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Subject: comp.os.msdos.programmer FAQ part 3 of 4
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Archive-name: msdos-programmer-faq/part3
Last-modified: 24 Sep 1993
(continued from part 2) (no warranty on the code or information)
If the posting date is more than six weeks in the past, see instructions
in part 4 of this list for how to get an updated copy.
Copyright (C) 1993 Stan Brown, Oak Road Systems. All rights reserved.
section 4. Disks and files
===========================
Subject: 401. What drive was the PC booted from?
Under DOS 4.0 or later, load 3305 hex into AX; do an INT 21. DL is
returned with an integer indicating the boot drive (1=A:, etc.).
Subject: 402. How can I boot from drive b:?
(rev: 9 Aug 1993) Downloadable shareware:
pd1:<msdos.dskutl>boot_b.zip from Simtel
/pc/bootutil/boot_b.zip from Garbo.
The included documentation says it works by writing a new boot
sector on a disk in your a: drive that redirects the boot to your
b: drive. (A similar utility is bboot.zip in the same directory at
Garbo only.)
If that doesn't work, you can always interchange your a: and b:
drives by switching ribbon cables and changing the setup in your
BIOS. From an article posted 27 Jan 1993 on another newsgroup:
Take the "ribbon" connector, as you call it, and switch them. To
double check, start at the end of the cable that connects to the
motherboard or floppy controller. Follow the cable until you get to
the first connector. Connect this to the drive you want to be b:.
After this, there should be a few lines on the cable that get
flipped left to right. (On most cables, they just cut the lines and
physically reverse them. It should be quite obvious from looking at
the cable.) Anyway, the connector after the pins get flipped
right to left is the connector for your a: drive.
Subject: 403. Which real and virtual disk drives are valid?
(rev: 15 Aug 1993) Use INT 21 function 29 (parse filename). Point
DS:SI at a null-terminated ASCII string that contains the drive
letter and a colon, point ES:DI at a 37-byte dummy FCB buffer, set
AX to 2900h, and do an INT 21. On return, AL is FF if the drive is
invalid, something else if the drive is valid. RAM disks and
SUBSTed drives are considered valid.
You can detect whether the drive is ASSIGNed by using INT 2F
AX=0601. To check whether the drive is SUBSTed, use INT 21 AX=4409;
or use INT 21 function 52 to test for both JOIN and SUBST. See Ralf
Brown's interrupt list.
Unfortunately, the b: drive is considered valid even on a single-
diskette system. You can check that special case by interrogating
the BIOS equipment byte at 0040:0010. Bits 7-6 contain the one less
than the number of diskette drives, so if those bits are zero you
know that b: is an invalid drive even though function 29 says it's
valid.
Following is some code originally posted by Doug Dougherty to test
valid drives (without regard to SUBST and JOIN), with SB's fix for
the b: special case, tested in Borland C++ 2.0 (in the small model):
#include <dos.h>
void drvlist(void) {
char *s = "A:", fcb_buff[37];
int valid;
for ( ; *s<='Z'; (*s)++) {
_SI = (unsigned) s;
_DI = (unsigned) fcb_buff;
_ES = _DS;
_AX = 0x2900;
geninterrupt(0x21);
valid = _AL != 0xFF;
if (*s == 'B' && valid) {
char far *equipbyte = (char far *)0x00400010UL;
valid = (*equipbyte & (3 << 6)) != 0;
}
printf("Drive '%s' is %sa valid drive.\n",
s, valid ? "" : "not ");
}
}
SB translated this to MSC 7.0 and tested it in small model:
#include <dos.h>
#include <stdio.h>
void drvlist(void) {
char *s = "A:", fcb_buff[37], *buff=fcb_buff;
int valid;
for ( ; *s<='Z'; (*s)++) {
__asm mov si,s __asm mov di,buff
__asm mov ax,ds __asm mov es,ax
__asm mov ax,0x2900 __asm int 21h
__asm xor ah,ah __asm mov valid,ax
valid = (valid != 0xFF);
if (*s == 'B' && valid) {
char far *equipbyte = (char far *)0x00400010UL;
valid = (*equipbyte & (3 << 6)) != 0;
}
printf("Drive '%s' is %sa valid drive.\n",
s, valid ? "" : "not ");
}
}
Subject: 404. How can I make my single floppy drive both a: and b:?
Under any DOS since DOS 2.0, you can put the command
assign b=a
into your AUTOEXEC.BAT file. Then, when you type "DIR B:" you'll no
longer get the annoying prompt to insert diskette B (and the even
more annoying prompt to insert A the next time you type "DIR A:").
You may be wondering why anybody would want to do this. Suppose you
use two different machines, maybe one at home and one at work. One
of them has only a 3.5" diskette drive; the other machine has two
drives, and b: is the 3.5" one. You're bound to type "dir b:" on
the first one, and get the nuisance message
Insert diskette for drive B: and press any key when ready.
But if you assign drive b: to point to a:, you avoid this problem.
Caution: there are a few commands, such as DISKCOPY, that will not
work right on ASSIGNed or SUBSTed drives. See the DOS manual for
the full list. Before typing one of those commands, be sure to turn
off the mapping by typing "assign" without arguments.
The DOS 5.0 manual says that ASSIGN is obsolete, and recommends the
equivalent form of SUBST: "subst b: a:\". Unfortunately, if this
command is executed when a: doesn't hold a diskette, the command
fails. ASSIGN doesn't have this problem, so under DOS 5.0 you
should disregard that particular bit of advice in the manual.
Subject: 405. How can I disable access to a drive?
(new: 15 Aug 1993) Reader Eric DeVolder writes that he has made
available a program to do this. It's downloadable from Simtel as
pd1:<msdos.dskutl>rmdriv20.zip from Simtel
/pc/sysutil/rmdriv20.zip at Garbo.
(existence verified; files not tested by SB)
Subject: 406. How can a batch file test existence of a directory?
(new: 28 Aug 1993) The standard way, which in fact is documented in
the DOS manual, is
if exist d:\path\nul goto found
Unfortunately, this is not entirely reliable. SB found it failed in
Pathworks (a/k/a PCSA, DEC's network that connects PCs and VAXes),
or on a MARS box that uses an OEM version of MS-DOS 5.0. Readers
have reported that it failed on Novell networks or on DR-DOS.
There appears to be no foolproof way to use pure batch commands to
test for existence of a directory. The real solution is to write a
program, which returns a value that your batch program can then test
with an if errorlevel. Reader Duncan Murdoch kindly posted the
following Turbo Pascal version:
program existdir;
{ Confirms the existence of a directory given on the command line.
Returns errorlevel 2 on error, 1 if not found, 0 if found. }
uses
dos;
var
s : searchrec;
begin
if paramcount <> 1 then
begin
writeln('Syntax: EXISTDIR directory');
halt(2);
end
else
begin
findfirst(paramstr(1),Directory,S);
while (Doserror = 0) and ((Directory and S.Attr) = 0) do
findnext(S);
if Doserror <> 0 then
begin
Writeln('Directory not found.');
halt(1);
end
else
begin
Writeln('Directory found.');
halt(0);
end;
end;
end.
Timo Salmi also has a Turbo Pascal version in his Turbo Pascal FAQ,
which is downloadable as
/pc/ts/tsfaqp15.zip at Garbo
pd1:<msdos.info>tsfaqp15.zip at Simtel.
Subject: 407. Why won't my C program open a file with a path?
You've probably got something like the following code:
char *filename = "c:\foo\bar\mumble.dat";
. . . fopen(filename, "r");
The problem is that \f is a form feed, \b is a backspace, and \m is
m. Whenever you want a backslash in a string constant in C, you
must use two backslashes:
char *filename = "c:\\foo\\bar\\mumble.dat";
This is a feature of every C compiler, because Dennis Ritchie
designed C this way. It's a problem only on MS-DOS systems, because
only DOS (and Atari ST/TT running TOS) uses the backslash in
directory paths. But even in DOS this backslash convention applies
_only_ to string constants in your source code. For file and
keyboard input at run time, \ is just a normal character, so users
of your program would type in file specs at run time the same way as
in DOS commands, with single backslashes.
Another possibility is to code all paths in source programs with /
rather than \ characters:
char *filename = "c:/foo/bar/mumble.dat";
Ralf Brown writes that "All versions of the DOS kernel accept either
forward or backslashes as directory separators. I tend to use this
form more frequently than backslashes since it is easier to type and
read." This applies to DOS function calls (and therefore to calls
to the file library of every programming language), but not to DOS
commands.
Subject: 408. How can I redirect printer output to a file?
(rev: 16 Aug 1993) Recommended: PRN2FILE from {PC Magazine},
downloadable as:
pd1:<msdos.printer>prn2file.zip at Simtel
/pc/printer/prn2file.zip at Garbo.
{PC Magazine} has given copies away as part of its utilities disks,
so you may already have a copy.
The directories mentioned above have lots of other utilities to
redirect printer output.
Subject: 409. How can I redirect the output of a batch file?
(new: 12 June 1993) Assuming the batch file is called batch.bat, to
send its output (stdout) to another file, just invoke COMMAND.COM as
a secondary command processor:
command /c batch parameters_if_any >outfile
Timo Salmi's notes on this and other batch tricks are downloadable:
pd1:<msdos.batutl>tsbat43.zip at Simtel
/pc/ts/tsbat43.zip at Garbo.
Subject: 410. How can I redirect stderr?
(new: 15 Aug 1993) Use freopen(..., stderr) and then execute the
desired command via system( ). There are downloadable versions of
programs to do this. Recommended by SB:
pd1:<msdos.sysutl>rdstderr.zip from Simtel.
Source code (in Turbo Pascal 4.0) and executable are included.
A C example is downloadable as
pd1:<msdos.c>redirect.c from Simtel.
SB compiled it with MSC 7.0, and it works fine with one exception:
Contrary to the included comments, redirected output starts writing
at the beginning of the output file rather than appending. That is
easily solved by adding "fseek(stderr, 0L, SEEK_END);" after the
freopen( ) call for stderr.
Subject: 411. How can my program open more files than DOS's limit of 20?
(rev: 12 Sep 1993) This is a summary of an article Ralf Brown posted
on 8 August 1992, with some additions from a Microsoft tech note.)
DOS imposes some limits. Once you overcome those, which is pretty
easy, you may have to take additional measures to overcome the
limitations built into your compiler's run-time library.
1) Limitations imposed by DOS
There are separate limits on files and file handles. For example,
DOS opens three files but five file handles: CON (stdin, stdout,
and stderr), AUX (stdaux), and PRN (stdprn).
The limit in FILES= in CONFIG.SYS is a system-wide limit on files
opened by all programs (including the three that DOS opens and any
opened by TSRs); each process has a limit of 20 handles (including
the five that DOS opens). Example: CONFIG.SYS has FILES=40. Then
program #1 will be able to open 15 file handles. Assuming that the
program actually does open 15 handles pointing to 15 different
files, other programs could still open a total of 22 files (40-3-15
= 22), though no one program could open more than 15 file handles.
If you're running DOS 3.3 or later, you can increase the per-process
limit of 20 file handles by a call to INT 21 function 67, Set Handle
Count. Your program is still limited by the system-wide limit on
open files, so you may also need to increase the FILES= value in
your CONFIG.SYS file (and reboot). The run-time library that you're
using may have a fixed-size table of file handles, so you may also
need to get source code for the module that contains the table,
increase the table size, and recompile it.
2) Limitations in Microsoft C run-time library
In Microsoft C the run-time library limits you to 20 file handles.
To change this, you must be aware of two limits:
- file handles used with _open( ), _read( ), etc.: Edit _NFILE_ in
CRT0DAT.ASM.
- stream files used with fopen( ), fread( ), etc.: Edit _NFILE_ in
_FILE.C for DOS or FILE.ASM for Windows/QuickWin. This must not
exceed the value of _NFILE_ in CRT0DAT.ASM.
(QuickWin uses the constant _WFILE_ in CRT0DAT.ASM and WFILE.ASM for
the maximum number of child text windows.)
After changing the limits, recompile using CSTARTUP.BAT. Microsoft
recommends that you first read README.TXT in the same directory.
3) Limitations in Borland C++ run-time library
(Reader Chin Huang provided this information on 12 Sep 1993.)
To increase the open file limit for a program you compile with
Borland C++ 3.1, edit the file _NFILE.H in the include directory and
change the _NFILE_ value. Compile and link the modules FILES.C and
FILES2.C from the lib directory into your program.
Subject: 412. How can I read, create, change, or delete the volume
label?
In DOS 5.0 (and possibly in 4.0 as well), there are actually two
volume labels: one, the traditional one, is an entry in the root
directory of the disk; and the other is in the boot record along
with the serial number (see next Q). The DIR and VOL commands
report the traditional label; the LABEL command reports the
traditional one but changes both of them.
In DOS 4.0 and later, use INT 21 function 69 to access the boot
record's serial number and volume label together; see the next Q.
Assume that by "volume label" you mean the traditional one, the one
that DIR and VOL display. Though it's a directory entry in the root
directory, you can't change it using the newer DOS file-access
functions (3C, 41, 43); instead, use the old FCB-oriented directory
functions. Specifically, you need to allocate a 64-byte buffer and
a 41- byte extended FCB (file control block). Call INT 21 AH=1A to
find out whether there is a volume label. If there is, AL returns 0
and you can change the label using DOS function 17 or delete it
using DOS function 13. If there's no volume label, function 1A will
return FF and you can create a label via function 16. Important
points to notice are that ? wildcards are allowed but * are not; the
volume label must be space filled not null terminated.
The following MSC 7.0 code worked for SB in DOS 5.0; the functions
it uses have been around since DOS 2.0. The function parameter is 0
for the current disk, 1 for a:, 2 for b:, etc. It doesn't matter
what your current directory is; these functions always search the
root directory for volume labels. (I didn't try to change the
volume label of any networked drives.)
// Requires DOS.H, STDIO.H, STRING.H
void vollabel(unsigned char drivenum) {
static unsigned char extfcb[41], dta[64], status, *newlabel;
int chars_got = 0;
#define DOS(buff,func) __asm { __asm mov dx,offset buff \
__asm mov ax,seg buff __asm push ds __asm mov ds,ax \
__asm mov ah,func __asm int 21h __asm pop ds \
__asm mov status,al }
#define getlabel(buff,prompt) newlabel = buff; \
memset(newlabel,' ',11); printf(prompt); \
scanf("%11[^\n]%n", newlabel, &chars_got); \
if (chars_got < 11) newlabel[chars_got] = ' ';
// Set up the 64-byte transfer area used by function 1A.
DOS(dta, 1Ah)
// Set up an extended FCB and search for the volume label.
memset(extfcb, 0, sizeof extfcb);
extfcb[0] = 0xFF; // denotes extended FCB
extfcb[6] = 8; // volume-label attribute bit
extfcb[7] = drivenum; // 1=A, 2=B, etc.; 0=current drive
memset(&extfcb[8], '?', 11); // wildcard *.*
DOS(extfcb,11h)
if (status == 0) { // DTA contains volume label's FCB
printf("volume label is %11.11s\n", &dta[8]);
getlabel(&dta[0x18], "new label (\"delete\" to delete): ");
if (chars_got == 0)
printf("label not changed\n");
else if (strncmp(newlabel,"delete ",11) == 0) {
DOS(dta,13h)
printf(status ? "label failed\n" : "label deleted\n");
}
else { // user wants to change label
DOS(dta,17h)
printf(status ? "label failed\n" : "label changed\n");
}
}
else { // no volume label was found
printf("disk has no volume label.\n");
getlabel(&extfcb[8], "new label (<Enter> for none): ");
if (chars_got > 0) {
DOS(extfcb,16h)
printf(status ? "label failed\n" : "label created\n");
}
}
} // end function vollabel
Subject: 413. How can I get the disk serial number?
Use INT 21. AX=6900 gets the serial number; AX=6901 sets it. See
Ralf Brown's interrupt list, or page 496 of {PC Magazine} July 1992,
for details.
This function also gets and sets the volume label, but it's the
volume label in the boot record, not the volume label that a DIR
command displays. See the preceding Q.
Subject: 414. What's the format of .OBJ, .EXE., .COM files?
Please see section 2, "Compile and link".
Subject: 415. How can I flush the software disk cache?
Please see "How can a program reboot my PC?" in section 7, "Other
software questions and problems".
section 5. Serial ports (COM ports)
===================================
Subject: 501. How do I set my machine up to use COM3 and COM4?
Unless your machine is fairly old, it's probably already set up.
After installing the board that contains the extra COM port(s),
check the I/O addresses in word 0040:0004 or 0040:0006. (In DEBUG,
type "D 40:4 L4" and remember that every word is displayed low
byte first, so if you see "03 56" the word is 5603.) If those
addresses are nonzero, your PC is ready to use the ports and you
don't need the rest of this answer.
If the I/O address words in the 0040 segment are zero after you've
installed the I/O board, you need some code to store these values
into the BIOS data segment:
0040:0004 word I/O address of COM3
0040:0006 word I/O address of COM4
0040:0011 byte (bits 3-1): number of serial ports installed
The documentation with your I/O board should tell you the port
addresses. When you know the proper port addresses, you can add
code to your program to store them and the number of serial ports
into the BIOS data area before you open communications. Or you can
use DEBUG to create a little program to include in your AUTOEXEC.BAT
file, using this script:
n SET_ADDR.COM <--- or a different name ending in .COM
a 100
mov AX,0040
mov DS,AX
mov wo [0004],aaaa <--- replace aaaa with COM3 address or 0
mov wo [0006],ffff <--- replace ffff with COM4 address or 0
and by [0011],f1
or by [0011],8 <--- use number of serial ports times 2
mov AH,0
int 21
<--- this line must be blank
rCX
1f
rBX
0
w
q
Subject: 502. How do I find the I/O address of a COM port?
(rev: 15 Aug 1993) Look in the four words beginning at 0040:0000
for COM1 through COM4. (The DEBUG command "D 40:0 L8" will do this.
Remember that words are stored and displayed low byte first, so a
word value of 03F8 will be displayed as F8 03.) If the value is
zero, that COM port is not installed (or you've got an old BIOS; see
the preceding Q). If the value is nonzero, it is the I/O address of
the transmit/receive register for the COM port. Each COM port
occupies eight consecutive I/O addresses (though only the first
seven are used by many chips).
Here's some C code to find the I/O address:
unsigned ptSel(unsigned comport) {
unsigned io_addr;
if (comport >= 1 && comport <= 4) {
unsigned far *com_addr = (unsigned far *)0x00400000UL;
io_addr = com_addr[comport-1];
}
else
io_addr = 0;
return io_addr;
}
You might also want to explore Port Finder, downloadable as
pd1:<msdos.sysutl>pf271.zip at Simtel
/pub/msdos/utilities/sysutl/pf271.zip at nic.funet.fi
I (SB) haven't tried it myself, but a posted article reviewed it
very favorably and said it also lets you swap ports around.
Subject: 503. But aren't the COM ports always at I/O addresses 3F8,
2F8, 3E8, and 2E8?
The first two are usually right (though not always); the last two
are different on many machines.
Subject: 504. How do I configure a COM port and use it to transmit data?
(rev: 17 Sep 1993) Do you want actual code, or do you want books
that explain what's going on?
1) Source code
First, check your compiler's run-time library. Many compilers offer
functions similar to Microsoft C's _bios_serialcom() or Borland's
bioscom(), which may meet your needs.
Second, check for downloadable resources at Simtel and Garbo. At
Simtel, pd1:<msdos.c>pcl4c34.zip (March 1993) is described as
"Asynchronous communications library for C"; Garbo has a whole
/pc/comm directory. Also, an extended example is in Borland's
TechFax TI445, downloadable as part of
pd1:<msdos.turbo-c>bchelp10.zip at Simtel
/pc/turbopas/bchelp10.zip at Garbo.
Though written by Borland, much of it is applicable to other forms
of C, and it should give you ideas for other programming languages.
2) Reference books
Highly recommended: Joe Campbell's {C Programmer's Guide to Serial
Communications}, ISBN 0-672-22584-0. He gives complete details on
how serial ports work, along with complete programs for doing polled
or interrupt-driver I/O. The book is quite thick, and none of it
looks like filler.
If Campbell's book is overkill for you, you'll find a good short
description of serial I/O in {DOS 5: A Developer's Guide}, ISBN
1-55851-177-6, by Al Williams.
Finally, a reader has recommended {Serial Communications Programming
in C/C++} by Mark Goodwin (ISBN 1558281983), with source code in the
book and on disk. Topics include the basics, various methods of
serial communications on the PC (with consideration of high-speed
modems), ANSI screen interface, file transfer protocols (Xmodem and
Ymodem), etc. There is code in C, and that code is extended into a
C++ class for those who use C++. There are also subroutines in
Assembly.
3) Downloadable information files
A "Serial Port FAQ" is occasionally posted to this newsgroup. You
can get a copy by ftp from pfsparc02.phil15.uni-sb.de. Look for
file names *Serial* in directory /pub/E-Technik/afd . (The archive
administrator warns that the ftp address may change, sometime in
the future, to etcip1.ee.uni-sb.de .) North American users should
access rtfm.mit.edu, directory /pub/usenet/comp.os.msdos.programmer,
file names T_S_P*_3.
section 6. Other hardware questions and problems
================================================
Subject: 601. Which 80x86 CPU is running my program?
(rev: 16 Aug 1993) According to an article posted by Michael
Davidson, Intel's approved code for distinguishing among 8086,
80286, 80386, and 80486 and for detecting the presence of an 80287
or 80387 is published in Intel's 486SX processor manual (order
number 240950-001). David Kirschbaum's improved version of this is
downloadable as
pd1:<msdos.sysutl>cpuid593.zip from Simtel
/pc/sysinfo/cpuid593.zip from Garbo.
According to an article posted by its author, WCPU knows the
differences between DX and SX varieties of 386 and 486 chips, and
can detect a math coprocessor and a Pentium. It's downloadable as
pd1:<msdos.sysinfo>wcpu050.zip at Simtel
/pc/sysinfo/wcpu050.zip at Garbo.
Subject: 602. How can a C program send control codes to my printer?
If you just fprintf(stdprn, ...), C will translate some of your
control codes. The way around this is to reopen the printer in
binary mode:
prn = fopen("PRN", "wb");
You must use a different file handle because stdprn isn't an lvalue.
By the way, PRN or LPT1 must not be followed by a colon in DOS 5.0.
There's one special case, Ctrl-Z (ASCII 26), the DOS end-of-file
character. If you try to send an ASCII 26 to your printer, DOS
simply ignores it. To get around this, you need to reset the
printer from "cooked" to "raw" mode. Microsoft C users must use int
21 function 44, "get/set device information". Turbo C and Borland
C++ users can use ioctl to accomplish the same thing:
ioctl(fileno(prn), 1, ioctl(fileno(prn),0) & 0xFF | 0x20, 0);
An alternative approach is simply to write the printer output into a
disk file, then copy the file to the printer with the /B switch.
A third approach is to bypass DOS functions entirely and use the
BIOS printer functions at INT 17. If you also fprintf(stdprn,...)
in the same program, you'll need to use fflush( ) to synchronize
fprintf( )'s buffered output with the BIOS's unbuffered.
By the way, if you've opened the printer in binary mode from a C
program, remember that outgoing \n won't be translated to carriage
return/line feed. Depending on your printer, you may need to send
explicit \n\r sequences.
Subject: 603. How can I redirect printer output to a file?
Please see section 4, "Disks and files", for the answer.
Subject: 604. Which video adapter is installed?
The technique below should work if your BIOS is not too old. It
uses three functions from INT 10, the BIOS video interrupt. (If
you're using a Borland language, you may not have to do this the
hard way. Look for a function called DetectGraph or something
similar.)
Set AH=12h, AL=0, BL=32h; INT 10h. If AL is 12h, you have a VGA.
If not, set AH=12h, BL=10h; INT 10h. If BL is 0,1,2,3, you have an
EGA with 64,128,192,256K memory. If not, set AH=0Fh; INT 10h. If
AL is 7, you have an MDA (original monochrome adapter) or Hercules;
if not, you have a CGA.
This worked when tested with a VGA, but SB had no other adapter
types to test it with.
Subject: 605. How do I switch to 43- or 50-line mode?
pd1:<msdos.screen>vidmode.zip, downloadable from Simtel, contains
.COM utilities and .ASM source code.
Subject: 606. How can I find the Microsoft mouse position and button
status?
Use INT 33 function 3, described in Ralf Brown's interrupt list.
The Windows manual says that the Logitech mouse is compatible with
the Microsoft one, so the interrupt will probably work the same.
Also, many files are downloadable from pd1:<msdos.mouse> at Simtel.
Subject: 607. How can I access a specific address in the PC's memory?
First check the library that came with your compiler. Many vendors
have some variant of peek and poke functions; in Turbo Pascal use
the pseudo-arrays Mem, MemW, and MemL. As an alternative, you can
construct a far pointer: use Ptr in Turbo Pascal, MK_FP in the
Turbo C family, and FP_OFF and FP_SEG in Microsoft C.
Caution: Turbo C and Turbo C++ also have FP_OFF and FP_SEG macros,
but they can't be used to construct a pointer. In Borland C++ those
macros work the same as in Microsoft C, but MK_FP is easier to use.
By the way, it's not useful to talk about "portable" ways to do
this. Any operation that is tied to a specific memory address is
not likely to work on another kind of machine.
Subject: 608. How can I read or write my PC's CMOS memory?
(rev: 24 Sep 1993) There are a great many public-domain utilities
that do this. These are downloadable from Simtel:
pd1:<msdos.at>
cmos14.zip 5965 920817 Saves/restores CMOS to/from file
cmoser11.zip 28323 910721 386/286 enhanced CMOS setup program
cmosram.zip 76096 920214 Save AT/386/486 CMOS data to file and restore
rom2.zip 15692 900131 Save AT and 386 CMOS data to file and restore
setup21.zip 18172 880613 Setup program which modifies CMOS RAM
viewcmos.zip 11068 900225 Display contents of AT CMOS RAM, w/C source
A program to check and display CMOS memory (but not write to it) is
downloadable as part of
/pc/ts/tsutle22.zip at Garbo
pd1:<msdos.sysutl>tsutle22.zip at Simtel.
Good reports of CMOS299.ZIP, available in the pc.dir directory of
cantva.canterbury.ac.nz [132.181.30.3], have been posted.
Of the above, SB's only experience is with CMOSRAM, which seems to
work fine. It contains an excellent (and witty) .DOC file that
explains the hardware involved and gives specific recommendations
for preventing disaster or recovering from it. It's $5 shareware.
Robert Jourdain's {Programmer's Problem Solver for the IBM PC, XT,
and AT} has code for accessing the CMOS RAM, according to an article
posted in this newsgroup.
Subject: 609. How can I access memory beyond 640K?
(rev: 14 Sep 1993) This is a legitimate FAQ, in that it is frequently
asked. But there is no single agreed-upon answer. Please see the
separate article called "How to access memory above 640K" in
comp.os.msdos.programmer and in faqp*.zip at Simtel and Garbo.
The 29 June 1993 issue (xii:12) of {PC Magazine} carries an article,
"How DOS Programs Can Use Over 1MB of RAM" on pages 302-304.
Subject: 610. Where can I find a list of 80x86 opcodes?
(new: 2 May 1993) It's part of a rather long file, the 8 Dec 1992
edition of the Info-IBMPC Digest (V92 #185), downloadable as
pd2:<archives.ibmpc>9212.1-txt at Simtel. (Note: pd2, not
pd1.) Opcodes for the 8086 through 80386 are listed.
section 7. Other software questions and problems
================================================
Subject: 701. How can a program reboot my PC?
(rev: 11 Sep 1993) You can generate a "cold" boot or a "warm" boot.
A cold boot is the same as turning the power off and on; a warm boot
is the same as Ctrl-Alt-Del and skips the power-on self test.
For a warm boot, store the hex value 1234 in the word at 0040:0072.
For a cold boot, store 0 in that word. Then, if you want to live
dangerously, jump to address FFFF:0000. Here's C code to do it:
/* WARNING: data loss possible */
void bootme(int want_warm) /* arg 0 = cold boot, 1 = warm */ {
void (far* boot)(void) = (void (far*)(void))0xFFFF0000UL;
unsigned far* type = (unsigned far*)0x00400072UL;
*type = (want_warm ? 0x1234 : 0);
(*boot)( );
}
What's wrong with that method? It will boot right away, without
closing files, flushing disk caches, etc. If you boot without
flushing a write-behind disk cache (if one is running), you could
lose data or even trash your hard drive.
There are two methods of signaling the cache to flush its buffers:
(1) simulate a keyboard Ctrl-Alt-Del in the keystroke translation
function of the BIOS (INT 15 function 4F; but see notes below), and
(2) issue a disk reset (DOS function 0D). Most disk-cache programs
hook one or both of those interrupts, so if you use both methods
you'll probably be safe.
When user code simulates a Ctrl-Alt-Del, one or more of the programs
that have hooked INT 15 function 4F can ask that the key be ignored by
clearing the carry flag. For example, HyperDisk does this when it
has started but not finished a cache flush. So if the carry flag
comes back cleared, the boot code has to wait a couple of clock
ticks and then try again. (None of this matters on older machines
whose BIOS can't support 101- or 102-key keyboards; see "What is the
SysRq key for?" in section 3, "Keyboard".)
C code that tries to signal the disk cache (if any) to flush is
given below. Turbo Pascal code by Timo Salmi that does more or less
the same job may be found at question 49 (as of this writing) in the
Turbo Pascal FAQ in comp.lang.pascal, and is downloadable in
FAQPAS2.TXT in
/pc/ts/tsfaqp15.zip at Garbo
pd1:<msdos.info>tsfaqp15.zip at Simtel.
Here's C code that reboots after trying to signal the disk cache:
#include <dos.h>
void bootme(int want_warm) /* arg 0 = cold boot, 1 = warm */ {
union REGS reg;
void (far* boot)(void) = (void (far*)(void))0xFFFF0000UL;
unsigned far* boottype = (unsigned far*)0x00400072UL;
char far* shiftstate = (char far*)0x00400017UL;
unsigned ticks;
int time_to_waste;
/* Simulate reception of Ctrl-Alt-Del: */
for (;;) {
*shiftstate |= 0x0C; /* turn on Ctrl & Alt */
reg.h.ah = 0x4F; /* see notes below */
reg.h.al = 0x53; /* 0x53 = Del's scan code */
reg.x.cflag = 1; /* sentinel for ignoring key */
int86(0x15, ®, ®);
/* If carry flag is still set, we've finished. */
if (reg.x.cflag)
break;
/* Else waste some time before trying again: */
reg.h.ah = 0;
int86(0x1A, ®, ®);/* system time into CX:DX */
ticks = reg.x.dx;
for (time_to_waste = 3; time_to_waste > 0; ) {
reg.h.ah = 0;
int86(0x1A, ®, ®);
if (ticks != reg.x.dx)
ticks = reg.x.dx , --time_to_waste;
}
}
/* Issue a DOS disk reset request: */
reg.h.ah = 0x0D;
int86(0x21, ®, ®);
/* Set boot type and boot: */
*boottype = (want_warm ? 0x1234 : 0);
(*boot)( );
}
Reader Timo Salmi reported (26 July 1993) that the INT 15 AH=4F call
may not work on older PCs (below AT, XT2, XT286), according to Ralf
Brown's interrupt list.
Reader Roger Fulton reported (1 July 1993) that INT 15 AH=4F call
above hangs even a modern PC "ONLY when ANSI.SYS [is] loaded high
using EMM386.EXE. (Other things loaded high with EMM386.EXE were
OK; ANSI.SYS loaded high with QEMM386.SYS was OK; ANSI.SYS loaded
low with EMM386.EXE installed was OK.)" His solution was to use
only the disk reset, INT 21 function 0D, which does flush SMARTDRV,
then wait five seconds in hopes that any other disk-caching
software would have time to flush its queue.
If you have a more bulletproof solution, please send it to the
editor.
Reader Per Bergland reported (10 Sep 1993) that the jump to
FFFF:0000 will not work in Windows or other protected-mode programs.
(For example, when the above reboot code ran in a DOS session under
Windows, a box with "waiting for system shutdown" appeared. The PC
hung and had to be reset by cycling power.) His solution, which does
a cold boot not a warm boot, is to pulse pin 0 of the 8042 keyboard
controller, which is connected to the CPU's "reset" line. He has
tested the following code on various Compaqs, and expects it will
work for any AT-class machine; he cautions that you must first flush
the disk cache as indicated above.
cli
@@WaitOutReady: { Busy-wait until 8042 is ready for new command}
in al,64h { read 8042 status byte}
test al,00000010b { Bit 1 of status indicates input buffer full }
jnz @@WaitOutReady
mov al,0FEh { Pulse "reset" = 8042 pin 0 }
out 64h,al
{ The PC will reboot now }
Subject: 702. How can I time events with finer resolution than the
system clock's 55 ms (about 18 ticks a second)?
(rev: 28 Aug 1993) The following files, among others, are
downloadable from Simtel:
pd1:<msdos.at>
atim.zip 4783 881126 Precision program timing for AT
pd1:<msdos.c>
millisec.zip 37734 911205 MSC/asm src for millisecond res timing
mschrt3.zip 53708 910605 High-res timer toolbox for MSC 5.1
msec_12.zip 8484 920320 High-def millisec timer v1.2 (C,ASM)
ztimer11.zip 77625 920428 Microsecond timer for C, C++, ASM
(also at Garbo as /pc/c/ztimer11.zip)
pd1:<msdos.turbo-c>
tchrt3.zip 53436 910606 High-res timer toolbox for Turbo C 2.0
tctimer.arc 20087 891030 High-res timing of events for Turbo C
(same as /pc/c/tctimer.zoo at Garbo; both are version 1.0)
For Turbo Pascal users, source and object code are downloadable in
pd1:<msdos.turbopas>bonus507.zip at Simtel
/pc/turbopas/bonus507.zip at Garbo.
Also see "Q: How is millisecond timing done?" in FAQPAS.TXT,
downloadable as
/pc/ts/tsfaqp15.zip at Garbo
pd1:<msdos.info>tsfaqp15.zip at Simtel.
Subject: 703. How can I find the error level of the previous program?
(rev: 16 Aug 1993) First, which previous program are you talking
about? If your current program ran another one, when the child
program ends its error level is available to the program that
spawned it. Most high-level languages provide a way to do this; for
instance, in Turbo Pascal it's Lo(DosExitCode) and the high byte
gives the way in which the child terminated. In Microsoft C, the
exit code of a synchronous child process is the return value of the
spawn-type function that creates the process.
If your language doesn't have a function to return the error code
of a child process, you can use INT 21 function 4D (get return
code). By the way, this will tell you the child's exit code and the
manner of its ending (normal, Ctrl-C, critical error, or TSR).
It's much trickier if the current program wants to get the error
level of the program that ran and finished before this one started.
G.A.Theall has published source and compiled code to do this; the
code is downloadable as
pd1:<msdos.batutl>errlvl13.zip at Simtel
/pc/batchutil/errlvl12.zip (an older version) at Garbo.
(The code uses undocumented features in DOS 3.3 through 5.0. Theall
says in the .DOC file that the values returned under 4DOS or other
replacements won't be right.)
Subject: 704. How can a program set DOS environment variables?
(rev: 13 June 1993) Program functions that read or write "the
environment" typically access only the program's copy of it. What
this Q really wants to do is to modify the active environment, the
one that is affected by SET commands in batch files or at the DOS
prompt. You need to do some programming to find the active
environment, and that depends on the version of DOS.
A fairly well-written article in {PC Magazine} 28 Nov 1989
(viii:20), pages 309-314, explains how to find the active
environment, and includes Pascal source code. The article hints at
how to change the environment, and suggests creating paths longer
than 128 characters as one application.
Now as for downloadable source code, there are many possibilities.
SB looked at some of these, and liked
pd1:<msdos.envutil>rbsetnv1.zip at Simtel
/pc/envutil/rbsetnv1.zip at Garbo
the best. It includes some utilities to manipulate the environment,
with source code in C. A newer program is
pd1:<msdos.batutl>strings2.zip at Simtel
part of /pc/pcmag/vol11n22.zip at Garbo,
which is the code from {PC Magazine} 22 Dec 1992 (xi:22).
You can also use a call to INT 2E, Pass Command to Interpreter for
Execution; see Ralf Brown's interrupt list for details and cautions.
Subject: 705. How can I change the switch character to - from /?
Under DOS 5.0, you can't -- not completely, anyway. INT 21 function
3700, get switch character, always returns a '/' (hex 2F) -- and the
DOS commands don't even call that function, but hard code '/' as the
switch character.
Some history: DOS used to let you change the switch character by
using SWITCHAR= in CONFIG.SYS or by calling DOS function 3701. DOS
commands and other programs called DOS function 3700 to find out the
switch character. If you changed the switch character to '-' (the
usual choice), you could then type "dir c:/c700 -p" rather than "dir
c:\c700 /p". Under DOS 4.0, the DOS commands ignored the switch
character but functions 3700 and 3701 still worked and could be used
by other programs. Under DOS 5.0, even those functions no longer
work, though all DOS functions still accept '/' or '\' in file
specs.
You can reactivate the functions to get and set switchar by using
programs like SLASH.ZIP or the sample TSR called SWITCHAR in
amisl091.zip (see "How can I write a TSR?", below.) DOS commands
will still use the slash, but non-DOS programs that call DOS func-
tion 3700 will use your desired switch character. (DOS replacements
like 4DOS may honor the switch character for internal commands.)
Some readers may wonder why this is even an issue. Making '-' the
switch character frees up the front slash to separate names in the
path part of a file spec. This is easier for the ten-fingered to
type, and it's one less difference to remember for commuters between
DOS and Unix. The switch character is the only issue, since all the
INT 21 functions accept '/' or '\' to separate directory names.
Subject: 706. Why does my interrupt function behave strangely?
(rev: 24 Sep 1993) Interrupt service routines can be tricky,
because you have to do some things differently from "normal"
programs. If you make a mistake, debugging is a pain because the
symptoms may not point at what's wrong. Your machine may lock up or
behave erratically, or just about anything else can happen. Here
are some things to look for. (See the next Q for general help
before you have a problem.)
First, did you fail to set up the registers at the start of your
routine? When your routine begins executing, you can count on
having CS point to your code segment and SS:SP point to some valid
stack (of unknown length), and that's it. In particular, an
interrupt service routine must set DS to DGROUP before accessing any
data in its data segments. (If you're writing in a high-level
language, the compiler may generate this code for you automatically;
check your compiler manual. For instance, in Borland and Microsoft
C, give your function the "interrupt" attribute.)
Did you remember to turn off stack checking when compiling your
interrupt server and any functions it calls? The stack during the
interrupt is not where the stack-checking code expects it to be.
(Caution: Some third-party libraries have stack checking compiled
in, so you can't call them from your interrupt service routine.)
Next, are you calling any DOS functions (INT 21, 25, or 26) in your
routine? DOS is not re-entrant. This means that if your interrupt
happens to be triggered while the CPU is executing a DOS function,
calling another DOS function will wreak havoc. (Some DOS functions
are fully re-entrant, as noted in Ralf Brown's interrupt list.
Also, your program can test, in a way too complicated to present
here, when it's safe to call non-re-entrant DOS functions. See INT
28 and functions 34, 5D06, 5D0B of INT 21; and consult {Undocumented
DOS} by Andrew Schulman. Your program must read both the "InDOS
flag" and the "critical error flag".)
Is a function in your language library causing trouble? Does it
depend on some initializations done at program startup that is no
longer available when the interrupt executes? Does it call DOS (see
preceding paragraph)? For example, in both Borland and Microsoft C
the memory-allocation functions (malloc, etc..) and standard I/O
functions (scanf, printf) call DOS functions and also depend on
setups that they can't get at from inside an interrupt. Many other
library functions have the same problem, so you can't use them
inside an interrupt function without special precautions.
Is your routine simply taking too long? This can be a problem if
you're hooking on to the timer interrupt, INT 1C or INT 8. Since
that interrupt expects to be called 18.2 times a second, your
routine -- plus any others hooked to the same interrupts -- must
execute in less than 55 ms. If they use even a substantial fraction
of that time, you'll see significant slowdowns of your foreground
program. A good discussion is downloadable as
pub/msdos/SIMTEL20-mirror/info/intshare.zip at ni.funet.fi
pd1:<msdos.info>intshare.zip at Simtel.
Did you forget to restore all registers at the end of your routine?
Did you chain improperly to the original interrupt? You need to
restore the stack to the way it was upon entry to your routine, then
do a far jump (not call) to the original interrupt service routine.
(The process is a little different in high-level languages.)
Subject: 707. How can I write a TSR (terminate-stay-resident utility)?
(rev: 20 June 1993) There are books, and there's code to download.
First, the books:
- Ray Duncan's {Advanced MS-DOS}, ISBN 1-55615-157-8, gives a brief
checklist intended for experienced programmers. The ISBN is for
the second edition, through DOS 4; but check to see whether the
DOS 5 version is available yet.
- {DOS 5: A Developer's Guide} by Al Williams, ISBN 1-55851-177-6,
goes into a little more detail, 90 pages worth!
- Pascal programmers might look at {The Ultimate DOS Programmer's
Manual} by John Mueller and Wallace Wang, ISBN 0-8306-3534-3, for
an extended example in mixed Pascal and assembler.
- For a pure assembler treatment, check Steven Holzner's {Advanced
Assembly Language}, ISBN 0-13-663014-6. He has a book with the
same title out from Brady Press, but it's about half as long as
this one.
Next, the code. Some of it is companion code to published articles,
which are also listed below:
- The Alternate Multiplex Interrupt Specification, downloadable as
pd1:<msdos.info>altmpx35.zip at Simtel
/pc/programming/altmpx35.zip at Garbo
/afs/cs/user/ralf/pub/altmpx35.zip at cs.cmu.edu
- Ralf Brown's assembly-language implementation of the spec, with
utilities in C, downloadable as
pd1:<msdos.asmutl>amisl091.zip at Simtel
/pc/c/amisl091.zip at Garbo
/afs/cs/user/ralf/pub/amisl091.zip at cs.cmu.edu
- Douglas Boling's MASM template for a TSR is downloadable as
pd1:<msdos.asmutl>template.zip at Simtel.
- A posted article mentions Boling's "Strategies and Techniques for
Writing State-of-the-Art TSRs that Exploit MS-DOS 5", Microsoft
Systems Journal, Jan-Feb 1992, Volume 7, Number 1, pages 41-59,
with examples downloadable in
pd1:<msdos.msjournal>msjv7-1.zip at Simtel
- code for Al Stevens's "Writing Terminate-and-Stay-Resident
Programs", Computer Language, February 1988, pages 37-48 and March
1988, pages 67-76 is downloadable as
pd1:<msdos.c>tsrc.zip at Simtel
- software examples to accompany Kaare Christian's "Using Microsoft
C Version 5.1 to Write Terminate-and-Stay-Resident Programs",
Microsoft Systems Journal, September 1988, Volume 3, Number 5,
pages 47-57 are downloadable as
pd1:<msdos.msjournal>msjv3-5.arc at Simtel
Finally, there are commercial products, of which TesSeRact (for
C-language TSRs) is one of the best known.
Subject: 708. How can I write a device driver?
Many books answer this in detail. Among them are {Advanced MS-DOS}
and {DOS 5: A Developer's Guide}, cited in the preceding Q.
Michael Tischer's {PC System Programming}, ISBN 1-55755-036-0, has
an extensive treatment, as does Dettman and Kyle's {DOS Programmer's
Reference: 2d Edition}, ISBN 0-88022-458-4. For a really in-depth
treatment, look for a specialized book like Robert Lai's {Writing
MS-DOS Device Drivers}, ISBN 0-201-13185-4.
Subject: 709. What can I use to manage versions of software?
(rev: 21 Aug 1993) A port of the Unix RCS utility is downloadable as
pd1:<msdos.gnuish>rcs55ax.zip (EXE and docs) from Simtel
pd1:<msdos.gnuish>rcs55as.zip (source) from Simtel
/pc/unix/alrcs5ex.zip (EXE and docs ?) from Garbo.
This is no longer limited to one-character extensions on filenames
(.CPP and .BAS are now OK).
An RCS56 is available at a number of archive sites, but it appears
to be unauthorized. In response to a query, Keith Petersen, Simtel
administrator, said that RCS56 was removed from Simtel at the
author's request because it did not contain source code and thus was
in violation of the GNU copyleft.
As for commercial software, SB posted a question asking for readers'
experiences in July 1993 and seven readers responded. PVCS from
Intersolv (formerly Polymake) got five positive reviews, though
several readers commented that it's expensive; RCS from MKS got one
positive and one negative review; Burton TLIB got one negative
review; DRTS from ILSI got one positive review.
Subject: 710. What's this "null pointer assignment" after my C program
executes?
(rev: 17 Sep 1993) Somewhere in your program, you assigned a value
_through_ a pointer without first assigning a value _to_ the
pointer. (This might have been something like a strcpy or memcpy
with a pointer as its first argument, not necessarily an actual
assignment statement.) Your program may look like it ran correctly,
but if you get this message you can be certain that there's a bug
somewhere.
Microsoft and Borland C, as part of their exit code (after a return
from your main function), check whether the location 0000 in your
data segment contains a different value from what you started with;
if so, they infer that you must have used an uninitialized pointer.
This implies that the message will appear at the end of execution of
your program regardless of where the error actually occurred.
To track down the problem, you can put exit( ) statements at various
spots in the program and narrow down where the uninitialized pointer
is being used by seeing which added exit( ) makes the null-pointer
message disappear. Or, in the debugger, set a watch at location
0000 in your data segment, assuming you're in small or medium model.
(If data pointers are 32 bits, as in the compact and large models, a
null pointer will overwrite the interrupt vectors at 0000:0000 and
probably lock up your machine.)
Under MSC/C++ 7.0, you can declare the undocumented library function
extern _cdecl _nullcheck(void);
and then sprinkle calls to _nullcheck( ) through your program at
regular intervals.
Borland's TechFax document #TI726 discusses the null pointer
assignment from a Borland point of view. It's one of many documents
downloadable as part of
pd1:<msdos.turbo-c>bchelp10.zip at Simtel
/pc/turbopas/bchelp10.zip at Garbo.
Subject: 711. How can my program tell if it's running under Windows?
(rev: 18 Apr 1993) Set AX=4680 and execute INT 2F. If AX contains
0, you're in Windows real mode or standard mode (or under the DOS
5.0 shell). Otherwise, set AX=1600 and INT 2F. If AL does not
contain 0 or 80, you're in Windows 386 enhanced mode. See {PC
Magazine} 24 Nov 1992 (xi:20), pages 492-493.
When Windows 3.0 or 3.1 is running, the DOS environment will contain
a definition of the string windir, in lower case.
For more information, see {PC Magazine} 26 May 1992 (xi:10) pages
345-346. A program, WINMODE, is available as part of
pd1:<msdos.pcmag>vol11n10.zip at Simtel
/pc/pcmag/vol11n10.zip at Garbo.
Subject: 712. How do I copyright software that I write?
(rev: 9 Sep 1993) The following is adapted (and greatly condensed)
from chapter 4 of the Chicago Manual of Style (13th edition, ISBN
0-226-10390-0). Disclaimer: This is not written by a lawyer, and
is not legal advice. Also, there are very likely to be differences
in copyright law among nations. No matter where you live, if
significant money may be involved, get legal advice.
That said, in the U.S. (at least), when you write something, you own
the copyright. (The most significant exception to programmers is
"works made for hire", i.e., something you write because your
employer or client pays you to. A contract, agreed in advance, can
vest the copyright in the programmer even if an employee.) You
don't have to register the work with the Copyright Office unless
(until) the copyright is infringed and you intend to bring suit;
however, it is easier to recover damages in court if you did
register the work within three months of publication.
From paragraph 4.16 of the Chicago Manual: "... the [copyright]
notice consists of three parts: (1) the symbol [C-in-a-circle]
(preferred because it also suits the requirements of the Universal
Copyright Convention), the word 'Copyright', or the abbreviation
'Copr.', (2) a date--the year of first publication, and (3) the name
of the copyright owner. Most publishers also add the phrase 'All
rights reserved' because it affords some protection in Central and
South American countries ...." Surprise: "(C)" is legally not the
same as the C-in-a-circle, so those of us who are ASCII-bound must
use the word or the abbreviation.
You can download a much more comprehensive treatment from the
Internet. Terry Carroll posts a six-part Copyright FAQ to
misc.legal, news.answers and other groups.
(continued in part 4)