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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