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.

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