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.