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GLOSSARY Release 60 Last change 03jan99 Copyright (c) 1989,1990,1991,1992,1993,1994,1995,1996,1997,1998,1999 Ralf Brown A20 (Address line 20) The 80286 and higher CPUs allow addresses in real mode to extend slightly beyond the one megabyte mark, which causes an incompatibility with some older programs which expect such addresses to wrap back to the beginning of the address space. For complete compatibility with the 8088, newer machines thus contain circuitry which permits the twenty-first address line (A20) to be disabled. The CPU then effectively has only twenty address lines in real mode, just as the 8088 does, and addresses which would extend beyond the one megabyte mark wrap to the beginning of the address space. See also High Memory Area, Real Mode. ABIOS (Advanced BIOS) The IBM XT/286 and PS/2 models with 80286 or higher processors contain two separate BIOSes. The ABIOS is a protected-mode BIOS which is used by OS/2. For machines without an ABIOS, such as the IBM AT, OS/2 loads the equivalent of the ABIOS from disk. see also CBIOS ACCESS.bus A further development of the I2C bus which forms the physical layer of the Universal Serial Bus. Because ACCESS.bus is used for communication with external devices, it includes power and ground wires in addition to I2C's clock and data lines, and permits twice the current draw (6ma) on the clock and data lines. see also I2C, SMBus. API (Application Program[ming] Interface) The defined set of calls which a program may make to interact with or request services of the operating system or environment under which it is running. Because the inputs and outputs of the calls are well-defined, a program using the API can continue using the identical calls even if the internal organization of the program providing the API changes. APL (A Programming Language) An interactive, mathematically- oriented language which is well-suited to manipulating matrices. Originally using greek letters and numerous special symbols, thus requiring a special display, versions are now available which use keywords in place of the special symbols. ASCIZ A NUL-terminated ASCII string. The ASCIZ string "ABC" consists of the four bytes 41h, 42h, 43h, and 00h. Unless otherwise specified, maximum lengths given in the interrupt list do not include the terminating NUL. AVATAR (Advanced Video Attribute Terminal Assembler and Recreator) A set of control codes which may be used to affect the output of characters to the screen on systems equipped with an appropriate driver. Similar in intent to ANSI sequences, AVATAR has shorter command sequences and provides additional PC-specific functionality. AVATAR is primarily used by the Opus and Maximus bulletin board systems (it was designed by one of the developers of the Opus system). BASIC (Beginner's All-purpose Symbolic Instruction Code) A programming language originally designed as a means of teaching FORTRAN. There are many variations of BASIC with differing capabilities; the majority are interpreted but compiled BASIC is becoming more popular. All genuine IBM personal computers (including the latest PS/2 models) come equipped with a cassette-based BASIC interpreter in ROM. BCD (Binary Coded Decimal) A method of data storage where two decimal digits are stored in each byte, one in the upper four bits and the other in the lower four bits. Since only the values 0 through 9 are used in each half of a byte, BCD values can be read as decimal numbers on a hexadecimal display of memory or a file. Big-Endian One of the two major ways of organizing multi-byte numeric values in memory. A big-endian layout places the most significant byte of the value in the lowest (first) memory location, i.e. 12345678h is stored as 12h 34h 56h 78h. Motorola processors are big-endian. Compare Little-Endian. BIOS (Basic Input/Output System) A set of standardized calls giving low-level access to the hardware. The BIOS is the lowest software layer above the actual hardware and serves to insulate programs (and operating systems) which use it from the details of accessing the hardware directly. BIOS Parameter Block The BIOS Parameter Block stores the low-level layout of a drive. See also INT 21h Function 53h. bit-banging The process of sending data to or reading data from a serial device by raising and lowering a data line in the proper sequence (writing), or polling a data line and determining the bit sequence represented by the detected transitions and their timing. It is equivalent to emulating a UART in software. Boot To start up the computer or operating system. The term "boot" is a contraction of "bootstrap", which in turn comes from the expression "to lift oneself by one's boot straps." The ROM BIOS on IBM PCs and compatibles reads in the first sector of the disk, which contains a short (less than 500 bytes) program that reads in a portion of the operating system, which in turn reads in the remainder of the operating system. See also IPL. Boot Drive The disk drive from which the operating system was booted. See also Boot. BPB see BIOS Parameter Block Breakpoint When debugging, a memory location which when accessed causes a break in the normal flow of execution and the invocation of the debugger. Used to let a program run at full speed until a certain instruction is reached or (less frequently) a particular data item is accessed or changed. Cache Caching is a method of increasing performance by keeping frequently-used data in a location which is more quickly accessed. The most common caches are disk caches (store disk sectors in RAM) and RAM caches (store portions of main memory in special high-speed RAM which may be accessed as fast as the CPU is capable of accessing memory). See also Delayed Write, Write-Through. Callback A call to a specified function made by the operating system or operating environment when a request (usually an asynchronous request) completes. This permits the calling program to continue operating while the request is processed yet still be aware of its completion immediately without the need to constantly poll the request's status. See also Callout. Callout A call made by the operating system, operating environment, or an application program on various events, which may be intercepted by other software which is interested in the current state of the system. See also Callback, External Device Interface. CAS see Communicating Applications Specification CBIOS (Compatibility BIOS) The IBM XT/286 and PS/2 models with 80286 or higher processors contain two separate BIOSes. The CBIOS is a real-mode BIOS which is compatible with the earlier products in the IBM PC family and PS/2 models with 8086 processors. See also ABIOS. CDS see Current Directory Structure CGA (Color/Graphics Adapter) One of the two video display boards introduced together with the original IBM PC. See also HGC, MDA. Clock Tick 1/18.2 second, or approximately 55 milliseconds. This is the rate at which the IBM PC's system clock is updated, derived by dividing 1.19 MHz (1/4 of the original PC's 4.77 MHz CPU clock) by 65536 (the highest divisor possible on the 8253/8254 timer chip). CMOS (Complementary Metal-Oxide-Semiconductor) A type of integrated circuit design known for its low power consumption. CMOS RAM A small amount (typically 64 or 128 bytes) of memory in the system's real-time clock chip that is preserved by the clock's battery and is used for storing configuration information. See also Real-Time Clock. Communicating Applications Specification DCA and Intel's standard programmatic interface for sending and receiving FAXes via any of a number of internal FAX boards. CP/M (Control Program for Microcomputers) An early operating system for micros based on the 8-bit Intel 8080 CPU (and later the compatible 8085 and Zilog Z80 CPUs). MSDOS version 1.0 was essentially a clone of CP/M for the Intel 8086. CP/M-86 One of the three operating systems offered by IBM for its original PC (the other two were MSDOS and the UCSD p-System). It has since evolved into DR-DOS version 6 and Novell DOS 7. CPU (Central Processing Unit) The microprocessor which executes programs on your computer. Current Directory Structure The data record used by DOS to keep track of the current directory on a drive; whether the drive is valid, network, SUBSTituted, or JOINed; and other pertinent information. See also INT 21h Function 52h. Cylinder The set of concentric tracks of data located at the same position on each data-bearing surface of the disk. A double-sided floppy will contain two tracks per cylinder. DAC (Digital-to-Analog Converter) A hardware device (in its simplest form, nothing more than a set of interconnected resistors) which converts a digital number into an analog signal whose voltage is proportional to the value of the digital number. VGA and later color video boards use DACs to convert color values into the analog signals sent to the display; sound boards normally use DACs as well. DCC (Display Combination Code) A number which indicates both the type of display adapter board and the type of monitor attached to the video board. Delayed Write A form of caching in which control is returned before the data is actually written to the storage media. See also Cache, Write-Through. Device Driver An interface module between the device-independent portions of the operating system and an actual hardware device which converts device-independent requests into the actual sequence of device operations to perform the requested action. IO.SYS contains the standard, built-in MSDOS device drivers such as CON, COM1, AUX, PRN, etc. See also INT 21h Function 52h and INT 2Fh Function 0802h. Device Driver Request Header The data structure passed to a device driver which contains the command to be executed, its parameters, and space for a returned status and data values. See INT 2Fh Function 0802h. DGIS (Direct Graphics Interface Standard) Direct Memory Access A method whereby peripherals may transfer data into or out of main memory without the involvement of the CPU. Disk Transfer Address The Disk Transfer Address indicates where functions which do not take an explicit data address will read or store data. Although the name implies that only disk accesses use this address, other functions use it as well. See INT 21h Function 4Eh for an example of the DTA's use. DLL see Dynamic Link Library DMA see Direct Memory Access DOS Extender A program which allows a program to run in protected mode while still retaining access to real-mode MSDOS services. See also Protected Mode. DOS Parameter List The DOS Parameter List is used to pass arguments to SHARE and network functions. See also INT 21h Function 5D00h. DOS Protected-Mode Interface An API which provides basic services for protected-mode programs to allocate memory, invoke real-mode software, etc. See also Virtual Control Program Interface. DOS Protected-Mode Services An API designed by Novell (and first released with Novell DOS 7) which allows TSRs to store most of themselves outside of the first megabyte of memory. DPB see Drive Paramter Block DPL see DOS Parameter List DPMI see DOS Protected-Mode Interface DPMS see DOS Protected-Mode Services DRAM (Dynamic Random Access Memory) RAM memory which essentially consists of a tiny capacitor for each bit of memory. Since capacitors do not hold a charge indefinitely, DRAM must be constantly refreshed to avoid losing its contents. Also, the process of reading the contents of the memory are destructive, meaning extra time must be spent restoring the contents of memory addresses which are accessed, so DRAM is slower than SRAM. See also Refresh, SRAM. Drive Parameter Block The DOS Drive Parameter Block stores the description of the media layout for a logical drive, as well as some housekeeping information. See also INT 21h Function 1Fh and INT 21h Function 32h. DTA see Disk Transfer Address DWORD Doubleword; four bytes. Commonly used to hold a 32-bit segment:offset or selector:offset address. Dynamic Link Library A collection of subroutines which are linked with a program at the time it is loaded into memory rather than permanently placed in the executable. This has the advantage of allowing a single copy of the subroutine library to reside on disk or in memory even when it is used by many programs. It also permits all programs using the DLL to be updated without recompiling simply by installing a new version of the library. EGA (Enhanced Graphics Adapter) IBM's second color video board for the IBM PC family, capable of a maximum resolution of 640x350 pixels in 16 simultaneous colors of a total of 64 possible colors. EISA (Enhanced Industry-Standard Architecture) A 32-bit superset of the IBM AT's expansion bus (which is now known as the ISA or Industry-Standard Architecture bus). EMS see Expanded Memory Specification EOI (End of Interrupt) A particular command sent to the interrupt controller to indicate that the interrupt has been handled by software and that new interrupts of the same or lower priority may now be signalled by the interrupt controller. ESDI (Enhanced Small Device Interface) A disk drive interface type which was briefly popular before IDE took over. An ESDI drive can transfer data between the drive and controller at 10, 15, or 20 megabits per second, which is faster than an MFM or RLL controller but slower than what is possible with an IDE or SCSI drive. See also IDE. Exception A signal by the CPU that some error condition has been encountered that it can not deal with without a program's intervention. The most commonly encountered exceptions on Intel processors are Exceptions 12 and 13 (decimal, how Intel specifies exception numbers), which are stack and general problems, respectively. Exception 13 is typically caused by a memory access which wraps from the end of a segment back to the beginning. Expanded Memory Specification A specification devised by Lotus, Intel, and Microsoft for accessing more than one megabyte of memory by bank-switching additional memory into the one megabyte real mode address space. Extended BIOS Data Area A block of memory, typically the 1K at the top of conventional memory, which is used to store additional data for use by the BIOS which does not fit into the 256-byte data area at segment 0040h. Extended File Control Block A DOS File Control Block which has had an additional seven bytes prepended to permit control of file attributes (which are stored in the appendage). See also FCB. Extended Memory Memory beyond the one megabyte address which is available only on 80286 and higher machines. Except for a small portion (the High Memory Area), extended memory is only accessible from protected mode. Extended Memory Specification A specification devised by Microsoft which allows multiple programs to share extended (above 1 megabyte) memory and noncontiguous memory above 640K. See also Upper Memory Block. External Device Interface A series of calls made by the DESQview multitasker on various "interesting" events, which may be intercepted by programs which wish to keep track of the current system state. See also Callout. FAT see File Allocation Table FCB see File Control Block File Allocation Table A data structure on disk that records which clusters are free, which are unusable, and which have been allocated. The clusters occupied by a file are linked into a list in the file allocation table, allowing DOS to find the contents of the file. File Control Block A data record in the calling program's address space which is used by DOS 1.x functions to record the state of an open file. See also INT 21h Function 13h. File Handle A small positive integer used to identify the previously-opened file on which a program wishes to perform an operation. Flush To force the copying of any data still stored in temporary buffers to its final destination. FM (Frequency Modulation) A method of encoding data as a series of magnetic flux reversals on disk or tape, commonly known as single-density recording. In frequency modulation, a series of clock pulses are written at regular intervals, with one data bit for each clock pulse. See also MFM, RLL. Formatting Preparing a storage medium (usually magnetic media such as a disk or tape) for storing data. Low-level or physical formatting writes all necessary housekeeping data to enable the storage device to read the media and may also initialize the storage units on the media to a known state. High-level or logical formatting writes data used by the operating system, such as allocation information and directories onto media which has already been physically formatted. Formatting programs often perform both a low-level and a high-level format. FOSSIL (Fido/Opus/Seadog Standard Interface Layer) A standardized API for performing serial I/O, originally used by the Fido and Opus bulletin- board software and Seadog bulletin-board mailer, but now in wider use. FTP (File Transfer Protocol) The standard protocol for copying files from one machine to another on a TCP/IP (Internet) network. Also the program of the same name with which a user may transfer files. Gather-Write see Scatter/Gather Handle A short identifier, usually a small integer or a pointer, for some other object which is maintained or controlled by the operating system or environment; a particular handle may be valid system-wide or may have meaning only for a particular process. See also File Handle. HGC (Hercules Graphics Card) A monochrome video adapter capable of 720x352 monochrome graphics. The HGC was the first non-IBM video adapter for the IBM PC. See also CGA, MDA. High Memory Area The first 65520 bytes (64K less 16 bytes) of extended memory. This area is accessible from real mode on the 80286 and higher processors because these processors do not wrap addresses at one megabyte as the 8088 and 8086 do. See also A20, INT 2Fh Functions 4A01h. HMA see High Memory Area Horizontal Retrace When a monitor has finished displaying a single scan line, it must move it electron beam(s) back to the left edge of the CRT, during which time it turns off the beam. On the original CGA (and some early clones), the only time one could access the display memory without causing "snow" was during the horizontal or vertical retrace periods, as the display adapter was not itself accessing the display memory during those times. See also Vertical Retrace. I2C (also IIC; the "2" is superscripted) Inter-Integrated Circuit Bus -- A moderate-speed serial communications bus originally invented by Philips in the early 1980s for consumer-electronics applications, such as inter-chip communication in a television set or high-end stereo. The I2C bus has recently appeared on PCs in video capture boards and similar devices, as well as (surprisingly) SDRAM DIMMs (for the on-board serial EEPROM). The ACCESS.bus is a derivative of the I2C bus which forms the physical layer of the Universal Serial Bus. Similary, the SMBus (System Management Bus) also uses I2C as its physical layer. IDE (Integrated Drive Electronics) A type of disk drive interface which essentially extends the PC's expansion bus all the way to the drive and places the drive controller on the disk drive itself. See also ESDI. IDT see Interrupt Descriptor Table IFS see Installable File System IIC see I2C. Installable File System An Installable File System which allows non-DOS format media to be used by DOS. In most ways, an IFS is very similar to a networked drive, although an IFS would typically be local rather than remote. See also INT 21h Function 52h. Interrupt Descriptor Table The protected-mode table of descriptors specifying the handlers for up to 256 interrupts, and how to transfer control to each one. See also Interrupt Vector Table. Interrupt Vector Table The real-mode table of 256 four-byte interrupt handler addresses, located in the lowest 1024 bytes of the address space. See also Interrupt Descriptor Table. IP (Internet Protocol) The lower level (transport layer) of the TCP/IP protocol suite. See also TCP, TCP/IP. IPC (Inter-Process Communication) Any one of numerous methods for allowing two or more separate processes to exchange data. IPL (Initial Program Load) See Boot. IPX (Internetwork Packet Exchange) A low-level layer of Novell's NetWare networking software. IRQ (Interrupt ReQuest) A hardware line connected to the interrupt controller chip which signals that a CPU interrupt should be generated. ISA (Industry-Standard Architecture) The expansion bus used by the IBM PC/AT. See also EISA. IVT see Interrupt Vector Table JFT see Job File Table Job File Table The Job File Table (also called Open File Table) stored in a program's PSP which translates handles into SFT numbers. See also INT 21h Function 26h. LCD (Liquid Crystal Display) List of Lists An internal DOS table of lists and other tables through which most DOS-internal data structures may be reached. See INT 21h Function 52h. Little-Endian One of the two major ways of organizing multi-byte numeric values in memory. A little-endian layout places the least significant byte of the value in the lowest (first) memory location, i.e. 12345678h is stored as 78h 56h 34h 12h. Intel processors are little-endian. Compare Big-Endian. LPT Abbreviation for Line PrinTer. MCB see Memory Control Block MCGA (Multi-Color Graphics Array) The low-end color adapter offered in IBM's early PS/2 series machines. MDA (Monochrome Display Adapter) A text-only video adapter introduced together with the original IBM PC. See also CGA, HGC. Memory Control Block The data structure containing the length and owner (among other things) of a portion of the memory managed by DOS. See INT 21h Function 52h. MFM (Modified Frequency Modulation) A method of encoding data as a series of magnetic flux reversals on disk or tape, commonly known as double-density recording. In contrast to FM, modified frequency modulation omits all clock pulses except those between pairs of zero bits. See also FM, RLL. Mickey The smallest increment of motion a mouse can sense. MIDI (Musical Instrument Digital Interface) A standardized interface for controlling musical instruments with a computer. Modem (contraction of MOdulator/DEModulator) MSR (Model-Specific Register) Additional, indirectly-accessible, registers containing control or status information about various aspects of the processor such as caches, performance counters, and the like. These registers, accessible via the RDMSR and WRMSR instructions, were added with the Pentium and later-model 486 processors. Multitasking Any of a number of methods by which multiple programs may execute concurrently, with rapid switching between the programs giving the appearance that all are executing simultaneously. MZ The letters M and Z appear in numerous places in DOS (memory control blocks, .EXE header, etc.); the conventional explanation is that these are the initials of Mark Zbikowski, one of the principal architects of MSDOS 2.0. NCB see Network Control Block NDIS (Network Driver Interface Specification) A hardware-independent network interface developed by Microsoft and 3com. See also Packet Driver, TCP/IP. NetBIOS One of a number of low-level device-independent network interfaces; the other major interfaces are Novell's IPX and the Internet's IP (Internet Protocol, the lower-level portion of TCP/IP). Network Control Block A Network Control Block used to pass requests to NetBIOS and receive status information from the NetBIOS handler. Network Redirector A program which permits access to network devices (disks, printers, etc.) using the MSDOS kernel network redirector interface. See also Network Shell, Redirector Interface. Network Shell A program which permits access to network devices (disks, printers, etc.) by intercepting DOS calls before they reach the DOS kernel and handling those operating on network devices while passing through actions on local devices. See also Network Redirector. NMI see Non-Maskable Interrupt Non-Maskable Interrupt An interrupt which can not be disabled by clearing the CPU's interrupt enable flag, unlike most normal interrupts. Non-maskable interrupts are typically used to signal calamities which require immediate action, such as a hardware failure or imminent loss of power. Non-Volatile RAM Memory which can be modified like normal RAM but does not lose its contents when the system's power is turned off. This memory may be powered by a battery when the system power if off, or it may be a type of memory which does not need electricity to maintain its contents, such as EEPROM or bubble memory. NVRAM see Non-Volatile RAM ODI (Open Data-link Interface) A hardware-independent network interface developed by Novell, Inc. See also NDIS, Packet Driver. OEM (Original Equipment Manufacturer) a company which purchases components that are resold as part of its own products under the company's own brand name, e.g. a Gateway 2000-branded monitor may actually be a Mag or NEC monitor. Open File Table see Job File Table Overscan Area The "border" between the edge of the area where graphics or text can be displayed and the actual edge of the area the video adapter can illuminate on the monitor's screen. Overscan Register On a display adapter, the control register which specifies the color to be displayed in the overscan area. Packet Driver Any one of the numerous drivers conforming to FTP Software's Packet Driver Specification, which provides a hardware-independent network interface. See also NDIS, ODI. Page Fault A CPU-generated signal, and the operating system's reaction to it, generated when a program accesses a page of virtual memory which is not located in RAM at the time. The operating system's response is to load in the required page, possibly writing some other page out to disk in order to make room. See also INT 0E in INTERRUP.LST. Page Register A peripheral register or I/O port used to extend the addressing range of some other register or I/O port. The prime example are the DMA page registers, which allow the DMA controller to address more than 64K (since the DMA controller only contains 16 address lines; this is the cause of the 64K DMA boundaries). Palette Register A memory location on the video controller which specifies the actual color displayed for a particular color number. Paragraph A 16-byte unit of memory. This size is the result of the 8086's segmentation model, where incrementing the segment by one advances the absolute location by 16 bytes. Park To move a hard disk's read/write heads to a position in which it is safe to turn off the power and transport the disk drive. Many drives also lock the heads into position when they are parked, providing additional protection from sudden movement. Pel see Pixel Pixel A picture element, the smallest addressable unit of a graphical display. Post Make known, either generally or to a specific handler, that a particular event of interest has occurred. POST see Power-On Self-Test Power-On Self-Test A brief examination of the system's functionality performed each time the system is turned on. Print Spooler see SPOOL. Program Segment Prefix The Program Segment Prefix is a 256-byte data area prepended to a program when it is loaded. It contains the command line that the program was invoked with, and a variety of housekeeping information for DOS. See also INT 21h Function 26h. Protected Mode One of the operating modes of the 80286 and higher Intel processors, in which addresses used by programs no longer correspond to physical addresses and the CPU enforces various protection mechanisms designed to prevent one program from disrupting other programs or the operating system. See also Real Mode, Virtual-86 Mode. PSP see Program Segment Prefix PWORD Six bytes. Used to hold an 80386 protected-mode "far" address, consisting of a segment selector and a 32-bit offset, or a Turbo Pascal "real" variable. See also DWORD, QWORD. QWORD (quad-word) Eight bytes. See also DWORD, PWORD. RAM (Random Access Memory) See also DRAM, SRAM. Real Mode One of the operating modes of the 80286 and higher Intel processors, and the only operating mode of the 8088, 8086, 80186, and 80188 processors. In this mode, all addresses used by programs correspond directly to real physical addresses (thus the full name, Real Address Mode) and there are no CPU-imposed protections between programs. See also Protected Mode, Virtual-86 Mode. Real-Time Clock A battery-powered clock which continues to maintain its time even while the system is powered down. On PCs, the real-time clock contains a small amount of battery-powered memory (set CMOS RAM). Redirector Interface The set of device-independent INT 2Fh function calls invoked by the MSDOS kernel to operate on devices it recognizes as network devices. These function calls provide a lower-level interface than the INT 21h calls made to DOS, allowing a program intercepting these functions to be simpler than one intercepting INT 21h calls. See INT 2Fh Functions 1100h through 1130h. Refresh The process of periodically rewriting the contents of a DRAM memory chip to keep it from fading. The term "refresh" is also commonly applied to redrawing the image on a CRT's phosphors. See also DRAM. RGB (Red-Green-Blue) The color specification mechanism normally used in computer displays, where colors are separated into their primary-color components. See also YUV. RLL (Run-Length Limited) A method of encoding data as a series of magnetic flux reversals on disk or tape. RLL can achieve higher data densities than MFM recording because it encodes the data such that (on average), fewer than one flux reversal per data bit is required (however, timing becomes more critical). RLL is actually an entire family of encoding methods, specified with two numbers indicating the minimum and maximum distances between one bits (flux transitions). The variant normally called RLL is RLL-2,7; RLL-1,7 and RLL-3,9 are also in use. MFM is in effect RLL-1,3. See also FM, MFM. ROM (Read-Only Memory) A memory for program storage which may not be changed by the program as it runs. RTC see Real-Time Clock Scan Code The actual key number sent by the keyboard, which differs from the key codes seen by application programs. The enhanced (101/102-key) keyboard actually sends different scan codes than the original (83/84-key) IBM keyboard, but these are normally translated by the keyboard controller into the scan codes used by the original keyboard before they become visible to programs. Scatter/Gather A technique in which the contiguous data of a disk sector or sectors is transferred to or from multiple non-contiguous areas of memory. When reading into multiple areas of memory, this is called a scatter-read; the opposing operation is called gather-write. Scatter-Read see Scatter/Gather SCSI (Small Computer Systems Interface) A system-independent expansion bus typically used to connect hard disks, tape drives, and CD-ROMs to a computer. A host adapter connects the SCSI bus to the computer's own bus. See also ESDI, IDE. SDA see Swappable Data Area Sector The smallest addressable unit of data on a disk; under MS-DOS, this is normally 512 bytes. See also Track. SFT see System File Table SMBus (System Management Bus) A derivative of the I2C bus used for communication between various components of a computer, such as smart batteries and their chargers. In contrast to I2C, SMBus specifies fixed voltage levels (instead of relative to the power supply voltage) and a 10 KHz minimum clock rate (I2C minimum is 0). SMBus also specifies several timings which are not required by I2C. See also I2C, ACCESS.bus. SMM (System Management Mode) A special CPU mode typically invoked on changes in power-supply status. In this mode, additional hidden memory becomes available for storing the CPU's state and a control program to deal with the needs of power management or other critical events. SPOOL (Simultaneous Peripheral Operation OnLine) The process of performing output to a slow peripheral such as a printer while other tasks continue running on the CPU. This term dates back to mainframe days before the invention of timesharing. SQL (Structured Query Language) SRAM (Static Random Access Memory) RAM which typically consists of one flip-flop per bit of memory. Unlike DRAMs, static RAM retains its contents as long as power is applied. Because there is no need to refresh the contents of memory addresses which are read, SRAM is faster than DRAM, but it is more expensive and typically is available in much smaller sizes than DRAM because each bit occupies more space on the chip. See also DRAM. SVGA (Super VGA) A video adapter capable of higher resolution (pixels and/or colors) than the 320x200x256 and 640x480x16 which IBM's VGA adapter is capable of producing. See also VESA. Swappable Data Area The portion of the DOS data segment containing all of the variables used internally by DOS to record the state of a function call in progress. See also INT 21h Function 5D06h and INT 21h Function 5D0Bh. System File Table A System File Table is a DOS-internal data structure used to maintain the state of an open file for the DOS 2+ handle functions, just as an FCB maintains the state for DOS 1.x functions. See also INT 21h Function 52h. TCP (Transmission Control Protocol) A higher level (session layer) of the TCP/IP protocol suite. See also IP, TCP/IP. TCP/IP The protocol suite originally developed by DARPA for use on its ARPAnet network, which is now known as the Internet. See also IP, TCP. TSR (Terminate and Stay Resident) A program which remains in memory after terminating in order to provide services to other programs or the user. The name comes from the name of the DOS function call used to remain in memory after termination. Track One of multiple concentric circular rings of data on a single data-bearing surface of a disk. Tracks at the same location on different surfaces form a cylinder. UART (Universal Asyncronous Receiver/Transmitter) A peripheral device which converts a sequence of bytes into a serial bit stream at a programmable rate, and vice versa. This device is what makes an RS-232 serial port function. See also USART. UMB see Upper Memory Block UNC (Universal Naming Convention) The standard way of describing network servers and their directories under MS-DOS and Windows NT. A name in UNC format consists of two backslashes followed by the server name, optionally followed by another backslash and a list of backslash-separated fields; for example \\SERVER1\SHARED-DIR\SUBDIR1\SUBDIR2\FILENAME.EXT. undocumented Information about a product which is not publicly available from the manufacturer, and must be determined by reverse-engineering (disassembly, trial-and-error, etc.). Undocumented information tends to change -- often dramatically -- between successive revisions of a product, since the manufacturer has no obligation to maintain compatibility in behavior which is not explicitly stated. Universal Serial Bus A high-speed serial interconnection providing data rates as high as 48 MHz (6 MB/s) between various devices inside (or even outside of) a PC. See also I2C. Upper Memory Block A noncontiguous section of allocatable memory located between the 640K and 1024K addresses. See also INT 21h Function 52h. USART (Universal Synchronous/Asynchronous Receiver/Transmitter) see also UART USB see Universal Serial Bus V86 see Virtual-86 Mode VCPI see Virtual Control Program Interface VDM see Virtual DOS Machine VDS see Virtual DMA Specification VDU (Video Display Unit) Mainframe-speak for computer monitor. Vertical Retrace When a monitor has finished displaying an image by sweeping its electron beam(s) over the face of the CRT, it has to move the beam back up to the top of the display. During the time this takes, the beam is turned off. The vertical retrace interval is a good time to change the displayed picture for smooth animation. See also Horizontal Retrace. VESA (Video Electronics Standards Association) An industry group which sets both hardware and software standards and recommendations. The term VESA is also used to denote compliance with the VESA SuperVGA BIOS Extensions, a standard set of video BIOS functions for accessing video modes of higher resolution than those defined by IBM. VGA (Video Graphics Array) The video adapter introduced with the IBM PS/2 series of computers. Virtual-86 Mode One of the operating modes of the 80386 and 80486 processors in which user programs run as if the CPU were in Real Mode, while providing the protection and address-mapping capabilities of Protected Mode to a supervisor program which oversees the virtual machine on which the user programs are running. This mode is called Virtual-86 because one or more virtual 8086 environments are run on a single CPU. See also Protected Mode, Real Mode, Virtual Machine. Virtual Control Program Interface A simple API for protected-mode programs to allocate memory and switch into or out of protected mode. See also DOS Protected-Mode Interface. Virtual DMA Specification A set of interrupt calls which permit the use of DMA even on systems running in protected or Virtual-86 mode with their address remapping, or systems such as Micro Channel PS/2s with multiple bus masters independently performing DMA operations. Virtual DOS Machine A special type of virtual machine provided by OS/2 version 2.0, in which a copy of MS-DOS or an MS-DOS compatible operating system (or even an incompatible 8086 operating system) is run and appears to have full control of the system. See also Virtual Machine, INT 21h Function 64h. Virtual Machine One method for multitasking programs is to virtualize the CPU and other hardware, giving the appearance of sole possession of the system to each program being run. Such a virtualized environment is called a virtual machine. See also Virtual-86 Mode, Virtual DOS Machine. virus A program which attaches itself to other programs for the purpose of duplicating itself. Viruses often (but not always) contain harmful code which is triggered by some event, after a certain number of reproductions, or on a specific date. See also worm. VM see Virtual Machine VxD A virtual device driver for Windows 3.x or 95. So called because nearly all of the Windows 3.0 drivers had names of the form "VdeviceD". See also device driver. WORM (Write Once, Read Many) A storage medium which may be written exactly once, but may not be altered once data is stored. worm A program which duplicates itself, typically across networks. In contrast to a virus, a worm does not attach itself to other programs, but can reproduce itself independently. See also virus. Write-Through One of two main types of caches, the write-through cache immediately writes any new information to the medium it is caching, so that the cache never contains information which is not already present on the cached device. See also cache, XBDA see Extended BIOS Data Area XDI see External Device Interface XGA (Extended Graphics Array) XMS see Extended Memory Specification YIQ see YUV YUV A color specification mechanism used in NTSC-type color television signals. Y represents luminance (overall brightness, the only part of the signal used by black-and-white televisions), while U and V are chrominance (color) information. Also called YIQ. See also RGB. --- end of file ---