EMS 4.0 may
conflict with the use of the new functions 16 through 30 (4Fh
through 5Dh) and functions 10 and 11. Functions 10 and 11 are
specific to the hardware on Intel expanded memory boards and may
not work correctly on all vendors' expanded memory boards.
Function 4Ah Reserved
LIM Function Call 11
This function was used in EMS 3.0, but was no longer documented in
EMS 3.2. It was formerly Get Page Translation Array. Use of this
function is discouraged, and in EMS 4.0 may conflict with the use
of the new functions (4Fh through 5Dh).
Function 4Bh Get Number of EMM Handles
LIM Function Call 12
The Get Handle Count function returns the number of open EMM
handles (including the operating system handle 0) in the system.
entry AH 4Bh
return AH error status: 00h, 80h, 81h, 84h
BX handle count (AH=00h) (including the operating system handle
[0]). max 255.
note 1) upward and downward compatible with EMS and EEMS 3.2.
2) uses registers AX and BX
Function 4Ch Get Pages Owned by Handle
LIM Function Call 13
Returns number of logical expanded memory pages allocated to a
specific EMM handle.
entry AH 4Ch
DX handle
return AH error status: 00h, 80h, 81h, 83h, 84h
BX pages allocated to handle, max 2048 because the EMM allows a
maximum of 2048 pages (32M bytes) of expanded memory.
note 1) This function is upward compatible with EMS and EEMS 3.2.
2) programmers should compare the number returned in BX with the maximum
number of pages returned by function 42h register DX, total number of
EMM pages. This should be an UNSIGNED comparison, just in case the spec
writers decide to use 16 bit unsigned numbers (for a maximum space of
one gigabyte) instead of signed numbers (for a maximum space of 512
megabytes). Unsigned comparisons will work properly in either case
3) uses registers AX and BX
Function 4Dh Get Pages for All Handles
LIM Function Call 14
Returns an array containing all active handles and the number of
logical expanded memory pages associated with each handle.
entry AH 4Dh
ES:DI pointer to 1020 byte array to receive information on an array of
structures where a copy of all open EMM handles and the number
of pages allocated to each will be stored.
return AH error status: 00h, 80h, 81h, 84h
BX number of active handles (1-255); array filled with 2-word
entries, consisting of a handle and the number of pages
allocated to that handle. (including the operating system handle
[0]). BX cannot be zero because the operating system handle is
always active and cannot be deallocated.
note 1) NOT COMPATIBLE with EMS or EEMS 3.2, since the new special OS handle
0000h is returned as part of the array. Unless benign use of this
information is used (such as displaying the handle and count of pages
associated with the handle) code should be changed to only work with
handles between 01h and FFh and to specifically ignore handle 00h.
2) The array consists of an array of 255 elements. The first word of each
element is the handle number, the second word contains the number of
pages allocated.
3) There are two types of handles, "standard" and "raw". The specification
does not talk about how this function works when both raw and standard
handles exist in a given system. There is no currently known way to
differentiate between a standard handle and a raw handle in EMS 4.0.
4) uses registers AX and BX
Function 4Eh Get or Set Page Map
LIM Function Call 15
Gets or sets the contents of the EMS page-mapping registers on the
expanded memory boards.
This group of four subfunctions is provided for context switching
required by operating environments and systems. These functions are
upward and downward compatible with both EMS and EEMS 3.2; in
addition, these functions now include the functionality of EEMS
function 6Ah ("function 43") involving all pages.
The size and contents of the map register array will vary from
system to system based on hardware vendor, software vendor, number
of boards and the capacity of each board in the system. Note the
array size can be determined by function 4Eh/03h.
Use these functions (except for 03h) instead of Functions 8 and 9
if you need to save or restore the mapping context but don't want
(or have) to use a handle.
00h Get Page Map
This call saves the mapping context for all mappable memory regions
(conventional and expanded) by copying the contents of the mapping
registers from each expanded memory board to a destination array.
The application must pass a pointer to the destination array.
entry AH 4Eh
AL 00h
ES:DI pointer to target array
return AH error status: 00h, 80h, 81h, 84h, 8Fh
note 1) uses register AX
2) does not use an EMM handle
01h Set Page Map
This call the mapping context for all mappable memory regions
(conventional and expanded) by copying the contents of a source
array into the mapping registers on each expanded memory board in
the system. The application must pass a pointer to the source array.
entry AH 4Eh
AL 01h
DS:SI pointer to source array
return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A3h
note 1) uses register AX
2) does not use an EMM handle
02h Get & Set Page Map
This call simultaneously saves the current mapping context and
restores a previous mapping context for all mappable memory regions
(both conventional and expanded). It first copies the contents of
the mapping registers from each expanded memory board in the system
into a destination array. Then the subfunction copies the contents
of a source array into the mapping registers on each of the
expanded memory boards.
entry AH 4Eh
AL 02h
DS:SI pointer to source array
ES:DI pointer to target array
return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A3h
note 1) uses register AX
03h Get Size of Page Map Save Array
entry AH 4Eh
AL 03h
return AH error status: 00h, 80h, 81h, 84h, 8Fh
AL size in bytes of array
note 1) this subfunction does not require an EMM handle
2) uses register AX
FUNCTIONS NEW TO EMS 4.0
Function 4Eh Get or Set Page Map
LIM Function Call 16
entry AH 4Eh
AL 00h if getting mapping registers
01h if setting mapping registers
02h if getting and setting mapping registers at once
03h if getting size of page-mapping array
DS:SI pointer to array holding information (AL=01/02)
ES:DI pointer to array to receive information (AL=00/02)
return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A3h
note 1) this function was designed to be used by multitasking operating systems
and should not ordinarily be used by appplication software.
Function 4Fh Get/Set Partial Page Map
LIM Function Call 16
These four subfunctions are provided for context switching required
by interrupt routines, operating environments and systems. This set
of functions provides extended functionality over the EEMS function
6Ah (function 43) involving subsets of pages. In EEMS, a subset of
pages could be specified by starting position and number of pages;
in this function a list of pages is specified, which need not be
contiguous.
Interrupt routines can use this function in place of functions 47h
and 48h, especially if the interrupt routine wants to use more than
the standard four physical pages.
AH 4Fh
AL subfunction
00h get partial page map
DS:SI pointer to structure containing list of
segments whose mapping contexts are to be saved
ES:DI pointer to array to receive page map
01h set partial page map
DS:SI pointer to structure containing saved partial
page map
02h get size of partial page map
BX number of mappable segments in the partial map
to be saved
return AH error status (00h): 00h, 80h, 81h, 84h, 8Bh, 8Fh, 0A3h
error status (01h): 00h, 80h, 81h, 84h, 8Fh, 0A3h
error status (02h): 00h, 80h, 81h, 84h, 8Bh, 8Fh
AL size of partial page map for subfunction 02h
DS:SI (call 00h) pointer to array containing the partial mapping
context and any additional information necessary to restore this
context to its original state when the program invokes a Set
subfunction.
note uses register AX
Function 50h Map/Unmap Multiple Pages
LIM Function Call 17
entry AH 50h
AL 00h (by physical page)
01h (by segment number)
CX contains the number of entries in the array. For example, if the
array contained four pages to map or unmap, then CX would
contain 4.
DX handle
DS:SI pointer to an array of structures that contains the information
necessary to map the desired pages.
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh, 8Fh
note 1) New function permits multiple logical-to-physical assignments to be made
in a single call.(faster than mapping individual pages)
2) The source map array is an array of word pairs. The first word of a
pair contains the logical page to map (0FFFFh if the physical page is
to be totally unmapped) and the second word of a pair contains the
physical page number (subfunction 00h) or the segment selector
(subfunction 01h) of the physical page in which the logical page shall
be mapped.
3) A map of available physical pages (by physical page number and segment
selectors) can be obtained using function 58h/00h, Get Mappable
Physical Address Array.
4) uses register AX
5) Both mapping and unmapping pages can be done simultaneously.
6) If a request to map or unmap zero pages is made, nothing is done and no
error is returned.
7) Pages can be mapped or unmapped using one of two methods. Both methods
produce identical results.
A) A logical page and a physical page at which the logical page is to
be mapped. This method is an extension of Function 5 (Map Handle
Page).
B) Specifys both a logical page and a corresponding segment address at
which the logical page is to be mapped. While functionally the same
as the first method, it may be easier to use the actual segment
address of a physical page than to use a number which only
represents its location. The memory manager verifies whether the
specified segment address falls on the boundary of a mappable
physical page. The manager then translates the segment address
passed to it into the necessary internal representation to map the
pages.
Function 51h Reallocate pages
LIM Function Call 18
This function allows an application to change the number of logical
pages allocated to an EMM handle.
entry AH 51h
BX number of pages desired at return
DX handle
return AH error status: 00h, 80h, 81h, 83h, 84h, 87h, 88h
BX number of pages now associated with handle
note 1) uses registers AX, BX
2) Logical pages which were originally allocated with Function 4 are called
pages and are 16K bytes long. Logical pages which were allocated with
Function 27 are called raw pages and might not be the same size as pages
allocated with Function 4.
3) If the status returned in BX is not zero, the value in BX is equal to
the number of pages allocated to the handle prior to calling this
function. This information can be used to verify that the request
generated the expected results.
Function 52h Get/Set Handle Attributes
LIM Function Call 19
entry AH 52h
AL subfunction
00h get handle attributes
01h set handle attributes
BL new attribute
00h make handle volatile
01h make handle non-volatile
02h get attribute capability
DX handle
return AH error status: (function 00h) 00h, 80h, 81h, 83h, 84h, 8Fh, 91h
error status: (function 01h) 00h, 80h, 81h, 83h, 84h, 8Fh, 90h,
91h
error status: (function 02h) 00h, 80h, 81h, 84h, 8Fh
AL attribute (for subfunction 00h)
00h handle is volatile
01h handle is nonvolatile
AL attribute capability (for subfunction 02h)
00h only volatile handles supported
01h both volatile and non-volatile supported
note 1) uses register AX
2) A volatile handle attribute instructs the memory manager to deallocate
both the handle and the pages allocated to it after a warm boot. If all
handles have the volatile attribute (default) at warm boot the handle
directory will be empty and all expanded memory will be initialized to
zero immediately after a warm boot.
3) If the handle's attribute has been set to non-volatile, the handle, its
name (if it is assigned one), and the contents of the pages allocated to
the handle are all maintained after a warm boot.
4) Most PCs disable RAM refresh signals for a considerable period during a
warm boot. This can corrupt some of the data in memory boards. Non-
volatile handles should not be used unless it is definitely known that
the EMS board will retain proper function through a warm boot.
5) subfunction 02h can be used to determine whether the memory manager can
support the non-volatile attribute.
6) Currently the only attribute supported is non-volatile handles and
pages, indicated by the least significant bit.
Function 53h Handle Name Functions
LIM Function Call 20
EMS handles may be named. Each name may be any eight characters.
At installation, all handles have their name initialized to ASCII
nulls (binary zeros). There is no restriction on the characters
which may be used in the handle name (ASCII chars 00h through
0FFh). A name of eight nulls (zeroes) is special, and indicates a
handle has no name. Nulls have no special significance, and they
can appear in the middle of a name. The handle name is 64 bits of
binary information to the EMM.
Functions 53h and 54h provide a way of setting and reading the
names associated with a particular handle. Function 53h manipulates
names by number.
When a handle is assigned a name, at least one character in the
name must be a non-null character in order to distinguish it from
a handle without a name.
00h Get Handle Name
This subfunction gets the eight character name currently
assigned to a handle.
The handle name is initialized to ASCII nulls (binary zeros)
three times: when the memory manager is installed, when a handle
is allocated, and when a handle is deallocated.
entry AH 53h
AL 00h
DX handle
ES:DI pointer to 8-byte handle name array into which the name
currently assigned to the handle will be copied.
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Fh
note uses register AX
01h Set Handle Name
This subfunction assigns an eight character name to a handle.
A handle can be renamed at any time by setting the handle's
name to a new value. When a handle is deallocated, its name is
removed (set to ASCII nulls).
entry AH 53h
AL 01h
DX handle
DS:SI pointer to 8-byte handle name array that is to be assigned to
the handle. The handle name must be padded with nulls if the
name is less than eight characters long.
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Fh, 0A1h
note uses register AX
Function 54h Handle Directory Functions
LIM Function Call 21
Function 54h manipulates handles by name.
00h Get Handle Directory
Returns an array which contains all active handles and the names
associated with each.
entry AH 54h
AL 00h
ES:DI pointer to 2550 byte target array
return AH error status: 00h, 80h, 81h, 84h, 8Fh
AL number of active handles
note 1) The name array consists of 10 byte entries; each entry has a word
containing the handle number, followed by the eight byte (64 bit) name.
2) uses register AX
3) The number of bytes required by the target array is:
10 bytes * total number of handles
4) The maximum size of this array is:
(10 bytes/entry) * 255 entries = 2550 bytes.
01h Search for Named Handle
Searches the handle name directory for a handle with a particular
name. If the named handle is found, this subfunction returns the
handle number associated with the name.
entry AH 54h
AL 01h
DS:SI pointer to an 8-byte string that contains the name of the
handle being searched for
return AH error status: 00h, 80h, 81h, 84h, 8Fh, A0h, 0A1h
DX handle number
note 1) uses registers AX and DX
02h Get Total Handles
Returns the total number of handles the EMM supports, including
the operating system handle (handle value 0).
entry AH 54h
AL 02h
return AH error status: 00h, 80h, 81h, 84h, 8Fh
BX total number of handles availible
note 1) This is NOT the current number of handles defined, but the maximum
number of handles that can be supported in the current environment.
2) uses registers AX and BX
Function 55h Alter Page Map and Jump (cross page branch)
LIM Function Call 22
Alters the memory mapping context and transfers control to the
specified address. Analogous to the FAR JUMP in the 8086 family
architecture. The memory mapping context which existed before
calling function is lost.
entry AH 55h
AL 00h physical page numbers provided by caller
01h segment addresses provided by caller
DX handle
DS:SI pointer to structure containing map and jump address
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh, 8Fh
note 1) Flags and all registers except AX are preserved across the jump.
2) uses register AX
3) Values in registers which don't contain required parameters maintain the
values across the jump. The values in registers (with the exception of
AX) and the flag state at the beginning of the function are still in the
registers and flags when the target address is reached.
4) Mapping no pages and jumping is not considered an error. If a request to
map zero pages and jump is made, control is transferred to the target
address, and this function performs a far jump.
Function 56h Alter Page Map and Call (cross page call)
LIM Function Call 23
00h and 01h
This subfunction saves the current memory mapping context,
alters the specified memory mapping context, and transfers
control to the specified address.
entry AH 56h
AL 00h physical page numbers provided by caller
01h segment addresses provided by caller
DS:SI pointer to structure containing page map and call address
DX handle
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Bh, 8Fh
note 1) Flags and all registers except AX are preserved to the called routine.
On return, flags and all registers except AX are preserved; AL is set to
zero and AX is undefined.
2) uses register AX
3) Values in registers which don't contain required parameters maintain
the values across the call. The values in registers (with the exception
of AX) and the flag state at the beginning of the function are still in
the registers and flags when the target address is reached.
4) Developers using this subfunction must make allowances for the
additional stack space this subfunction will use.
02h Get Page Map Stack Space Size
Since the Alter Page Map & Call function pushes additional
information onto the stack, this subfunction returns the number of
bytes of stack space the function requires.
entry AH 56h
AL 02h
return: BX number of bytes of stack used per call
AH error status: 00h, 80h, 81h, 84h, 8Fh
note 1) if successful, the target address is called. Use a RETF to return and
restore mapping context
2) uses registers AX, BX
Function 57h Move/Exchange Memory Region
LIM Function Call 24
00h Move Memory Region
Moves data between two memory areas. Includes moves between paged
and non-paged areas, or between two different paged areas.
entry AH 57h
AL 00h
SI offset to request block
DS segment selector to request block
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Fh, 92h, 93h, 94h,
95h, 96h, 98h, 0A2h
note 1) uses register AX
01h Exchange Memory Region
Exchanges data between two memory areas. Includes exchanges between
paged and non-paged areas, or between two different paged areas.
entry AH 57h
AL 01h
DS:SI pointer to the data structure which contains the source and
destination information for the exchange.
return AH error status: 00h, 80h, 81h, 83h, 84h, 8Ah, 8Fh, 93h, 94h, 95h,
96h, 97h, 98h, 0A2h
note 1) The request block is a structure with the following format:
dword region length in bytes
byte 0=source in conventional memory
1=source in expanded memory
word source handle
word source offset in page or selector
word source logical page (expanded) or selector (conventional)
byte 0=target in conventional memory
1=target in expanded memory
word target handle
word target offset in page or selector
word target logical page (expanded) or selector (conventional)
2) Expanded memory allocated to a handle is considered to be a linear
array, starting from logical page 0 and progressing through logical page
1, 2, ... n, n+1, ... up to the last logical page in the handle.
3) uses register AX
Function 58h Mappable Physical Address Array
LIM Function Call 25
These functions let you obtain a complete map of the way physical
memory is laid out in a vendor independent manner. This is a
functional equivalent of EEMS function 68h ("function 41"). EEMS
function 60h ("function 33") is a subset call of 68h.
00h Get Array
Returns an array containing the segment address and physical page
number for each mappable physical page in a system. This array
provides a cross reference between physical page numbers and the
actual segment addresses for each mappable page in the system.
entry AH 58h
AL 00h
ES:DI pointer to target array
return AH error status: 00h, 80h, 81h, 84h, 8Fh
CX entries in target array
note 1) The information returned is in an array composed of word pairs. The
first word is the physical page's segment selector, the second word the
physical page number. Note that values are not necessarily returned in a
particular order, either ascending/decending segment selector values or
as ascending/decending physical page number.
2) For compatibility with earlier EMS specifications, physical page zero
contains the segment selector value returned by function 41h, and
physical pages 1, 2 and 3 return segment selector values that corrospond
to the physical 16 KB blocks immediately following physical page zero.
3) uses registers AX and CX
4) The array is sorted in ascending segment order. This does not mean that
the physical page numbers associated with the segment addresses are
also in ascending order.
01h Get Physical Page Address Array Entries.
Returns a word which represents the number of entries in the
array returned by the previous subfunction. This number also
indicates the number of mappable physical pages in a system.
entry AH 58h
AL 01h
return AH error status: 00h, 80h, 81h, 84h, 8Fh
CX number of entries returned by 58h/00h
note 1) multiply CX by 4 for the byte count.
2) uses registers AX and CX
Function 59h Get Expanded Memory Hardware Information
LIM Function Call 26
These functions return information specific to a given hardware
implementation and to use of raw pages as opposed to standard
pages. The intent is that only operating system code ever need use
these functions.
00h Get EMS Hardware Info
Returns an array containing expanded memory hardware configuration
information for use by an operating system.
entry AH 59h
AL 00h
ES:DI pointer to 10 byte target array
The target array has the following format:
word: raw page size in paragraphs (multiples of 16 bytes)
word: number of alternate register sets
word: size of page maps (function 4Eh [15])
word: number of alternate registers sets for DMA
word: DMA operation -- see full specification
return AH error status: 00h, 80h, 81h, 84h, 8Fh, 0A4h
note 1) uses register AX
2) This function is for use by operating systems only.
3) This function can be disabled at any time by the operating system.
01h Get Unallocated Raw Page Count
Returns the number of unallocated non-standard length mappable
pages as well as the total number of non-standard length mappable
pages of expanded memory
entry AH 59h
AL 01h
return AH error status: 00h, 80h, 81h, 84h, 8Fh
BX unallocated raw pages availible for use
DX total raw 16k pages of expanded memory
note 1) uses registers AX, BX, CX
2) An expanded memory page which is a sub-multiple of 16K is termed a raw
page. An operating system may deal with mappable physical page sizes
which are sub-multiples of 16K bytes.
3) If the expanded memory board supplies pages in exact multiples of 16K
bytes, the number of pages this function returns is identical to the
number Function 3 (Get Unallocated Page Count) returns. In this case,
there is no difference between a page and a raw page.
Function 5Ah Allocate Raw Pages
LIM Function Call 27
Allocates the number of nonstandard size pages that the operating
system requests and assigns a unique EMM handle to these pages.
entry AH 5Ah
AL 00h allocate standard pages
01h allocate raw pages
BX number of pages to allocate
return AH error status: 00h, 80h, 81h, 84h, 85h, 87h, 88h
DX unique raw EMM handle (1-255)
note 1) it is intended this call be used only by operating systems
2) uses registers AX and DX
3) For all functions using the raw handle returned in DX, the length of
the physical and logical pages allocated to it are some nonstandard
length (that is, not 16K bytes).
4) this call is primarily for use by operating systems or EMM drivers
supporting hardware with a nonstandard EMS page size.
Function 5Bh Alternate Map Register Set - DMA Registers
LIM Function Call 28
entry AH 00h Get Alternate Map Register Set
01h Set Alternate Map Register Set
BL new alternate map register set number
ES:DI pointer to map register context save area if
BL=0
02h Get Alternate Map Save Array Size
03h Allocate Alternate Map Register Set
04h Deallocate Alternate Map Register Set
BL number of alternate map register set
05h Allocate DMA Register Set
06h Enable DMA on Alternate Map Register Set
BL DMA register set number
DL DMA channel number
07h Disable DMA on Alternate Map Register Set
BL DMA register set number
08h Deallocate DMA Register Set
BL DMA register set number
return AH status: 00h, 02h 00h, 80h, 84h, 81h, 8Fh, 0A4h
01h 00h, 80h, 81h, 84h, 8Fh, 9Ah, 9Ch, 9Dh,
0A3h, 0A4h
03h, 05h 00h 80h 81h 84h, 8Fh, 9Bh, 0A4h
04h 00h, 80h, 81h, 84h, 8Fh, 9Ch, 9Dh, 0A4h
06h, 07h 00h, 80h, 81h, 84h, 8Fh, 9Ah, 9Ch, 9Dh, 9Eh,
9Fh, 0A4h
BL current active alternate map register set number if nonzero
(AL=0)
BL number of alternate map register set; zero if not supported
(AL=3)
DX array size in bytes (subfunction 02h)
ES:DI pointer to a map register context save area if BL=0 (AL=0)
note 1) this call is for use by operating systems only, and can be enabled
or disabled at any time by the operating system
2) This set of functions performs the same functions at EEMS function 6Ah
subfunctions 04h and 05h ("function 43").
3) 00h uses registers AX, BX, ES:DI
01h uses register AX
02h uses registers AX and DX
03h uses registers AX and BX
04h uses register AX
05h uses registers AX, BX
06h uses register AX
07h uses register AX
Function 5Ch Prepare EMS Hardware for Warm Boot
LIM Function Call 29
Prepares the EMM hardware for a warm boot.
entry AH 5Ch
return AH error status: 00h, 80h, 81h, 84h
note 1) uses register AX
2) this function assumes that the next operation that the operating system
performs is a warm boot of the system.
3) in general, this function will affect the current mapping context, the
alternate register set in use, and any other expanded memory hardware
dependencies which need to be initialized at boot time.
4) if an application decides to map memory below 640K, the application must
trap all possible conditions leading to a warm boot and invoke this
function before performing the warm boot itself.
Function 5Dh Enable/Disable OS Function Set Functions
LIM Function Call 30
Lets the OS allow other programs or device drivers to use the OS
specific functions. This capability is provided only for an OS
which manages regions of mappable conventional memory and cannot
permit programs to use any of the functions which affect that
memory, but must be able to use these functions itself.
entry AH 5Dh
AL 00h enable OS function set
01h disable OS function set
02h return access key (resets memory manager, returns access
key at next invocation)
BX,CX access key returned by first invocation
return BX,CX access key, returned only on first invocation of function
AH status 00h, 80h, 81h, 84h, 8Fh, 0A4h
note 1) this function is for use by operating systems only. The operating system
can disable this function at any time.
2) 00h uses registers AX, BX, CX
01h uses registers AX, BX, CX
02h uses register AX
3) 00h, 01h: The OS/E (Operating System/Environment) functions these
subfunctions affect are:
Function 26. Get Expanded Memory Hardware Information.
Function 28. Alternate Map Register Sets.
Function 30. Enable/Disable Operating System Functions.
Function 5Eh Unknown
LIM Function call (not defined under LIM)
Function 5Fh Unknown
LIM Function call (not defined under LIM)
Function 60h EEMS - Get Physical Window Array
LIM Function call (not defined under LIM)
entry AH 60h
ES:DI pointer to buffer
return AH status
AL number of entries
buffer at ES:DI filled
Function 61h Generic Accelerator Card Support
LIM Function Call 34
Contact AST Research for a copy of the Generic Accelerator Card
Driver (GACD) Specification
note Can be used by accelerator card manufacturer to flush RAM cache,
ensuring that the cache accurately reflects what the processor would
see without the cache.
Function 68h EEMS - Get Addresses of All Page Frames in System
LIM Function Call (not defined under LIM)
entry AH 68h
ES:DI pointer to buffer
return AH status
AL number of entries
buffer at ES:DI filled
note Equivalent to LIM 4.0 function 58h
Function 69h EEMS - Map Page Into Frame
LIM Function Call (not defined under LIM)
entry AH 69h
AL frame number
BX page number
DX handle
return AH status
note Similar to EMS function 44h
Function 6Ah EEMS - Page Mapping
LIM Function Call (not defined under LIM)
entry AH 6Ah
AL 00h save partial page map
CH first page frame
CL number of frames
ES:DI pointer to buffer which is to be filled
01h restore partial page map
CH first page frame
CL number of frames
DI:SI pointer to previously saved page map
02h save and restore partial page map
CH first page frame
CL number of frames
ES:DI buffer for current page map
DI:SI new page map
03h get size of save array
CH first page frame
CL number of frames
return AL size of array in bytes
04h switch to standard map register setting
05h switch to alternate map register setting
06h deallocate pages mapped to frames in conventional memory
CH first page frame
CL number of frames
return AH status
note Similar to LIM function 4Eh, except that a subrange of pages can
be specified
EXPANDED MEMORY MANAGER ERROR CODES
EMM error codes are returned in AH after a call to the EMM (int 67h).
code meaning
00h function successful
80h internal error in EMM software (possibly corrupted driver)
81h hardware malfunction
82h EMM busy (dropped in EEMS 3.2)
83h invalid EMM handle
84h function requested not defined - unknown function code in AH.
85h no more EMM handles availible
86h error in save or restore of mapping context
87h more pages requested than exist
88h allocation request specified more logical pages than currently
availible in system (request does not exceed actual physical number of
pages, but some are already allocated to other handles); no pages
allocated
89h zero pages; cannot be allocated (dropped in EMS 4.0)
8Ah logical page requested to be mapped outside range of logical pages
assigned to handle
8Bh illegal page number in mapping request (valid numbers are 0 to 3)
8Ch page-mapping hardware state save area full
8Dh save of mapping context failed; save area already contains context
associated with page handle
8Eh retore of mapping context failed; save area does not contain context
for requested handle
8Fh subfunction parameter not defined (unknown function)
LIM 4.0 extended error codes:
90h attribute type undefined
91h warm boot data save not implemented
92h move overlaps memory
93h move/exchange larger than allocated region
94h conventional/expanded regions overlap
95h logical page offset outside of logical page
96h region larger than 1 MB
97h exchange source/destination overlap
98h source/destination undefined or not supported
99h (no status assigned)
9Ah alternate map register sets supported, specified set is not
9Bh all alternate map & DMA register sets allocated
9Ch alternate map & DMA register sets not supported
9Dh alternate map register or DMA set not defined, allocated or is currently
defined set
9Eh dedicated DMA channels not supported
9Fh dedicated DMA channels supported; specifed channel is not
0A0h named handle could not be found
0A1h handle name already exists
0A2h move/exchange wraps around 1 MB boundry
0A3h data structure contains corrupted data
0A4h access denied
This is a user-supported technical reference. If you find this information
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INDEX
DOS TECHNICAL INFORMATION
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
CHAPTER 1.
SOME HISTORY
THE OPERATING SYSTEM HIERARCHY
DOS STRUCTURE
DOS Initialization
CHAPTER 2.
SYSTEM MEMORY MAP - OVERALL
PC Port Assignment, Intel 8088, 80C88, 8086, 80286, 80386 CPUs
Reserved Memory Locations in the IBM PC
At Absolute Addresses
The IBM PC System Interrupts (Overview)
The IBM-PC System Interrupts (in detail)
Interrupt 00h Divide by Zero (processor error).
Interrupt 01h Single step
Interrupt 02h Non-maskable interrupt
Interrupt 03h Breakpoint
Interrupt 04h Divide overflow
Interrupt 05h Print Screen
Interrupt 06h Reserved by IBM
Interrupt 07h Reserved by IBM
Interrupt 08h Timer
Interrupt 09h Keyboard
Interrupt 0Ah EGA Vertical Retrace
Interrupt 0Bh Communications Controller (serial port) hdw. entry
Interrupt 0Ch Communications Controller (serial port) hdw. entry
Interrupt 0Dh Alternate Printer, PC/AT 80287
Interrupt 0Eh Diskette
Interrupt 0Fh Reserved by IBM
CHAPTER 3. THE PC ROM BIOS
Interrupt 10h Video I/O
Function 00h Determine or Set Video State
01h Set Cursor Type
02h Set Cursor Position
03h Read Cursor Position
04h Read Light Pen
05h Select Active Page
06h Scroll Page Up
07h Scroll Page Down
08h Read Character Attribute
09h Write Character and Attribute
0Ah Write Character
0Bh Set Color Palette
0Ch Write Dot
0Dh Read Dot
0Eh Write TTY
0Fh Return Current Video State
10h Set Palette Registers
11h Character Generator Routine (EGA and after)
12h Alternate Select (EGA and after)
13h Write String
14h Load LCD Character Font
15h Return Physical Display Parameters
1Ah Display Combination Code
1Bh Functionality/State Information
1Ch Save/Restore Video State
70h Get Video RAM Address
71h Get INCRAM Addresses
72h Scroll Screen Right
73h Scroll Screen Left
81h DESQview video - Get something?
82h DESQview - Get Current Window Info
F0h Microsoft Mouse driver EGA support - Read One Register
F1h Microsoft Mouse driver EGA support - Write One Register
F2h Microsoft Mouse driver EGA support - Read Register Range
F3h Microsoft Mouse driver EGA support - Write Register Range
F4h Microsoft Mouse driver EGA support - Read Register Set
F5h Microsoft Mouse driver EGA support - Read Register Set
F6h Microsoft Mouse driver EGA support
F7h Microsoft Mouse driver EGA support
FAh Microsoft Mouse driver EGA support - Interrogate Driver
FEh Get Alternate Screen Buffer Address (text mode only)
FFh Update Real Display (text mode only)
Interrupt 11h Equipment Check
Interrupt 12h Memory Size
Interrupt 13h Disk I/O
Function 00h Reset
01h Get Status of disk system
02h Read Sectors
03h Write Sectors
04h Verify
05h Format Track
06h Hard Disk
07h Hard Disk
08h Read Drive Parameters
09h Initialize Two Fixed Disk Base Tables
0Ah Read Long (Hard disk)
0Bh Write Long
0Ch Seek To Cylinder
0Dh Alternate Disk Reset
0Eh Read Sector Buffer
0Fh Write sector buffer
10h Test For Drive Ready
11h Recalibrate Drive
12h Controller RAM Diagnostic
13h Drive Diagnostic
14h Controller Internal Diagnostic
15h Get Disk Type
16h Change of Disk Status (diskette)
17h Set Disk Type for Format (diskette)
18h Set Media Type For Format (diskette)
19h Park Hard Disk Heads
1Ah ESDI Hard Disk - Format
Interrupt 14h Initialize and Access Serial Port For Int 14
Function 01h Send Character in AL to Comm Port DX (0 or 1)
02h Wait For A Character From Comm Port DX
03h Fetch the Status of Comm Port DX (0 or 1)
04h Extended Initialize
05h Extended Communication Port Control
Interrupt 15h Cassette I/O
Function 00h Turn Cassette Motor On
01h Turn Cassette Motor Off
02h Read Blocks From Cassette
03h Write Data Blocks to Cassette
0Fh ESDI Format Unit Periodic Interrupt
10h TopView API Function Calls
20h PRINT.COM (DOS internal)
21h Power-On Self Test (POST) Error Log
40h Read/Modify Profiles
41h Wait On External Event
42h Request System Power Off
43h Read System Status
44h (De)activate Internal Modem Power
4Fh Keyboard Intercept
80h Device Open
81h Device Close
82h Program Termination
83h Event Wait
84h Read Joystick Input Settings
85h System Request (SysReq) Key Pressed
86h Elapsed Time Wait
88h Extended Memory Size Determine
89h Switch Processor to Protected Mode
91h Set Flag and Complete Interrupt
C0h Get System Configuration
C1h System
C2h Pointing Device BIOS Interface (DesQview 2.x)
C3h Enable/Disable Watchdog Timeout
C4h Programmable Option Select
DEh DesQview Services
Interrupt 16h Keyboard I/O
Function 00h Get Keyboard Input
01h Check Keystroke Buffer
02h Shift Status
03h Keyboard
04h Keyboard Click Toggle
05h Keyboard Buffer Write
10h Get Enhanced Keystroke And Read
11h Check Enhanced Keystroke
12h Extended Get Shift Status
F0h Set CPU speed (Compaq 386)
Interrupt 17h Printer
Function 00h Print Character/send AL to printer DX (0, 1, or 2)
01h Initialize Printer
02h Printer Status
Interrupt 18h ROM BASIC
Interrupt 19h Bootstrap Loader
Interrupt 1Ah Time of Day
Function 00h Read System Time Counter
01h Set Clock
02h Read Real Time Clock Time
03h Set Real Time Clock Time
04h Read Real Time Clock Date
05h Set Real Time Clock Date
06h Set Real Time Clock Alarm
07h Reset Real Time Clock Alarm
08h Set Real Time Clock Activated Power On Mode
09h Read Real Time Clock Alarm Time and Status
0Ah Read System-Timer Day Counter
0Bh Set System-Timer Day Counter
80h Set Up Sound Multiplexor
Interrupt 1Bh Control-Break
Interrupt 1Ch Timer Tick
Interrupt 1Dh Vector of Video Initialization Parameters.
Interrupt 1Eh Vector of Diskette Controller Parameters
Interrupt 1Fh Pointer to Graphics Character Extensions (Graphics Set 2)
Interrupt 20h PROGRAM TERMINATE
Interrupt 20h DOS - Terminate Program
Interrupt 20h Minix - Send/Receive Message
CHAPTER 4. DOS INTERRUPTS AND FUNCTION CALLS
DOS Registers
Interrupts
Interrupt 21h Function Request (Overview)
Calling the DOS Services
Interrupt 21h Function Request (in detail)
Function 00h Program Terminate
01h Keyboard Input
02h Display Output
03h Auxiliary Input
04h Auxiliary Output
05h Printer Output
06h Direct Console I/O
07h Direct Console Input Without Echo
08h Console Input Without Echo
09h Print String
0Ah Buffered Keyboard Input
0Bh Check Standard Input Status
0Ch Clear Keyboard Buffer and Invoke a Kbd Function
0Dh Disk Reset
0Eh Select Disk
0Fh Open File
10h Close File
11h Search for First Entry
12h Search for Next Entry
13h Delete File
14h Sequential Read
15h Sequential Write
16h Create File
17h Rename File
18h Unknown
19h Current Disk
1Ah Set Disk Transfer Address
1Bh Allocation Table Information
1Ch Allocation Table Information for Specific Device
1Dh Unknown
1Eh Unknown
1Fh Read DOS Disk Block (default drive)
20h Unknown
21h Random Read
22h Random Write
23h File Size
24h Set Relative Record Field
25h Set Interrupt Vector
26h Create New Program Segment
27h Random Block Read
28h Random Block Write
29h Parse Filename
2Ah Get Date
2Bh Get Date
2Ch Get Time
2Dh Set Time
2Eh Set/Reset Verify Switch
2Fh Get Disk Transfer Address (DTA)
30h Get DOS Version Number
31h Terminate Process and Stay Resident
32h Read DOS Disk Block
33h Ctrl-Break Check
34h Return INDOS Flag
35h Get Vector
36h Get Disk Free Space
37h Get/Set Switch Character (SWITCHAR)
38h Return Country Dependent Information
39h Create Subdirectory (MKDIR)
3Ah Remove Subdirectory (RMDIR)
3Bh Change Durrent Directory (CHDIR)
3Ch Create a File (CREAT)
3Dh Open a File
3Eh Close a File Handle
3Fh Read From a File or Device
40h Write to a File or Device
41h Delete a File from a Specified Directory (UNLINK)
42h Move File Read/Write Pointer (LSEEK)
43h Change File Mode (CHMOD)
44h I/O Control for Devices (IOCTL)
45h Duplicate a File Handle (DUP)
46h Force a Duplicate of a Handle (FORCDUP)
47h Get Current Directory
48h Allocate Memory
49h Free Allocated Memory
4Ah Modify Allocated Memory Blocks (SETBLOCK)
4Bh Load or Execute a Program (EXEC)
4Ch Terminate a Process (EXIT)
4Dh Get Return Code of a Subprocess (WAIT)
4Eh Find First Matching File (FIND FIRST)
4Fh Find Next Matching File (FIND NEXT)
50h Set PSP
51h Get PSP
52h IN-VARS
53h Translate BPB
54h Get Verify Setting
55h Create Child PSP
56h Rename a File
57h Get or Set Timestamp of a File
58h Get/Set Allocation Strategy (DOS 3.x)
59h Get Extended Error Code
5Ah Create Unique Filename
5Bh Create a New File
5Ch Lock/Unlock File Access
5Dh Network - Partial
5Eh Network Printer
5Fh Network Redirection
60h Parse Pathname
61h Unknown
62h Get Program Segment Prefix (PSP) Address
63h Get Lead Byte Table (DOS 2.25)
64h Unknown
65h Get Extended Country Information (DOS 3.3)
66h Get/Set Global Code Page Table (DOS 3.3)
67h Set Handle Count (DOS 3.3)
68h Commit File (DOS 3.3)
69h Disk Serial Number DOS 4.0 (US)
6Ah unknown (DOS 4.0?)
6Bh unknown (DOS 4.0?)
6Ch Extended Open/Create DOS 4.0 (US)
89h DOS_Sleep
Aftermarket Application Installed Function Calls, Used by NetWare
Function B6h-FFh Novell NetWare
CHAPTER 5. Interrupts 22h Through 86h
Interrupt 22h Terminate Address
Interrupt 23h Ctrl-Break Exit Address
Interrupt 24h Critical Error Handler
Interrupt 25h Absolute Disk Read
Interrupt 26h Absolute Disk Write
Interrupt 27h Terminate And Stay Resident
Interrupt 28h (not documented by Microsoft)
Interrupt 29h (not documented by Microsoft)
Interrupt 2Ah Microsoft Networks - Session Layer Interrupt
Interrupt 2Bh (not documented by Microsoft)
Interrupt 2Ch (not documented by Microsoft)
Interrupt 2Dh (not documented by Microsoft)
Interrupt 2Eh (undocumented by Microsoft)
Interrupt 2Fh Multiplex Interrupt
Interrupt 30h (not a vector!) far jump instruction for CP/M-style calls
Interrupt 31h Unknown
Interrupt 32h Unknown
Interrupt 33h Used by Microsoft Mouse Driver
Interrupt 34h Turbo C/Microsoft languages - Floating Point emulation
Interrupt 35h Turbo C/Microsoft languages - Floating Point emulation
Interrupt 36h Turbo C/Microsoft languages - Floating Point emulation
Interrupt 37h Turbo C/Microsoft languages - Floating Point emulation
Interrupt 38h Turbo C/Microsoft languages - Floating Point emulation
Interrupt 39h Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Ah Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Bh Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Ch Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Dh Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Eh Turbo C/Microsoft languages - Floating Point emulation
Interrupt 3Fh Overlay manager interrupt (Microsoft LINK.EXE)
Interrupt 40h Hard Disk BIOS
Interrupt 41h Hard Disk Parameters
Interrupt 42h Pointer to screen BIOS entry
Interrupt 43h Pointer to EGA initialization parameter table
Interrupt 44h Pointer to EGA graphics character table
Interrupt 45h Reserved by IBM (not initialized)
Interrupt 46h Pointer to second hard disk, parameter block
Interrupt 47h Reserved by IBM (not initialized)
Interrupt 48h Cordless Keyboard Translation
Interrupt 49h Non-keyboard Scan Code Translation Table Address
Interrupt 4Ah Real-Time Clock Alarm
Interrupt 4Bh Reserved by IBM (not initialized)
Interrupt 4Ch Reserved by IBM (not initialized)
Interrupt 4Dh Reserved by IBM (not initialized)
Interrupt 4Eh Reserved by IBM (not initialized)
Interrupt 4Fh Reserved by IBM (not initialized)
Interrupt 50-57 IRQ0-IRQ7 relocated by DesQview
Interrupt 58h Reserved by IBM (not initialized)
Interrupt 59h Reserved by IBM (not initialized)
Interrupt 5Ah Reserved by IBM (not initialized)
Interrupt 5Bh Reserved by IBM (not initialized)
Interrupt 5Ah Cluster Adapter BIOS entry address
Interrupt 5Bh Reserved by IBM (not initialized) (cluster adapter?)
Interrupt 5Ch NETBIOS interface entry port
Interrupt 5Dh Reserved by IBM (not initialized)
Interrupt 5Eh Reserved by IBM (not initialized)
Interrupt 5Fh Reserved by IBM (not initialized)
Interrupt 60h-67h User Program Interrupts (availible for general use)
Interrupt 67h Used by Lotus-Intel-Microsoft Expanded Memory Specification
Interrupt 68h Not Used (not initialized)
Interrupt 69h Not Used (not initialized)
Interrupt 6Ah Not Used (not initialized)
Interrupt 6Bh Not Used (not initialized)
Interrupt 6Ch System Resume Vector (Convertible) (not initialized on PC)
Interrupt 6Dh Not Used (not initialized)
Interrupt 6Fh Not Used (not initialized)
Interrupt 70h IRQ 8, Real Time Clock Interrupt
Interrupt 71h IRQ 9, Redirected to IRQ 8
Interrupt 72h IRQ 10 (AT, XT/286, PS/2) Reserved
Interrupt 73h IRQ 11 (AT, XT/286, PS/2) Reserved
Interrupt 74h IRQ 12 Mouse Interrupt (AT, XT/286, PS/2)
Interrupt 75h IRQ 13, Coprocessor Error, BIOS Redirect to int 2 (NMI) (AT)
Interrupt 76h IRQ 14, Hard Disk Controller (AT, XT/286, PS/2)
Interrupt 77h IRQ 15 (AT, XT/286, PS/2) Reserved
Interrupt 78h Not Used
Interrupt 79h Not Used
Interrupt 7Ah Novell NetWare - LOW-LEVEL API
Interrupt 7Bh-7Fh Not Used
Interrupt 80h-85h Reserved by BASIC
Interrupt 86h Relocated by NETBIOS int 18
Interrupt 86h-F0h Used by BASIC when BASIC interpreter is running
Interrupt E4h Logitech Modula-2 v2.0 MONITOR
Interrupt F1h-FFh (absolute addresses 3C4-3FF)
Interrupt F8h Set Shell Interrupt (OEM)
Interrupt F9h First of 8 SHELL service codes, reserved for OEM shell (WINDOW);
Interrupt FAh USART ready (RS-232C)
Interrupt FBh USART RS ready (keyboard)
Interrupt FCh Unknown
Interrupt FDh reserved for user interrupt
Interrupt FEh AT/XT286/PS50+ - destroyed by return from protected mode
Interrupt FFh AT/XT286/PS50+ - destroyed by return from protected mode
CHAPTER 6. DOS CONTROL BLOCKS AND WORK AREAS
The Disk Transfer Area (DTA)
DOS Program Segment
STANDRD FILE CONTROL BLOCK
EXTENDED FILE CONTROL BLOCK
MEMORY CONTROL BLOCKS
CONTROL BLOCK
MEMORY CONTROL BLOCKS
CHAPTER 7. DOS File Structure
File Management Functions
FCB FUNCTION CALLS
HANDLE FUNCTION CALLS
SPECIAL FILE HANDLES
ASCII and BINARY MODE
FILE I/O IN BINARY (RAW) MODE
FILE I/O IN ASCII (COOKED) MODE
NUMBER OF OPEN FILES ALLOWED
RESTRICTIONS ON FCB USAGE
RESTRICTIONS ON HANDLE USAGE
ALLOCATING SPACE TO A FILE
MSDOS / PCDOS DIFFERENCES
.EXE FILE STRUCTURE
THE RELOCATION TABLE
"NEW" .EXE FORMAT (Microsoft Windows and OS/2)
CHAPTER 8. DOS DISK INFORMATION
THE DOS AREA
THE BOOT RECORD
THE DOS FILE ALLOCATION TABLE (FAT)
USE OF THE 12 BIT FILE ALLOCATION TABLE
USE OF THE 16 BIT FILE ALLOCATION TABLE
DOS DISK DIRECTORY
DIRECTORY ENTRIES
THE DATA AREA
HArd DISK LAYOUT
SYSTEM INITIALIZATION
THE BOOT SEQUENCE
BOOT RECORD/PARTITION TABLE
HARD DISK TECHNICAL INFORMATION
DETERMINING FIXED DISK ALLOCATION
CHAPTER 9. INSTALLABLE DEVICE DRIVERS
DEVICE DRIVER FORMAT
TYPES OF DEVICES
DEVICE HEADER
POINTER TO NEXT DEVICE HEADER FIELD
ATTRIBUTE FIELD
POINTER TO STRATEGY AND INTERRUPT ROUTINES
NAME/UNIT FIELD
CREATING A DEVICE DRIVER
INSTALLING DEVICE DRIVERS
INSTALLING CHARACTER DEVICES
INSTALLING BLOCK DEVICES
REQUEST HEADER
UNIT CODE FIELD
COMMAND CODE FIELD
STATUS FIELD
DEVICE DRIVER FUNCTIONS
INIT
MEDIA CHECK
MEDIA DESCRIPTOR
BUILD BPB (BIOS Parameter Block)
INPUT / OUTPUT
NONDESTRUCTIVE INPUT NO WAIT
STATUS
FLUSH
OPEN or CLOSE (3.x)
REMOVABLE MEDIA (DOS 3.x)
THE CLOCK$ DEVICE
CHAPTER 10. LOTUS-INTEL-MICROSOFT EXPANDED MEMORY SPECIFICATION
The Expanded Memory Manager
History
Page Frames
Expanded Memory Services
AST/Quadram/Ashton-Tate Enhanced EMM
Calling the Manager
Common EMS s (hex calls)
1 (40h) Get Manager Status
2 (41h) Get Page Frame Segment
3 (42h) Get Number of Pages
4 (43h) Get Handle and Allocate Memory
5 (44h) Map Memory
6 (45h) Release Handle and Memory
7 (46h) Get EMM Version
8 (47h) Save Mapping Context
9 (48h) Restore Mapping Context
10 (49h) Reserved
11 (4Ah) Reserved
12 (4Bh) Get Number of EMM Handles
13 (4Ch) Get Pages Owned By Handle
14 (4Dh) Get Pages for All Handles
15 (4Eh) Get Or Set Page Map
new LIM 4.0 specification:
16 (4Fh) Get/Set Partial Page Map
17 (50h) Map/Unmap Multiple Pages
18 (51h) Reallocate Pages
19 (52h) Handle Attribute Functions
20 (53h) Get Handle Name
21 (54h) Get Handle Directory
22 (55h) Alter Page Map & Jump
23 (56h) Alter Page Map & Call
24 (57h) Move Memory Region
25 (58h) Get Mappable Physical Address Array
26 (59h) Get Expanded Memory Hardware
27 (5Ah) Allocate Raw Pages
28 (5Bh) Get Alternate Map Register Set
29 (5Ch) Prepare Expanded Memory Hardware
30 (5Dh) Enable OS/E Function Set
31 (5Eh) Unknown
32 (5Fh) Unknown
33 (60h) Unknown
34 (61h) AST Generic Accelerator Card Support
(68h) EEMS - Get Addresses of All Page Frames in System
(69h) EEMS - Map Page Into Frame
(6Ah) EEMS - Page Mapping
Expanded Memory Manager Error Codes
LIM 4.0 extended error codes
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
These scan codes are generated by pressing a key on the PC's keyboard. This
is the 'make' code. A 'break' code is generated when the key is released. The
break scancode is 128 higher than the make code, and is generated by setting
bit 7 of the scan code byte to 1.
IBM PC KEYBOARD EXTENDED CODES
Normal Shift Control Alt
ESC 1
1 2 0;120
2 3 0;121
3 4 0;122
4 5 0;123
5 6 0;124
6 7 0;125
7 8 0;126
8 9 0;127
9 10 0;128
0 11 0;129
- 12 0;130
= 13 0;131
TAB 15 0;15(backtab)
backtab none 0;15
RETURN 28
Normal Shift Control Alt NumLock
Home 0;71 0;119 none 7
UpArrow 0;72 none none 8
PgUp 0;73 0;132 none 9
gray - 0;74 0;74
LArrow 0;75 0;115 none 4
keypad 5 none none none 5
RArrow 0;77 0;116 6
gray + 0;78 0;78
End 0;79 0;117 none 1
DnArrow 0;80 2
PgDn 0;81 0;118 none 3
Ins 0;82 none 11
Del 0;83 0;128 none 52
PrtSc 55 0;114
L shift 42
R shift 54
alt key 56
capslock 58
spacebar 57
control key 29
numlock 69
scrollock 70
; 39
[ 26
] 27
" 40
\ 43
/ 53
, 51
. 52
IBM PC KEYBOARD EXTENDED CODES
Normal Shift Control Alt
a = 30 0;30
b = 48 0;48
c = 46 0;46
d = 32 0;32
e = 18 0;18
f = 33 0;33
g = 34 0;34
h = 35 0;35
i = 23 0;23
j = 36 0;36
k = 37 0;37
l = 38 0;38
m = 50 0;50
n = 49 0;49
o = 24 0;24
p = 25 0;25
q = 16 0;16
r = 19 0;19
s = 31 0;31
t = 20 0;20
u = 22 0;22
v = 47 0;47
w = 17 0;17
x = 45 0;45
y = 21 0;21
z = 44 0;44
Normal Shift Control Alt
F1 = 0;59 0;84 0;94 0;104
F2 = 0;60 0;85 0;95 0;105
F3 = 0;61 0;86 0;96 0;106
F4 = 0;62 0;87 0;97 0;107
F5 = 0;63 0;88 0;98 0;108
F6 = 0;64 0;89 0;99 0;109
F7 = 0;65 0;90 0;100 0;110
F8 = 0;66 0;91 0;101 0;111
F9 = 0;67 0;92 0;102 0;112
F10 = 0;68 0;93 0;103 0;113
"Enhanced" 101/102 key keyboard scancodes
Normal Shift Control Alt
F11 = 0;152 0;162 0;172 0;182 |
F12 = 0;153 0;163 0;173 0;183 | Tandy?
F11 = 0;133 0;135 0;137 0;139
F12 = 0;134 0;136 0;138 0;140
alt-home 0;151
UpArr 0;141 0;152
Ctrl - 0;142
Ctrl 5 0;143
Ctrl + 0;144
DnArr 0;145 0;160
Ins 0;146 0;162
Del 0;147 0;163
Tab 0;148 0;165
/ 0;149 0;164
Ctrl-* 0;150
alt-Enter 0;166
alt-PgUp 0;153
alt-LArr 0;154
alt-RArr 0;155
alt-End 0;156
alt-PgDn 0;161
BIOS keystroke codes, hexadecimal
Key Normal Shift Control Alt
Esc 011B 011B 011B --
1! 0231 '1' 0221 '!' -- 7800
2@ 0332 '2' 0340 '@' 0300 7900
3# 0433 '3' 0423 '#' -- 7A00
4$ 0534 '4' 0524 '