This Glossary contains computer and electronics terms that
are applicable to the subject matter in this book. This
Glossary is meant to be as comprehensive as possible on the
subject of upgrading or repairing PCs. Many terms correspond
to the latest technology in disk interfaces, modems, video and
display equipment, and many standards that govern the PC
industry. Although a glossary is a resource not designed to be
read from beginning to end, you should find that scanning
through this one is interesting, if not enlightening, with
respect to some of the newer PC technology.
The computer industry is filled with acronyms used as
shorthand for a number of terms. This Glossary defines many
acronyms, as well as the term on which the acronym is based.
The definition of an acronym usually is included under the
acronym. For example, Video Graphics Array is defined
under the acronym VGA rather than under Video
Graphics Array. This organization makes it easier to look
up a term--IDE, for example--even if you do not know in
advance what it stands for (Integrated Drive Electronics).
For additional reference, Que's Computer User's
Dictionary is a comprehensive, general-purpose computer
dictionary of computer terminology.
The Web sites can also help you with terms that are not
included in this chapter:
http://zeppo.cnet.com/Resources/Info/Glossary/
http://www-edlab.ucdavis.edu/ed180/hardwarepracticum.html
10Base2 IEEE standard for baseband
Ethernet at 10Mbps over RG-58 coaxial cable to a maximum
distance of 185 meters. Also known as Thin
Ethernet.
10Base5 IEEE standard for baseband
Ethernet at 10Mbps over thick coaxial cable to a maximum
distance of 500 meters. Also known as Thick
Ethernet.
10BaseT A 10Mbps CSMA/CD Ethernet local
area network that works on Category 3 or better twisted-pair
wiring that is very similar to standard telephone cabling. The
10BaseT Ethernet local area networks work on a "star"
configuration in which the wire from each workstation routes
directly to a 10BaseT hub. Hubs may be joined together.
100BaseT A 100Mbps CSMA/CD Ethernet local
area network that works on Category 5 twisted-pair wiring. The
100BaseT Ethernet local area networks work on a "star"
configuration in which the wire from each workstation routes
directly to a central 100BaseT hub. This is the new standard
for 100Mbps Ethernet.
100BaseVG The joint Hewlett
Packard-AT&T proposal for Fast Ethernet running at 100
million bits per second. It uses four pairs of Category 5
cable, using the 10BaseT twisted-pair wiring scheme to
transmit or receive. The 100BaseVG splits the signal across
the four wire pairs at 25MHz each. This standard has not found
favor with corporations and has been almost totally replaced
by 100BaseT.
80286 An Intel microprocessor with 16-bit
registers, a 16-bit data bus, and a 24-bit address bus. Can
operate in real and protected virtual modes.
80287 An Intel math coprocessor designed
to perform floating-point math with much greater speed and
precision than the main CPU. The 80287 can be installed in
most 286- and some 386DX-based systems, and it adds more than
50 new instructions to what is available in the primary CPU
alone.
80386 See 80386DX.
80386DX An Intel microprocessor with
32-bit registers, a 32-bit data bus, and a 32-bit address bus.
This processor can operate in real, protected virtual, and
virtual real modes.
80386SX An Intel microprocessor with
32-bit registers, a 16-bit data bus, and a 24-bit address bus.
This processor, designed as a low-cost version of the 386DX,
can operate in real, protected virtual, and virtual real
modes.
80387 An Intel math coprocessor designed
to perform floating-point math with much greater speed and
precision than the main CPU. The 80387 can be installed in
most 386DX-based systems, and it adds more than 50 new
instructions to what is available in the primary CPU
alone.
80486 See 80486DX.
80486DX An Intel microprocessor with
32-bit registers, a 32-bit data bus, and a 32-bit address bus.
The 486DX has a built-in cache controller with 8K of cache
memory as well as a built-in math coprocessor equivalent to a
387DX. The 486DX can operate in real, protected virtual, and
virtual real modes.
80486DX2 A version of the 486DX with an
internal clock doubling circuit that causes the chip to run at
twice the motherboard clock speed. If the motherboard clock is
33MHz, the DX2 chip will run at 66MHz. The DX2 designation
applies to chips sold through the OEM market, while a retail
version of the DX2 is sold as an overdrive processor.
80486DX4 A version of the 486DX with an
internal clock-tripling circuit that causes the chip to run at
three times the motherboard clock speed. If the motherboard
clock is 33.33MHz, the DX4 chip will run at 100MHz.
80486SX An Intel microprocessor with
32-bit registers, a 32-bit data bus, and a 32-bit address bus.
The 486SX is the same as the 486DX except that it lacks the
built-in math coprocessor function and was designed as a
low-cost version of the 486DX. The 486SX can operate in real,
protected virtual, and virtual real modes. Many 80486SX chips
were 80486DX chips whose floating-point processor failed
Intel's tests.
8086 An Intel microprocessor with 16-bit
registers, a 16-bit data bus, and a 20-bit address bus. This
processor can operate only in real mode.
8087 An Intel math coprocessor designed
to perform floating-point math with much greater speed and
precision than the main CPU. The 8087 can be installed in most
8086- and 8088-based systems and adds more than 50 new
instructions to what is available in the primary CPU
alone.
8088 An Intel microprocessor with 16-bit
registers, an 8-bit data bus, and a 20-bit address bus. This
processor can operate only in real mode and was designed as a
low- cost version of the 8086.
8514/A An analog video display adapter
from IBM for the PS/2 line of personal computers. Compared to
previous display adapters such as EGA and VGA, it provides a
high resolution of 1,024x768 pixels with as many as 256 colors
or 64 shades of gray. It provides a video coprocessor that
performs two-dimensional graphics functions internally, thus
relieving the CPU of graphics tasks. It uses an interlaced
monitor and scans every other line whenever the screen is
refreshed.
abend Short for abnormal end. Used
when the execution of a program or task is terminated
unexpectedly because of a bug or crash.
AC Alternating current. The frequency is
measured in cycles per seconds (cps) or hertz (Hz). The
standard value running through a wall outlet is 120 volts at
60Hz through a fuse or circuit breaker that usually can handle
about 20 amps.
accelerator board An add-in board
replacing the computer's CPU with circuitry that enables the
system to run faster.
access time The time that elapses from
the instant information is requested to the point that
delivery is completed. Usually described in nanoseconds (ns)
for memory chips. The IBM PC requires memory chips with an
access time of 200 ns, and the AT requires 150 ns chips. For
hard disk drives, access time is described in milliseconds
(ms). Most manufacturers rate average access time on a hard
disk as the time required for a seek across one-third of the
total number of cylinders plus one-half of the time for a
single revolution of the disk platters (latency).
accumulator A register (temporary
storage) where the result of an operation is formed.
active high Designates a digital signal
that has to go to a high value to produce an effect.
Synonymous with positive true.
active low Designates a digital signal
that has to go to a low value to produce an effect. Synonymous
with negative true.
actuator The device that moves a disk
drive's read/write heads across the platter surfaces. Also
known as an access mechanism.
adapter The device that serves as an
interface between the system unit and the devices attached to
it. Used by IBM to be synonymous with circuit board, circuit
card, or card.
adapter description files (ADF) Refers to
the setup, configuration files, and drivers necessary to
install an adapter card, such as a network adapter card.
address bus One or more electrical
conductors used to carry the binary-coded address from the
microprocessor throughout the rest of the system.
address Refers to where a particular
piece of data or other information is found in the computer.
Also can refer to the location of a set of instructions.
aliasing Undesirable visual effects
(sometimes called artifacts) in computer-generated
images caused by inadequate sampling techniques. The most
common effect is jagged edges along diagonal or curved object
boundaries. See dithering.
alphanumeric characters A character set
that contains only letters (A-Z) and digits (0-9). Other
characters, such as punctuation marks, also may be
allowed.
ampere The basic unit for measuring
electrical current. Also called amp.
analog loopback A modem self-test in
which data from the keyboard is sent to the modem's
transmitter, modulated into analog form, looped back to the
receiver, demodulated into digital form, and returned to the
screen for verification.
analog signals Continuously variable
signals in which the slightest change may be significant.
Analog circuits are more subject to distortion and noise but
are capable of handling complex signals with relatively simple
circuitry. An alternative to analog is digital, in
which signals are in only one of two states.
analog video A video signal represented
by an infinite number of smooth gradations between given video
levels. By contrast, a digital video signal assigns a finite
set of levels. See also digital video.
analog The representation of numerical
values by physical variables such as voltage, current, and so
on; continuously variable quantities whose values correspond
to the quantitative magnitude of the variables.
analog-to-digital converter An electronic
device that converts analog signals to digital form.
AND gate A logic gate in which the output
is 1 only if all inputs are 1.
AND A logic operator having the property
that if P is a statement, Q is a statement, R is a statement,
..., then the AND of P, Q, R, ... is true if all statements
are true and is false if any statement is false.
animation The process of displaying a
sequential series of still images to achieve the effect of
continuous motion.
ANSI American National Standards
Institute. A non-governmental organization founded in 1918 to
propose, modify, approve, and publish data processing
standards for voluntary use in the United States. Also, the
U.S. representative to the International Standards
Organization (ISO) in Paris and the International
Electrotechnical Commission (IEC). For more information, see
the vendor list. Contact ANSI, 1430 Broadway, New York, NY
10018.
answer mode A state in which the modem
transmits at the predefined high frequency of the
communications channel and receives at the low frequency. The
transmit/receive frequencies are the reverse of the calling
modem, which is in originate mode.
anti-aliasing Software adjustment to make
diagonal or curved lines appear smooth and continuous in
computer-generated images. See also aliasing.
APA All points addressable. A mode in
which all points of a displayable image can be controlled by
the user or a program.
API Application Program Interface. A
system call (routine) that gives programmers access to the
services provided by the operating system. In IBM-compatible
systems, the ROM BIOS and DOS together present an API that a
programmer can use to control the system's hardware.
APM Advanced Power Management. A
specification sponsored by Intel and Microsoft originally
proposed to extend the life of batteries in battery-powered
computers. The APM enables application programs, the system
BIOS, and the hardware to work together to reduce power
consumption. An APM-compliant BIOS provides built-in power
management services to the operating system. The application
software communicates power-saving data via predefined APM
interfaces.
arbitration A method by which multiple
devices attached to a single bus can bid or arbitrate to get
control of that bus.
archive bit The bit in a file's attribute
byte that sets the archive attribute. Tells whether the file
has been changed since it last was backed up.
archive medium A storage medium (floppy
disk, tape cartridge, or removable cartridge) to hold files
that need not be accessible instantly.
ARCnet Attached Resource Computer
Network. A baseband, token-passing local area network
technology offering a flexible bus/star topology for
connecting personal computers. Operating at 2.5Mbit/sec, it is
one of the oldest LAN systems and was popular in low-cost
networks. Originally developed by John Murphy of Datapoint
Corporation, although ARCnet interface cards are available
from a variety of vendors.
areal density A calculation of the bit
density (bits per inch, or BPI) multiplied by the track
density (tracks per inch, or TPI), which results in a figure
indicating how many bits per square inch are present on the
disk surface.
ARQ Automatic repeat request. A general
term for error-control protocols that feature error detection
and automatic retransmission of defective blocks of data.
ASCII character A 1-byte character from
the ASCII character set, including alphabetic and numeric
characters, punctuation symbols, and various graphics
characters.
ASCII American Standard Code for
Information Interchange. A standard 7-bit code created in 1965
by Robert W. Bemer to achieve compatibility among various
types of data processing equipment. The standard ASCII
character set consists of 128 decimal numbers, ranging from 0
through 127, assigned to letters, numbers, punctuation marks,
and the most common special characters. In 1981, IBM
introduced the extended ASCII character set with the IBM PC,
extending the code to 8 bits and adding characters from 128
through 255 to represent additional special mathematical,
graphical, and foreign characters.
ASME American Society of Mechanical
Engineers (http://www.asme.org/).
ASME International has nearly 600 codes and standards in
print, and its many committees involve more than 3,000
individuals, mostly engineers but not necessarily members of
the society. The standards are used in more than 90 countries
throughout the world.
aspect ratio The measurement of a film or
television viewing area in terms of relative height and width.
The aspect ratio of most modern motion pictures varies from
3:5 to as large as 3:7, which creates a problem when a
wide-format motion picture is transferred to the more
square-shaped television screen, which has an aspect ratio of
3:4.
assemble To translate a program expressed
in an assembler language into a computer machine language.
assembler language A computer-oriented
language whose instructions are usually in one-to-one
correspondence with machine language instructions.
asymmetrical modulation A duplex
transmission technique that splits the communications channel
into one high-speed channel and one slower channel. During a
call under asymmetrical modulation, the modem with the
greatest amount of data to transmit is allocated the
high-speed channel. The modem with less data is allocated the
slow, or back, channel (450 bps). The modems dynamically
reverse the channels during a call if the volume of data
transfer changes.
asynchronous communication Data
transmission in which the length of time between transmitted
characters may vary. Timing is dependent on the actual time
for the transfer to take place, as opposed to synchronous
communication, which is timed rigidly by an external clock
signal. Because the receiving modem must be signaled when the
data bits of a character begin and end, start and stop bits
are added to each character.
ATA The AT Attachment interface. An IDE
disk interface standard introduced in March 1989 that defines
a compatible register set and a 40-pin connector and its
associated signals. See also IDE.
ATA-2 The second-generation AT Attachment
interface specification. This version defines faster transfer
modes and Logical Block Addressing schemes to allow high-
performance large-capacity drives. Also called Fast
ATA, Fast ATA-2, and Enhanced IDE
(EIDE).
ATAPI The AT Attachment Packet Interface.
A specification that defines device-side characteristics for
an IDE-connected peripheral, such as CD-ROM or tape drives.
ATAPI is essentially an adaptation of the SCSI command set to
the IDE interface.
ATM Asynchronous Transfer Mode. A high
bandwidth, low-delay, packet-like switching and multiplexing
technique. Usable capacity is segmented into fixed-size cells,
consisting of header and information fields allocated to
services on demand. ATM will be the basis for the future
broadband network, in view of its flexibility and suitability
for both transmission and switching.
attribute byte A byte of information,
held in the directory entry of any file, that describes
various attributes of the file, such as whether it is
read-only or if it has been backed up since it last was
changed. Attributes can be set by the DOS ATTRIB
command.
audio frequencies Frequencies that can be
heard by the human ear (approximately 20 to 20,000Hz).
audio A signal that can be heard, such as
through the speaker of the PC. Many PC diagnostics tests use
both visual (on-screen) codes and audio signals.
auto answer A setting in modems enabling
them to answer incoming calls over the phone lines
automatically.
auto dial A feature in modems enabling
them to dial phone numbers without human intervention.
AUTOEXEC.BAT A special batch file that
DOS executes at startup. Contains any number of DOS commands
that are executed automatically.
automatic head parking Disk drive head
parking performed whenever the drive is powered off. Found in
all hard disk drives with a voice-coil actuator.
average access time The average time it
takes a disk drive to begin reading any data placed anywhere
on the drive. This includes the average seek time,
which is when the heads are moved, as well as the
latency, which is the average amount of time required
for any given data sector to pass underneath the heads.
Together these make up the average access time.
average latency The average time required
for any byte of data stored on a disk to rotate under the disk
drive's read/write head. Equal to one-half the time required
for a single rotation of a platter.
average seek time The average amount of
time it takes to move the heads from one random cylinder
location to another, usually including any head settling time.
In many cases, the average seek time is tested across
one-third of the total number of cylinders for consistency in
measurement.
AVI Audio Video Interleave. A storage
technique developed by Microsoft for its Video for Windows
product that combines audio and video into a single frame or
track, saving valuable disk space and keeping audio
synchronized with the corresponding video.
backup disk Contains information copied
from another disk. Used to make sure that original information
is not destroyed or altered.
backup The process of duplicating a file
or library onto a separate piece of media. Good insurance
against loss of an original.
bad sector A disk sector that cannot hold
data reliably because of a media flaw or damaged format
markings.
bad track table A label affixed to the
casing of a hard disk drive that tells which tracks are flawed
and cannot hold data. The listing is entered into the
low-level formatting program.
balanced signal Refers to signals
consisting of equal currents moving in opposite directions.
When balanced or nearly balanced signals pass through
twisted-pair lines, the electromagnetic interference effects
such as crosstalk caused by the two opposite currents largely
cancel each other out. Differential signaling is a
method that uses balanced signals.
balun Short for
balanced/unbalanced. A type of transformer that enables
balanced cables to be joined with unbalanced cables.
Twisted-pair (balanced) cables, for example, can be joined
with coaxial (unbalanced) cables if the proper balun
transformer is used.
bandwidth Generally, the measure of the
range of frequencies within a radiation band required to
transmit a particular signal. Measures in millions of cycles
per second the difference between the lowest and highest
signal frequencies. The bandwidth of a computer monitor is a
measure of the rate that a monitor can handle information from
the display adapter. The wider the bandwidth, the more
information the monitor can carry and the greater the
resolution. This term is also used to describe the
data-carrying capacity of a given network circuit. The
bandwidth of a network circuit is a measure of the rate at
which a network can handle information. The higher the
bandwidth, the more information the network can carry.
bank The collection of memory chips that
make up a block of memory readable by the processor in a
single bus cycle. This block, therefore, must be as large as
the data bus of the particular microprocessor. In PC systems,
the processor data bus (and therefore the bank size) is
usually 8, 16, 32, or 64 bits wide. Optionally, some systems
also incorporate an optional parity bit for each 8 data bits,
resulting in a total of 9, 18, 36, or 72 bits (respectively)
for each bank.
bar code The code used on consumer
products and inventory parts for identification purposes.
Consists of bars of varying thicknesses that represent
characters and numerals that are read with an optical reader.
The most common version is called the Universal Product
Code (UPC).
baseband The transmission of digital
signals over a limited distance. ARCnet and Ethernet local
area networks use baseband signaling. Contrasts with
broadband transmission, which refers to the
transmission of analog signals over a greater distance.
BASIC Beginner's All-purpose Symbolic
Instruction Code. A popular computer programming language.
Originally developed by John Kemeny and Thomas Kurtz in the
mid-1960s at Dartmouth College. Normally an interpretive
language, meaning that each statement is translated and
executed as it is encountered; can be a compiled language, in
which all the program statements are compiled before
execution.
batch file A set of commands stored in a
disk file for execution by the operating system. A special
batch file called AUTOEXEC.BAT is executed by IBM DOS
each time the system is started. All DOS batch files have a
BAT file extension.
baud rate See baud.
baud A unit of signaling speed denoting
the number of discrete signal elements that can be transmitted
per second. The word baud is derived from the name of
J.M.E. Baudot (1845-1903), a French pioneer in the field of
printing telegraphy and the inventor of Baudot code. Although
technically inaccurate, baud rate commonly is used to
mean bit rate. Because each signal element or baud may
translate into many individual bits, bits per second
(bps) normally differs from baud rate. A rate of 2,400
baud means that 2,400 frequency or signal changes per second
are being sent, but each frequency change may signal several
bits of information. Most people are surprised to learn that
2,400 and 1,200 bps modems transmit at 600 baud, and that
9,600 and 14,400 bps modems transmit at 2,400 baud.
Baudot code A 5-bit code used in many
types of data communications, including teletype (TTY), radio
teletype (RTTY), and telecommunications devices for the deaf
(TDD). Baudot code has been revised and extended several
times.
bay An opening in a computer cabinet that
holds disk drives.
BBS Bulletin board system. A computer
that operates with a program and a modem to enable other
computers with modems to communicate with it, often on a
round-the-clock basis. Thousands of PC IBM- and Apple-related
BBSes offer a wealth of information and public-domain software
that can be downloaded.
Betacam A half-inch video recording
format developed by Sony that offers near 1-inch tape quality
on a portable system.
bezel A cosmetic panel that covers the
face of a drive or some other device.
Bezier curve A mathematical method for
describing a curve, often used in illustration and CAD
programs to draw complex shapes.
bi-directional Refers to lines over which
data can move in two directions, such as a data bus or a
telephone line. Also refers to the capability of a printer to
print from right to left and from left to right
alternately.
binary Refers to the computer numbering
system that consists of two numerals, 0 and 1. Also called
base-2.
BIOS Basic Input/Output System. The part
of an operating system that handles the communications between
the computer and its peripherals. Often burned into read-only
memory (ROM) chips.
bisynchronous Binary synchronous control.
An earlier protocol developed by IBM for software applications
and communicating devices operating in synchronous
envi-ronments. The protocol defines operations at the link
level of communications--for example, the format of data
frames exchanged between modems over a phone line.
bit density Expressed as bits per inch
(BPI). Defines how many bits can be written onto one linear
inch of a track. Sometimes also called linear density.
bit depth The number of bits used to
describe the color of each pixel on a computer display. For
example, a bit depth of two means that the monitor can display
only black and white pixels; a bit depth of four means the
monitor can display 16 different colors; a bit depth of eight
allows for 256 colors, and so on.
bit map A method of storing graphics
information in memory in which a bit devoted to each pixel
(picture element) on-screen indicates whether that pixel is on
or off. A bit map contains a bit for each point or dot on a
video display screen and allows fine resolution because any
point or pixel on-screen can be addressed. A greater number of
bits can be used to describe each pixel's color, intensity,
and other display characteristics.
bit Binary digit. Represented logically
by 0 or 1 and electrically by 0 volts and (typically) 5 volts.
Other methods are used to represent binary digits physically
(tones, different voltages, lights, and so on), but the logic
is always the same.
blank or blanking interval A period in
which no video signal is received by a monitor, while the
videodisc or digital video player searches for the next video
segment or frame to display.
block diagram The logical structure or
layout of a system in graphics form. Does not necessarily
match the physical layout and does not specify all the
components and their interconnections.
block A string of records, words, or
characters formed for technical or logical reasons to be
treated as an entity.
BMP Bit MaP. A Windows graphics format
that may be device-dependent or independent.
Device-independent BMP files (DIB) are coded for translation
to a wide variety of displays and printers.
BNC British National Connector. A type of
connector plug and jack system. Originally designed in England
for television set antennas, the BNC is a type of connector
designed for use with coaxial cabling. Male and female BNCs
are available. Although the term is redundant, BNCs usually
are referred to as BNC connectors. Often used in local
area network cabling systems that use coaxial cable, such as
Ethernet and ARCnet, and also used frequently for video
cabling systems.
Boolean operation Any operation in which
each of the operands and the result take one of two
values.
boot record A one-sector record that
tells the computer's built-in operating system (BIOS) the most
fundamental facts about a disk and DOS. Instructs the computer
how to load the operating system files into memory, thus
booting the machine.
boot To load a program into the computer.
The term comes from the phrase "pulling a boot on by the
bootstrap."
bootstrap A technique or device designed
to bring itself into a desired state by means of its own
action. The term is used to describe the process by which a
device such as a PC goes from its initial power-on condition
to a running condition without human intervention.
bps Bits per second. The number of binary
digits, or bits, transmitted per second. Sometimes confused
with baud.
bridge In local area networks, an
interconnection between two similar networks. Also the
hardware equipment used to establish such an
interconnection.
broadband A term used to describe analog
transmission. Requires modems for connecting terminals and
computers to the network. Using frequency division
multiplexing, many different signals or sets of data can be
transmitted simultaneously. The alternative transmission
scheme is baseband, or digital, transmission.
broadcast quality In the U.S., a standard
of 525 lines of video picture information at a rate of 60Hz.
See also NTSC format.
bubble memory A special type of
nonvolatile read/write memory introduced by Intel where
magnetic regions are suspended in crystal film and data is
maintained when the power is off. A typical bubble memory chip
contains about 512K, or more than 4 million bubbles. Failed to
catch on because of slow access times measured in several
milliseconds. Has found a niche use as solid-state "disk"
emulators in environments in which conventional drives are
unacceptable, such as military or factory use.
buffer A block of memory used as a
holding tank to store data temporarily. Often positioned
between a slower peripheral device and the faster computer.
All data moving between the peripheral and the computer passes
through the buffer. A buffer enables the data to be read from
or written to the peripheral in larger chunks, which improves
performance. A buffer that is x bytes in size usually
holds the last x bytes of data that moved between the
peripheral and CPU. This method contrasts with that of a
cache, which adds intelligence to the buffer so that
the most often accessed data rather than the last accessed
data remains in the buffer (cache). A cache can improve
performance greatly over a plain buffer.
bug An error or a defect in a
program.
burn-in The operation of a circuit or
equipment to establish that components are stable and to
screen for failures.
bus master An intelligent device that,
when attached to the Micro Channel, EISA, VLB, or PCI bus, can
bid for and gain control of the bus to perform its specific
task.
bus A linear electrical signal pathway
over which power, data, and other signals travel. It is
capable of connecting to three or more attachments. A bus is
generally considered to be distinct from radial or
point-to-point signal connections. The term comes from the
Latin omnibus meaning "for all." When used to describe
a topology, bus always implies a linear structure.
byte A collection of bits that makes up a
character or other designation. Generally, a byte is 8 data
bits. When referring to system RAM, an additional parity
(error-checking) bit also is stored (see parity),
making the total 9 bits.
C A high-level computer programming
language frequently used on mainframes, minis, and PC computer
systems.
cache An intelligent buffer. By using an
intelligent algorithm, a cache contains the data that is
accessed most often between a slower peripheral device and the
faster CPU.
CAM Common Access Method. A committee
formed in 1988 that consists of a number of computer
peripheral suppliers and is dedicated to developing standards
for a common software interface between SCSI peripherals and
host adapters. The CAM committee also has set a standard for
IDE drives called the ATA interface.
capacitor A device consisting of two
plates separated by insulating material and designed to store
an electrical charge.
card A printed circuit board containing
electronic components that form an entire circuit, usually
designed to plug into a connector or slot. Sometimes called an
adapter.
carpal tunnel syndrome A painful hand
injury that gets its name from the narrow tunnel in the wrist
that connects the ligament and bone. When undue pressure is
put on the tendons, they can swell and compress the median
nerve, which carries impulses from the brain to the hand,
causing numbness, weakness, tingling, and burning in the
fingers and hands. Computer users get carpal tunnel syndrome
primarily from improper keyboard ergonomics that result in
undue strain on the wrist and hand.
carrier detect signal A modem inte erface
signal that indicates to the attached data terminal equipment
(DTE) that it is receiving a signal from the distant modem.
Defined in the RS-232 specification. Same as the received
line-signal detector.
carrier A continuous-frequency signal
capable of being either modulated or impressed with another
information-carrying signal. The reference signal used for the
transmission or reception of data. The most common use of this
signal with computers involves modem communications over phone
lines. The carrier is used as a signal on which the
information is superimposed.
cathode ray tube (CRT) A device that
contains electrodes surrounded by a glass sphere or cylinder
and displays information by creating a beam of electrons that
strikes a phosphor coating inside the display unit. This
device is most commonly used in computer monitors and
terminals.
CAV Constant Angular Velocity. An optical
disk recording format where the data is recorded on the disk
in concentric circles. CAV disks are rotated at a constant
speed. This is similar to the recording technique used on
floppy disk drives. CAV limits the total recorded capacity
compared to CLV (Constant Linear Velocity), which is also used
in optical recording.
CCITT An acronym for the Comité
Consultatif Internationale de Télégraphique et Téléphonique
(the International Telegraph and Telephone Consultative
Committee). Renamed ITU (International Telecommunications
Union). See ITU.
CCS Common Command Set. A set of SCSI
commands specified in the ANSI SCSI-1 Standard X3.131-1986
Addendum 4.B. All SCSI devices must be capable of using the
CCS in order to be fully compatible with the ANSI SCSI-1
standard.
CD Compact Disc or compact audio disc. A
4.75-inch (12cm) optical disc that contains information
encoded digitally in the constant linear velocity (CLV)
format. This popular format for high-fidelity music offers 90
decibels signal/noise ratio, 74 minutes of digital sound, and
no degradation of quality from playback. The standards for
this format (developed by NV Philips and Sony Corporation) are
known as the Red Book. The official (and rarely-used)
designation for the audio-only format is CD-DA (compact
disc-digital audio). The simple audio format is also known as
CD-A (compact disc-audio). A smaller (3-inch) version of the
CD is known as CD-3.
CD Video A CD format introduced in 1987
that combined 20 minutes of digital audio and 6 minutes of
analog video on a standard 4.75-inch CD. Upon introduction,
many firms renamed 8-inch and 12-inch videodiscs as CDV in an
attempt to capitalize on the consumer popularity of the audio
CD. The term fell out of use in 1990 and was replaced in some
part by laser disc.
CD+G Compact Disc-Graphics. A CD format
that includes extended graphics capabilities as written into
the original CD-ROM specifications. Includes limited video
graphics encoded into the CD subcode area. Developed and
marketed by Warner New Media.
CD+MIDI Compact Disc-Musical Instrument
Digital Interface. A CD format that adds digital audio,
graphics information, and musical instrument digital interface
(MIDI) specifications and capabilities to the CD+G format.
Developed and marketed by Warner New Media.
CD-DA Compact Disc Digital Audio. Also
known as Red Book Audio and is the digital sound format
used by audio CDs. CD-DA uses a sampling rate of 44.1KHz and
stores 16 bits of information for each sample. CD audio is not
played through the computer, but through a special chip in the
CD-ROM drive. Fifteen minutes of CD-DA sound can require about
80M. The highest quality sound that can be used by multimedia
PCs is the CD-DA format at 44.1KHz sample rate. See also
CD.
CD-I Compact Disc-Interactive. A compact
disk format released in October 1991 that provides audio,
digital data, still graphics, and motion video. The standards
for this format (developed by NV Philips and Sony Corporation)
are known as the Green Book.
CD-R Compact Disc Recordable, sometimes
called CD-Writable. CD-R disks are compact discs that can be
recorded and read as many times as desired. CD-R is part of
the Orange Book standard defined by ISO. CD-R technology is
used for mass production of multimedia applications. CD-R
discs can be compatible with CD-ROM, CD-ROM XA, and CD audio.
Orange Book specifies multi-session capabilities, which allows
data recording on the disk at different times in several
recording sessions. Kodak's Photo CD is an example of CD-R
technology, and it fits up to 100 digital photographs on a
single CD. Multi-session capability allows several rolls of
35mm film to be added to a single disc on different
occasions.
CD-ROM Compact Disc-Read Only Memory. A
4.75-inch laser-encoded optical memory storage medium with the
same constant linear velocity (CLV) spiral format as audio CDs
and some videodiscs, CD-ROMs can hold about 650M of data.
CD-ROMs require more error-correction information than the
standard prerecorded compact audio disc. The standards for
this format (developed by NV Philips and Sony Corporation) are
known as the Yellow Book. See also CD-ROM
XA.
CD-ROM drive A device that retrieves data
from a CD-ROM disc; differs from a standard audio CD player by
the incorporation of additional error-correction circuitry.
CD-ROM drives usually can also play music from audio CDs.
CD-ROM XA Compact Disc Read Only Memory
eXtended Architecture. The XA standard was developed jointly
by Sony, Philips, and Microsoft in 1988 and is now part of the
Yellow Book standard. XA is a built-in feature of newer CD-ROM
drives, and it supports simultaneous sound playback with data
transfer. Non-XA drives support either sound playback or data
transfer, but not both simultaneously. XA also provides for
data compression right on the disk, which can also increase
data transfer rates.
CD-WO Compact Disc-Write Once. A variant
on CD-ROM that can be written to once and read many times;
developed by NV Philips and Sony Corporation. Also known as
CD-WORM (CD-write once/read many). Standards for this
format are known as the Orange Book.
CD-WORM See CD-WO.
ceramic substrate A thin, flat, fired
ceramic part used to hold an IC chip (usually made of
beryllium oxide or aluminum oxide).
CERN Conseil Européen pour la Recherche
Nucléaire (The European Laboratory for Particle Physics). The
site in Geneva where the World Wide Web was created in
1989.
CGA Color Graphics Adapter. A type of PC
video display adapter introduced by IBM on August 12, 1981,
that supports text and graphics. Text is supported at a
maximum resolution of 80x25 characters in 16 colors with a
character box of 8x8 pixels. Graphics is supported at a
maximum resolution of 320x200 pixels in 16 colors or 640x200
pixels in two colors. The CGA outputs a TTL (digital) signal
with a horizontal scanning frequency of 15.75KHz and supports
TTL color or NTSC composite displays.
CGI Common Gateway Interface. An API
(Application Programming Interface) for HTTP that provides the
server with the capability to run scripts or compiled
applications when requested.
channel A path along which signals can be
sent.
character A representation, coded in
binary digits, of a letter, number, or other symbol.
checksum Short for summation
check, a technique for determining whether a package of
data is valid. The package, a string of binary digits, is
added up and compared with the expected number.
chip carrier A ceramic or plastic package
that carries an integrated circuit.
chip Another name for an IC, or
integrated circuit. Housed in a plastic or ceramic
carrier device with pins for making electrical
connections.
CHS Cylinder Head Sector. The term used
to describe the non-translating scheme used by the BIOS to
access IDE drives that are less than or equal to 528M in
capacity. See also LBA.
CIF Common Image Format. The standard
sample structure that represents the picture information of a
single frame in digital HDTV, independent of frame rate and
sync/blank structure. The uncompressed bit rate for
transmitting CIF at 29.97 frames/sec is 36.45Mbps.
circuit board A collection of circuits
gathered on a sheet of plastic, usually with all contacts made
through a strip of pins. The circuit board usually is made by
chemically etching metal-coated plastic.
circuit A complete electronic path.
CISC Complex instruction-set computer.
Refers to traditional computers that operate with large sets
of processor instructions. Most modern computers, including
the Intel 80xxx processors, are in this category. CISC
processors have expanded instruction sets that are complex in
nature and require several to many execution cycles to
complete. This structure contrasts with RISC (reduced
instruction-set computer) processors, which have far fewer
instructions that execute quickly.
clean room A dust-free room where certain
electronic components (such as hard disk drives) must be
manufactured and serviced to prevent contamination. Rooms are
rated by Class numbers. A Class 100 clean room must have fewer
than 100 particles larger than 0.5 microns per cubic foot of
space.
clock speed A measurement of the rate at
which the clock signal for a device oscillates, usually
expressed in millions of cycles per second (MHz).
clock The source of a computer's timing
signals. Synchronizes every operation of the CPU.
clone An IBM-compatible computer system
that physically as well as electrically emulates the design of
one of IBM's personal computer systems, usually the AT or XT.
For example, an AT clone has parts (motherboard, power supply,
and so on) that are physically interchangeable with the same
parts in the IBM AT system.
cluster Also called allocation
unit. A group of sectors on a disk that forms a
fundamental unit of storage to the operating system. Cluster
or allocation unit size is determined by DOS when the disk is
formatted.
CLV Constant Linear Velocity. An optical
recording format where the spacing of data is consistent
throughout the disk, and the rotational speed of the disk
varies, depending on what track is being read. Additionally,
more sectors of data are placed on the outer tracks compared
to the inner tracks of the disk, which is similar to Zone
Recording on hard drives. CLV drives will adjust the
rotational speed to maintain a constant track velocity as the
diameter of the track changes. CLV drives rotate faster near
the center of the disk and slower toward the edge. Rotational
adjustment maximizes the amount of data that can be stored on
a disk. CD audio and CD-ROM use CLV recording.
CMOS Complementary Metal-Oxide
Semiconductor. A type of chip design that requires little
power to operate. In an AT-type system, a battery-powered CMOS
memory and clock chip is used to store and maintain the clock
setting and system configuration information.
coated media Hard disk platters coated
with a reddish iron-oxide medium on which data is
recorded.
coaxial cable Also called coax
cable. A data-transmission medium noted for its wide
bandwidth, immunity to interference, and high cost compared to
other types of cable. Signals are transmitted inside a fully
shielded environment, in which an inner conductor is
surrounded by a solid insulating material and then an outer
conductor or shield. Used in many local area network systems
such as Ethernet and ARCnet.
COBOL Common business-oriented language.
A high-level computer programming language. The business
world's preferred programming language on mainframe computer
systems, it has never achieved popularity on smaller
computers.
code page switching A DOS feature in
versions 3.3 and later that changes the characters displayed
on-screen or printed on an output device. Primarily used to
sup- port foreign-language characters. Requires an EGA or
better video system and an IBM-compatible graphics
printer.
CODEC CODer-DECoder. A device that
converts voice signals from their analog form to digital
signals acceptable to more modern digital PBXs and digital
transmission systems. It then converts those digital signals
back to analog so that you can hear and understand what the
other party is saying.
coercivity A measurement in units of
oersteads of the amount of magnetic energy to switch or
"coerce" the flux change in the magnetic recording media.
High-coercivity disk media require a stronger write
current.
Color Graphics Adapter See
CGA.
COM port A serial port on a PC that
conforms to the RS-232 standard. See also RS-232.
COMDEX The largest international computer
trade show and conference in the world. COMDEX/Fall is held in
Las Vegas during October, and COMDEX/Spring usually is held in
Chicago or Atlanta during April.
COMMAND.COM An operating system file that
is loaded last when the computer is booted. The command
interpreter or user interface and program-loader portion of
DOS.
command An instruction that tells the
computer to start, stop, or continue an operation.
common mode noise Noise or electrical
disturbances that can be measured between a current- or
signal-carrying line and its associated ground. Common mode
noise is frequently introduced to signals between separate
computer equipment components through the power distribution
circuits. It can be a problem when single-ended signals are
used to connect different equipment or components that are
powered by different circuits.
common The ground or return path for an
electrical signal. If a wire, it usually is colored black.
compiler A program that translates a
program written in a high-level language into its equivalent
machine language. The output from a compiler is called an
object program.
complete backup A backup of all
information on a hard disk, including the directory tree
structure.
composite video Television picture
information and sync pulses combined. The IBM Color Graphics
Adapter (CGA) outputs a composite video signal.
composite video The complete visual wave
form of the color video signal, composed of chrominance and
luminance picture information; blanking pedestal; field, line,
and color sync pulses; and field equalizing pulses. See also
RGB.
computer Device capable of accepting
data, applying prescribed processes to this data, and
displaying the results or information produced.
computer-based training CBT. The use of a
computer to deliver instruction or training; also known as
Computer-Aided (or Assisted) Instruction (CAI), Computer-Aided
Learning (CAL), Computer-Based Instruction (CBI), and
Computer-Based Learning (CBL).
CONFIG.SYS A file that can be created to
tell DOS how to configure itself when the machine starts up.
Can load device drivers, set the number of DOS buffers, and so
on.
configuration file A file kept by
application software to record various aspects of the
software's configuration, such as the printer it uses.
console The unit in your system, such as
a terminal or a keyboard, with which you communicate with the
computer.
contiguous Touching or joined at the edge
or boundary, in one piece.
continuity In electronics, an unbroken
pathway. Testing for continuity normally means testing to
determine whether a wire or other conductor is complete and
unbroken (by measuring 0 ohms). A broken wire shows infinite
resistance (or infinite ohms).
control cable The wider of the two cables
that connect an ST-506/412 or ESDI hard disk drive to a
controller card. A 34-pin cable that carries commands and
acknowledgments between the drive and controller.
controller card An adapter holding the
control electronics for one or more devices such as hard
disks. Ordinarily occupies one of the computer's slots.
controller The electronics that control a
device such as a hard disk drive and intermediate the passage
of data between the device and the computer.
convergence Describes the capability of a
color monitor to focus the three colored electron beams on a
single point. Poor convergence causes the characters on-screen
to appear fuzzy and can cause the user to have headaches and
eyestrain.
coprocessor An additional computer
processing unit designed to handle specific tasks in
conjunction with the main or central processing unit.
core An "old-fashioned" term for computer
memory.
CP/M Control Program/Microcomputer. An
operating system created by Gary Kildall, the founder of
Digital Research. Created for the old 8-bit microcomputers
that used the 8080, 8085, and Z-80 microprocessors. Was the
dominant operating system in the late 1970s and early 1980s
for small computers used in a business environment.
cps Characters per second. A data
transfer rate generally estimated from the bit rate and the
character length. At 2,400 bps, for example, 8-bit characters
with start and stop bits (for a total of 10 bits per
character) are transmitted at a rate of approximately 240 cps.
Some protocols, such as V.42 and MNP, employ advanced
techniques such as longer transmission frames and data
compression to increase cps.
CPU Central Processing Unit. The
computer's microprocessor chip; the brains of the outfit.
Typically, an IC using VLSI (very-large-scale integration)
technology to pack several different functions into a tiny
area. The most common electronic device in the CPU is the
transistor, of which several thousand to several million or
more are found.
crash A malfunction that brings work to a
halt. A system crash usually is caused by a software
malfunction, and ordinarily you can restart the system by
rebooting the machine. A head crash, however, entails
physical damage to a disk and probable data loss.
CRC Cyclic Redundancy Checking. An
error-detection technique consisting of a cyclic algorithm
performed on each block or frame of data by both sending and
receiving modems. The sending modem inserts the results of its
computation in each data block in the form of a CRC code. The
receiving modem compares its results with the received CRC
code and responds with either a positive or negative
acknowledgment. In the ARQ protocol implemented in high-speed
modems, the receiving modem accepts no more data until a
defective block is received correctly.
crosstalk The electromagnetic coupling of
a signal on one line with another nearby signal line.
Crosstalk is caused by electromagnetic induction, where a
signal traveling through a wire creates a magnetic field that
induces a current in other nearby wires.
CRT Cathode-Ray Tube. A term used to
describe a television or monitor screen tube.
current The flow of electrons, measured
in amperes (amp)s.
cursor The small flashing hyphen that
appears on-screen to indicate the point at which any input
from the keyboard will be placed.
cyclic redundancy checking See
CRC.
cylinder The set of tracks on a disk that
are on each side of all the disk platters in a stack and are
the same distance from the center of the disk. The total
number of tracks that can be read without moving the heads. A
floppy drive with two heads usually has 160 tracks, which are
accessible as 80 cylinders. A typical 4G hard disk has 10
platters with 20 heads (19 for data and one servo head) and
4,000 cylinders, in which each cylinder is composed of 19
tracks.
D/A Converter DAC. A device that converts
digital signals to analog form.
daisy chain Stringing up components in
such a manner that the signals move serially from one to the
other. Most microcomputer multiple-disk drive systems are
daisy-chained. The SCSI bus system is a daisy-chain
arrangement, in which the signals move from computer to disk
drives to tape units, and so on.
daisywheel printer An impact printer that
prints fully formed characters one at a time by rotating a
circular print element composed of a series of individual
spokes, each containing two characters that radiate from a
center hub. Produces letter-quality output.
DAT Digital Audio Tape. A small cassette
tape for storing large amounts of digital information, it is
sometimes called 4mm tape. DAT technology emerged in
Europe and Japan in 1986 as a way to produce high-quality,
digital audio recordings. One DAT cassette can hold anywhere
from 1G to 8G of data.
data cable Generically, a cable that
carries data. Specific to HD connections, the narrower (20
pin) of two cables that connect an ST-506/412 or ESDI hard
disk drive to a controller card.
data communications A type of
communication in which computers and terminals can exchange
data over an electronic medium.
data compression A technique where
mathematical algorithms are applied to the data in a file to
eliminate redundancies and therefore reduce the size of the
file. There are two types of compression: lossy and lossless.
Lossy compression deletes some of the original
(uncompressed) data needed to reconstruct a file and is
normally used only for graphic image or sound files, where the
loss of some resolution or information is acceptable.
Lossless compression completely maintains the integrity
of the original file, allowing it to be reconstructed exactly,
and is most commonly used for program or data files.
data separator A device that separates
data and clock signals from a single encoded signal pattern.
Usually the same device does both data separation and
combination and is sometimes called an endec, or
encoder/decoder.
data transfer rate The maximum rate at
which data can be transferred from one device to another.
data Groups of facts processed into
information. A graphic or textual representation of facts,
concepts, numbers, letters, symbols, or instructions used for
communication or processing.
DC Direct current, such as that provided
by a power supply or batteries.
DC-600 Data Cartridge 600, a data-storage
medium invented by 3M in 1971 that uses a quarter-inch-wide
tape 600 feet in length.
DCE Data Communications Equipment. The
hardware that performs the communication--usually a dial-up
modem that establishes and controls the data link through the
telephone network. See also DTE.
DDE Dynamic Data Exchange. A form of
interprocess communications used by Microsoft Windows to
support the exchange of commands and data between two
applications running simultaneously. This capability has been
enhanced further with Object Linking and Embedding (OLE).
DEBUG The name of a utility program
included with DOS, used for specialized purposes such as
altering memory locations, tracing program execution, patching
programs and disk sectors, and performing other low-level
tasks.
decibel (dB) A logarithmic measure of the
ratio between two powers, voltages, currents, sound
intensities, and so on. Signal-to-noise ratios are expressed
in decibels.
dedicated line A user-installed telephone
line that connects a specified number of computers or
terminals within a limited area, such as a single building.
The line is a cable rather than a public-access telephone
line. The communications channel also may be referred to as
nonswitched because calls do not go through telephone company
switching equipment.
dedicated servo surface In
voice-coil-actuated hard disk drives, one side of one platter
given over to servo data that is used to guide and position
the read/write heads.
default Any setting assumed at start-up
or reset by the computer's software and attached devices and
operational until changed by the user. An assumption the
computer makes when no other parameters are specified. When
you type DIR without specifying the drive to search,
for example, the computer assumes that you want it to search
the default drive. The term is used in software to describe
any action the computer or program takes on its own with
embedded values.
defragmentation The process of
rearranging disk sectors so that files are stored on
consecutive sectors in adjacent tracks.
density The amount of data that can be
packed into a certain area on a specific storage media.
desktop A personal computer that sits on
a desk.
device driver A memory-resident program
loaded by CONFIG.SYS that controls an unusual device, such as
an expanded memory board.
Dhrystone A benchmark program used as a
standard figure of merit, indicating aspects of a computer
system's performance in areas other than floating-point math
performance. Because the program does not use any
floating-point operations, performs no I/O, and makes no
operating system calls, it is most useful for measuring the
processor performance of a system. The original Dhrystone
program was developed in 1984 and was written in Ada, although
the C and Pascal versions became more popular by 1989.
diagnostics Programs used to check the
operation of a computer system. These programs enable the
operator to check the entire system for any problems and to
indicate in what area the problems lie.
differential An electrical signaling
method where a pair of lines are used for each signal in
"push-pull" fashion. In most cases, differential signals are
balanced so that the same current flows on each line in
opposite directions. This is unlike single-ended signals,
which use only one line per signal referenced to a single
ground. Differential signals have a large tolerance for
common-mode noise and little crosstalk when used with
twisted-pair wires, even in long cables. Differential
signaling is expensive because two pins are required for each
signal.
digital loopback A test that checks the
modem's RS-232 interface and the cable that connects the
terminal or computer and the modem. The modem receives data
(in the form of digital signals) from the computer or terminal
and immediately returns the data to the screen for
verification.
digital signals Discrete, uniform
signals. In this book, the term refers to the binary digits 0
and 1.
digitize To transform an analog wave to a
digital signal that a computer can store. Conversion to
digital data and back is performed by a Digital to Analog
Converter (DAC), often a single chip device. How closely a
digitized sample represents an analog wave depends on the
number of times the amplitude of a wave is measured and
recorded (the rate of digitization) as well as the number of
different levels that can be specified at each instance. The
number of possible signal levels is dictated by the resolution
in bits.
DIP Dual Inline Package. A family of
rectangular, integrated-circuit flat packages that have leads
on the two longer sides. Package material is plastic or
ceramic.
DIP switch A tiny switch (or group of
switches) on a circuit board. Named for the form factor of the
carrier device in which the switch is housed.
direct memory access (DMA) A process by
which data moves between a disk drive (or other device) and
system memory without direct control of the central processing
unit, thus freeing up the CPU for other tasks.
directory An area of a disk that stores
the titles given to the files saved on the disk and serves as
a table of contents for those files. Contains data that
identifies the name of a file, the size, the attributes
(system, hidden, read-only, and so on), the date and time of
creation, and a pointer to the location of the file. Each
entry in a directory is 32 bytes long.
disc Flat, circular, rotating medium that
can store various types of information, both analog and
digital. "Disc" is often used in reference to optical storage
media, while "disk" refers to magnetic storage media. Disc is
often used as a short form for videodisc or compact audio disc
(CD).
disk Alternative spelling for "disc" that
generally refers to magnetic storage medium on which
information can be accessed at random. Floppy disks and hard
disks are examples.
diskette An alternate reference to a
floppy disk.
dithering The process of creating more
colors and shades from a given color palette. In monochrome
displays or printers, dithering will vary the black and white
dot patterns to simulate shades of gray. Gray scale dithering
is used to produce different shades of gray when the device
can produce only limited levels of black or white outputs.
Color screens or printers use dithering to create colors by
mixing and varying the dot sizing and spacing.
DLL (dynamic link library) An
executable driver program module for Microsoft Windows that
can be loaded on demand and linked in at runtime and
subsequently unloaded when the driver is no longer needed.
DMA See Direct Memory Access.
DMI Desktop Management Interface. DMI is
an operating system- and protocol- independent standard for
managing desktop systems and servers.
docking station Equipment that enables a
laptop or notebook computer to use peripherals and accessories
normally associated with desktop systems.
DOS Disk Operating System. A collection
of programs stored on the DOS disk that contain routines
enabling the system and user to manage information and the
hardware resources of the computer. DOS must be loaded into
the computer before other programs can be started.
dot pitch A measurement of the width of
the dots that make up a pixel. The smaller the dot pitch, the
sharper the image.
dot-matrix printer An impact printer that
prints characters composed of dots. Characters are printed one
at a time by pressing the ends of selected wires against an
inked ribbon and paper.
double density (DD) An indication of the
storage capacity of a floppy drive or disk in which eight or
nine sectors per track are recorded using MFM encoding. See
MFM.
down-time Operating time lost because of
a computer malfunction.
DPMI DOS Protected Mode Interface. An
industry standard interface that allows DOS applications to
execute program code in the protected mode of the 286 or
higher Intel processor. The DPMI specification is available
from Intel.
DRAM Dynamic Random Access Memory. The
most common type of computer memory, DRAM can be made very
inexpensively compared to other types of memory. DRAM chips
are small and inexpensive because they normally require only
one transistor and a capacitor to represent each bit. The
capacitors must be energized every 15ms or so (hundreds of
times per second) in order to maintain their charges. DRAM is
volatile, meaning it will lose data with no power or without
regular refresh cycles.
drive A mechanical device that
manipulates data storage media.
driver A program designed to interface a
particular piece of hardware to an operating system or other
standard software.
DSM Digital Storage Media. A digital
storage or transmission device or system.
DTE Data Terminal (or Terminating)
Equipment. The device, usually a computer or terminal, that
generates or is the final destination of data. See also
DCE.
duplex Indicates a communications channel
capable of carrying signals in both directions.
DVI Digital Video Interactive. A standard
that was originally developed at RCA Laboratories and sold to
Intel in 1988. The DVI integrates digital motion, still video,
sound, graphics, and special effects in a compressed format.
The DVI is a highly sophisticated hardware compression
technique used in interactive multimedia applications.
Dvorak keyboard A keyboard design by
August Dvorak that was patented in 1936 and approved by ANSI
in 1982. Provides increased speed and comfort and reduces the
rate of errors by placing the most frequently used letters in
the center for use by the strongest fingers. Finger motions
and awkward strokes are reduced by more than 90 percent in
comparison with the familiar QWERTY keyboard. The Dvorak
keyboard has the five vowel keys, AOEUI, together under the
left hand in the center row, and the five most frequently used
consonants, DHTNS, under the fingers of the right hand.
EBCDIC Extended Binary Coded Decimal
Interchange Code. An IBM-developed 8-bit code for the
representation of characters. It allows 256 possible character
combinations within a single byte. EBCDIC is the standard code
on IBM mini-computers and mainframes, but not on the IBM
microcomputers, where ASCII is used instead.
edit The process of rearranging data or
information.
EEPROM Electrically Erasable Programmable
Read Only Memory. A type of non- volatile memory chip used to
store semi-permanent information in a computer such as the
BIOS. An EEPROM can be erased and reprogrammed directly in the
host system without special equipment. This is used so
manufacturers can upgrade the ROM code in a system by
supplying a special program that erases and reprograms the
EEPROM chip with the new code. Also called a Flash
ROM.
EGA Enhanced Graphics Adapter. A type of
PC video display adapter first introduced by IBM on September
10, 1984, that supports text and graphics. Text is supported
at a maximum resolution of 80x25 characters in 16 colors with
a character box of 8x14 pixels. Graphics is supported at a
maximum resolution of 640x350 pixels in 16 (from a palette of
64) colors. The EGA outputs a TTL (digital) signal with a
horizontal scanning frequency of 15.75, 18.432, or 21.85KHz,
and supports TTL color or TTL monochrome displays.
EIA Electronic Industries Association,
which defines electronic standards in the United States.
EIDE Enhanced Integrated Drive
Electronics. A specific Western Digital implementation of the
ATA-2 specification. See also ATA-2.
EISA Extended Industry Standard
Architecture. An extension of the Industry Standard
Architecture (ISA) bus developed by IBM for the AT. The EISA
design was led by Compaq Corporation. Later, eight other
manufacturers (AST, Epson, Hewlett-Packard, NEC, Olivetti,
Tandy, Wyse, and Zenith) joined Compaq in a consortium founded
September 13, 1988. This group became known as the "gang of
nine." The EISA design was patterned largely after IBM's Micro
Channel Architecture (MCA) in the PS/2 systems, but unlike
MCA, EISA allows for backward compatibility with older plug-in
adapters.
electronic mail A method of transferring
messages from one computer to another.
electrostatic discharge (ESD) The
grounding of static electricity. A sudden flow of electricity
between two objects at different electrical potentials. ESD is
a primary cause of integrated circuit damage or failure.
embedded servo data Magnetic markings
embedded between or inside tracks on disk drives that use
voice-coil actuators. These markings enable the actuator to
fine-tune the position of the read/write heads.
EMM Expanded Memory Manager. A driver
that provides a software interface to expanded memory. EMMs
were originally created for expanded memory boards, but can
also use the memory management capabilities of the 386 or
higher processors to emulate an expanded memory board.
EMM386.EXE is an example of an EMM that comes with DOS.
EMS Expanded Memory Specification.
Sometimes also called the LIM spec, because it was
developed by Lotus, Intel, and Microsoft. Provides a way for
microcomputers running under DOS to access additional memory.
EMS memory management provides access to a maximum of 32M of
expanded memory through a small (usually 64K) window in
conventional memory. EMS is a cumbersome access scheme
designed primarily for pre-286 systems that could not access
extended memory.
emulator A piece of test apparatus that
emulates or imitates the function of a particular chip.
encoding The protocol by which data is
carried or stored by a medium.
encryption The translation of data into
unreadable codes to maintain security.
endec (encoder/decoder) A device that
takes data and clock signals and combines or encodes them
using a particular encoding scheme into a single signal for
transmission or storage. The same device also later separates
or decodes the data and clock signals during a receive or read
operation. Sometimes called a data separator.
Energy Star A certification program
started by the Environmental Protection Agency. Energy Star
certified computers and peripherals are designed to draw less
than 30 watts of electrical energy from a standard 110-volt AC
outlet during periods of inactivity. Also called Green
PCs.
Enhanced Graphics Adapter See
EGA.
Enhanced Small Device Interface See
ESDI.
EPROM Erasable Programmable Read-Only
Memory. A type of read-only memory (ROM) in which the data
pattern can be erased to allow a new pattern. Usually is
erased by ultraviolet light and recorded by a
higher-than-normal voltage programming signal.
equalization A compensation circuit
designed into modems to counteract certain distortions
introduced by the telephone channel. Two types are used: fixed
(compromise) equalizers and those that adapt to channel
conditions (adaptive). Good-quality modems use adaptive
equalization.
error control Various techniques that
check the reliability of characters (parity) or blocks of
data. V.42, MNP, and HST error-control protocols use error
detection (CRC) and retransmission of error frames (ARQ).
error message A word or combination of
words to indicate to the user that an error has occurred
somewhere in the program.
ESDI Enhanced Small Device Interface. A
hardware standard developed by Maxtor and standardized by a
consortium of 22 disk drive manufacturers on January 26, 1983.
A group of 27 manufacturers formed the ESDI steering committee
on September 15, 1986, to enhance and improve the
specification. A high-performance interface used primarily
with hard disks, ESDI provides for a maximum data transfer
rate to and from a hard disk of between 10 and 24Mbit/sec.
Ethernet A type of network protocol
developed in the late 1970s by Bob Metcalf at Xerox
Corporation and endorsed by the IEEE. One of the oldest LAN
communications protocols in the personal computing industry,
Ethernet networks use a collision- detection protocol to
manage contention.
expanded memory Otherwise known as EMS
memory, memory that conforms to the EMS specification.
Requires a special device driver and conforms to a standard
developed by Lotus, Intel, and Microsoft.
eXtended graphics array See
XGA.
extended memory Direct
processor-addressable memory that is addressed by an Intel (or
compatible) 286, 386, or 486 processor in the region beyond
the first megabyte. Addressable only in the processor's
protected mode of operation.
extended partition A nonbootable DOS
partition containing DOS volumes. Starting with DOS v3.3, the
DOS FDISK program can create two partitions that serve DOS: an
ordinary, bootable partition (called the primary partition)
and an extended partition, which may contain as many as 23
volumes from D: through Z:.
extra-high density (ED) An indication of
the storage capacity of a floppy drive or disk in which 36
sectors per track are recorded using a vertical recording
technique with MFM encoding.
Fast ATA Fast AT Attachment interface.
Also called Fast ATA-2, these are specific Seagate and
Quantum implementations of the ATA-2 interface. See also
ATA-2.
FAT File Allocation Table. A table held
near the outer edge of a disk that tells which sectors are
allocated to each file and in what order.
FDISK The name of the disk partitioning
program under several operating systems to create the Master
Boot Record and allocate partitions for the operating system's
use.
FIFO First-in first-out. A method of
storing and retrieving items from a list, table, or stack so
that the first element stored is the first one retrieved.
file A collection of information kept
somewhere other than in random-access memory.
file attribute Information held in the
attribute byte of a file's directory entry.
file name The name given to the disk
file. For DOS, it must be one to eight char- acters long and
may be followed by a file name extension, which can be one to
three characters long. Windows 95 eases these constraints by
allowing file names of up to 255 characters.
firmware Software contained in a
read-only memory (ROM) device. A cross between hardware and
software.
fixed disk Also called a hard
disk, a disk that cannot be removed from its controlling
hardware or housing. Made of rigid material with a magnetic
coating and used for the mass storage and retrieval of
data.
Flash ROM A type of EEPROM developed by
Intel that can be erased and reprogrammed in the host system.
See EEPROM.
floating-point unit (FPU) Sometimes
called the math coprocessor; handles the more complex
calculations of the processing cycle.
floppy tape A tape standard that uses
drives connecting to an ordinary floppy disk controller.
flow control A mechanism that compensates
for differences in the flow of data input to and output from a
modem or other device.
FM encoding Frequency modulation
encoding. An outdated method of encoding data on the disk
surface that uses up half the disk space with timing
signals.
folder In a graphical user interface, a
simulated file folder that holds documents (text, data, or
graphics), applications, and other folders. A folder is like a
DOS subdirectory.
form factor The physical dimensions of a
device. Two devices with the same form factor are physically
interchangeable. The IBM PC, XT, and XT Model 286, for
example, all use power supplies that are internally different
but have exactly the same form factor.
FORMAT The DOS format program that
performs both low- and high-level formatting on floppy disks
but only high-level formatting on hard disks.
formatted capacity The total number of
bytes of data that can fit on a formatted disk. The
unformatted capacity is higher because space is lost defining
the boundaries between sectors.
formatting Preparing a disk so that the
computer can read or write to it. Checks the disk for defects
and constructs an organizational system to manage information
on the disk.
FORTRAN Formula translator. A high-level
programming language for programs dealing primarily with
mathematical formulas and expressions similar to algebra and
used primarily in scientific and technical applications. One
of the oldest languages still widely used because of its
compact notation, the many mathematical subroutines available,
and the ease with which arrays, matrices, and loops can be
handled. FORTRAN was written in 1954 by John Backus at IBM;
the first successful FORTRAN program was executed by Harlan
Herrick.
frame buffer A memory device that stores,
pixel by pixel, the contents of an image. Frame buffers are
used to refresh a raster image. Sometimes they incorporate
local processing ability. The "depth" of the frame buffer is
the number of bits per pixel, which determines the number of
colors or intensities that can be displayed.
frame rate The speed at which video
frames are scanned or displayed--30 frames per second for
NTSC, 25 frames a second for PAL/SECAM.
frame A data communications term for a
block of data with header and trailer information attached.
The added information usually includes a frame number, block
size data, error-check codes, and start/end indicators.
frame A single, complete picture in a
video or film recording. A video frame consists of two
interlaced fields of either 525 lines (NTSC) or 625 lines
(PAL/SECAM), running at 30 frames per second (NTSC) or 25
frames per second (PAL/SECAM).
FTP File Transfer Protocol. A method of
transferring files over the Internet. FTP can be used to
transfer files between two machines on which the user has
accounts. Anonymous FTP can be used by a user to retrieve a
file from a server without having an account on that
server.
full duplex Signal flow in both
directions at the same time. In microcomputer communications,
also may refer to the suppression of the online local
echo.
full-height drive A drive unit that is
3.25 inches high, 5.75 inches wide, and 8 inches deep.
full-motion video A video sequence
displayed at full television standard resolutions and frame
rates. In the U.S., this would equate to NTSC video at 30
frames per second.
function keys Special-purpose keys that
can be programmed to perform various operations. Serve many
different functions, depending on the program being used.
gas-plasma display Commonly used in
portable systems, a type of display that operates by exciting
a gas, usually neon or an argon-neon mixture, through the
application of a voltage. When sufficient voltage is applied
at the intersection of two electrodes, the gas glows an
orange-red. Because gas-plasma displays generate light, they
require no backlighting.
gateway Officially, an
application-to-application conversion program or system. For
example, an e-mail gateway would convert between SMTP
(Internet) e-mail format to MHS (Novell) e-mail format. The
term gateway is also used as a slang term for router. See also
router.
genlocking The process of aligning the
data rate of a video image with that of a digital device to
digitize the image and enter it into computer memory. The
machine that performs this function is known as a
genlock.
GIF Graphics Interchange Format. A
popular raster graphics file format developed by CompuServe
that handles 8-bit color (256 colors) and uses the LZW method
to achieve compression ratios of approximately 1.5:1 to
2:1.
giga A multiplier indicating 1 billion
(1,000,000,000) of some unit. Abbreviated as g or G. When used
to indicate a number of bytes of memory storage, the
multiplier definition changes to 1,073,741,824. One gigabit,
for example, equals 1,000,000,000 bits, and one gigabyte
equals 1,073,741,824 bytes.
gigabyte (G) A unit of information
storage equal to 1,073,741,824 bytes.
Green Book The standard for Compact
Disc-Interactive (CD-I). Philips developed CD-I technology for
the consumer market, to be connected to a television instead
of a computer monitor. CD-I is not a computer system but a
consumer device. CD-I disks require special code and are not
compatible with standard CD-ROMs. A CD-ROM cannot be played on
the CD-I machine, but Red Book audio can be played on CD-I
devices.
GUI Graphical User Interface. A type of
program interface that enables users to choose commands and
functions by pointing to a graphical icon using either a
keyboard or pointing device such as a mouse. Windows and OS/2
are the most popular GUIs available for PC systems.
half duplex Signal flow in both
directions but only one way at a time. In microcomputer
communications, may refer to activation of the online local
echo, which causes the modem to send a copy of the transmitted
data to the screen of the sending computer.
half-height drive A drive unit that is
1.625 inches high and either 5.75 or 4 inches wide and 4 or 8
inches deep.
half toning A process that uses dithering
to simulate a continuous tone image such as a photograph or
shaded drawing using various sizes of dots. Newspapers,
magazines, and many books use half toning. The human eye will
merge the dots to give the impression of gray shades.
hard disk A high-capacity disk storage
unit characterized by a normally non- removable, rigid
substrate media. The platters in a hard disk normally are
constructed of aluminum or glass.
hard error An error in reading or writing
data that is caused by damaged hardware.
hardware Physical components that make up
a microcomputer, monitor, printer, and so on.
HDLC High-Level Data Link Control. A
standard protocol developed by the ISO for software
applications and communicating devices operating in
synchronous environments. Defines operations at the link level
of communications--for example, the format of data frames
exchanged between modems over a phone line.
head actuator The device that moves
read/write heads across a disk drive's platters. Most drives
use a stepper-motor or a voice-coil actuator.
head crash A (usually) rare occurrence in
which a read/write head strikes a platter surface with
sufficient force to damage the magnetic medium.
head parking A procedure in which a disk
drive's read/write heads are moved to an unused track so that
they will not damage data in the event of a head crash or
other failure.
head seek The movement of a drive's
read/write heads to a particular track.
head A small electromagnetic device
inside a drive that reads, records, and erases data on the
media.
heat sink A mass of metal attached to a
chip carrier or socket for the purpose of dissipating
heat.
helical scan A type of recording
technology that has vastly increased the capacity of tape
drives. Invented for use in broadcast systems and now used in
VCRs. Conventional longitudinal recording records a track of
data straight across the width of a single-track tape. Helical
scan recording packs more data on the tape by positioning the
tape at an angle to the recording heads. The heads spin to
record diagonal stripes of information on the tape.
hexadecimal number A number encoded in
base 16, such that digits include the letters A through F as
well as the numerals 0 through 9 (for example, 8BF3, which
equals 35,827 in base 10).
Hi8 video The high-quality extension of
the Video 8 (or 8mm) format, which features higher luminance
resolution.
hidden file A file that is not displayed
in DOS directory listings because the file's attribute byte
holds a special setting.
high density (HD) An indication of the
storage capacity of a floppy drive or disk in which 15 or 18
sectors per track are recorded using MFM encoding.
high sierra format A standard format for
placing files and directories on CD-ROMs, proposed by an ad
hoc committee of computer vendors, software developers, and
CD-ROM system integrators. (Work on the format proposal began
at the High Sierra Hotel at Lake Tahoe, Nevada.) A revised
version of the format was adopted by the ISO as ISO 9660.
high-definition television (HDTV) Any one
of a variety of video formats offering greater visual accuracy
(or resolution) than current NTSC, PAL, or SECAM broadcast
standards. Current formats generally range in resolution from
655 to 2,125 scanning lines, having an aspect ratio of 5:3 (or
1.67:1) and a video bandwidth of 30 to 50MHz (5+ times greater
than NTSC standard). Digital HDTV has a bandwidth of 300MHz.
HDTV is subjectively comparable to 35mm film.
high-level formatting Formatting
performed by the DOS FORMAT program. Among other things, it
creates the root directory and FATs.
history file A file created by utility
software to keep track of earlier use of the software. Many
backup programs, for example, keep history files describing
earlier backup sessions.
HMA High Memory Area. The first 64K of
extended memory, which is controlled typically by the
HIMEM.SYS device driver. Real-mode programs can be loaded into
the HMA to conserve conventional memory. Normally DOS 5.0 and
higher use the HMA exclusively to reduce the DOS conventional
memory footprint.
home page A top-level Web document that
relates to an individual or an organization. Other pages in
the document are accessible by links from the home page.
HPT High-pressure tin. A PLCC socket that
promotes high forces between socket contacts and PLCC contacts
for a good connection.
HST High-speed technology. The USRobotics
proprietary high-speed modem-signaling scheme, developed as an
interim protocol until the V.32 protocol could be implemented
in a cost-effective manner. Incorporates trellis-coded
modulation for greater immunity from variable phone-line
conditions and asymmetrical modulation for more efficient use
of the phone channel at speeds of 4,800 bps and above. The
forward channel transmits at either 9,600 bps (older designs)
or 14,400 bps, and the reverse channel transmits at 450 bps.
This technique eliminated the need for the V.32
echo-cancellation hardware that was more costly at the time
HST was developed. HST also incorporates MNP- compatible
error-control procedures adapted to the asymmetrical
modulation.
HTML HyperText Markup Language. A
language used to describe and format plain-text files on the
Web. HTML is based on pairs of tags which enable you to mix
graphics with text, change the appearance of text, and create
hypertext documents with links to other documents.
HTTP HyperText Transfer Protocol. The
protocol that describes the rules that a browser and server
use to communicate over the World Wide Web. HTTP allows a Web
browser to request HTML documents from a Web server.
Huffman coding A technique that minimizes
the average number of bytes required to represent the
characters in a text. Huffman coding works for a given
character distribution by assigning short codes to frequently
occurring characters and longer codes to infrequently
occurring characters.
hypertext A technology that allows for
quick and easy navigation between and within large documents.
Hypertext links are pointers to other sections within the same
document, other documents, or other resources such as FTP
sites, images, or sounds.
Hz An abbreviation for hertz, a frequency
measurement unit used internationally to indicate one cycle
per second.
I/O Input/output. A circuit path that
enables independent communications between the processor and
external devices.
IBMBIO.COM One of the DOS system files
required to boot the machine. The first file loaded from disk
during the boot that contains extensions to the ROM BIOS.
IBMDOS.COM One of the DOS system files
required to boot the machine. Contains the primary DOS
routines. Loaded by IBMBIO.COM, it in turns loads
COMMAND.COM.
IC Integrated circuit. A complete
electronic circuit contained on a single chip. May consist of
only a few transistors, capacitors, diodes, or resistors, or
thousands of them, and generally is classified according to
the complexity of the circuitry and the approximate number of
circuits on the chip. SSI (small-scale integration) equals 2
to 10 circuits. MSI (medium-scale integration) equals 10 to
100 circuits. LSI (large-scale integration) equals 100 to
1,000 circuits. VLSI (very-large-scale integration) equals
1,000 to 10,000 circuits. ULSI (ultra-large-scale integration)
equals more than 10,000 circuits.
IDE Integrated Drive Electronics.
Describes a hard disk with the disk controller circuitry
integrated within it. The first IDE drives commonly were
called hard cards. Also refers to the ATA interface
standard, the standard for attaching hard disk drives to
ISA bus IBM-compatible computers. IDE drives typically operate
as though they were standard ST-506/412 drives. See also
ATA.
incremental backup A backup of all files
that have changed since the last backup.
initiator A device attached to the SCSI
bus that sends a command to another device (the target) on the
SCSI bus. The SCSI host adapter plugged into the system bus is
an example of a SCSI initiator.
inkjet printer A type of printer that
sprays one or more colors of ink on the paper. Can produce
output with quality approaching that of a laser printer at a
lower cost.
input Data sent to the computer from the
keyboard, telephone, video camera, another computer, paddles,
joysticks, and so on.
instruction Program step that tells the
computer what to do for a single operation.
integrated circuit See IC.
interface A communications device or
protocol that enables one device to communicate with another.
Matches the output of one device to the input of the other
device.
interlacing A method of scanning
alternate lines of pixels on a display screen. The odd lines
are scanned first from top to bottom and left to right. The
electron gun goes back to the top and makes a second pass,
scanning the even lines. Interlacing requires two scan passes
to construct a single image. Because of this additional
scanning, interlaced screens often seem to flicker unless a
long persistence phosphor is used in the display.
interleave ratio The number of sectors
that pass beneath the read/write heads before the "next"
numbered sector arrives. When the interleave ratio is 3:1, for
example, a sector is read, two pass by, and then the next is
read. A proper interleave ratio, laid down during low-level
formatting, enables the disk to transfer information without
excessive revolutions due to missed sectors.
internal command In DOS, a command
contained in COMMAND.COM so that no other file must be loaded
in order to perform the command. DIR and
COPY are two examples of internal commands.
internal drive A disk or tape drive
mounted inside one of a computer's disk drive bays (or a hard
disk card, which is installed in one of the computer's
slots).
Internet A computer network that joins
many government, university, and private computers together
over phone lines. The Internet traces its origins to a network
set up in 1969 by the Department of Defense. You can connect
to the Internet through many online services such as
CompuServe, BIX, and America Online. Internet computers use
the TCP/IP communications protocol. There are several million
hosts on the Internet. A host is a mainframe, mini, or
workstation that directly supports the Internet protocol (the
IP in TCP/IP).
interpreter A program for a high-level
language that translates and executes the program at the same
time. The program statements that are interpreted remain in
their original source language, the way the programmer wrote
them--that is, the program does not need to be compiled before
execution. Interpreted programs run slower than compiled
programs and always must be run with the interpreter loaded
into memory.
interrupt A suspension of a process, such
as the execution of a computer program, caused by an event
external to that process and performed in such a way that the
process can be resumed. An interrupt can be caused by internal
or external conditions, such as a signal indicating that a
device or program has completed a transfer of data.
interrupt vector A pointer in a table
that gives the location of a set of instructions that the
computer should execute when a particular interrupt
occurs.
IO.SYS One of the DOS system files
required to boot the machine. The first file loaded from disk
during the boot. Contains extensions to the ROM BIOS.
IPX Internet Packet eXchange. Novell
NetWare's native LAN communications protocol used to move data
between server and/or workstation programs running on
different network nodes. The IPX packets are encapsulated and
carried by the packets used in Ethernet and the similar frames
used in Token-Ring networks.
IRQ Interrupt Request. Physical
connections between external hardware devices and the
interrupt controllers. When a device such as a floppy
controller or a printer needs the attention of the CPU, an IRQ
line is used to get the attention of the system to perform a
task. On PC and XT IBM-compatible systems, eight IRQ lines are
included, numbered IRQ0 through IRQ7. On the AT and PS/2
systems, 16 IRQ lines are numbered IRQ0 through IRQ15. IRQ
lines must be used only by a single adapter in the ISA bus
systems, but Micro-Channel Architecture (MCA) adapters can
share interrupts.
ISDN Integrated Services Digital Network.
An international telecommunications standard that enables a
communications channel to carry digital data simultaneously
with voice and video information.
ISO International Standards Organization.
The ISO, based in Paris, develops standards for international
and national data communications. The U.S. representative to
the ISO is the American National Standards Institute
(ANSI).
ISO 9660 An international standard that
defines file systems for CD-ROM disks, independent of the
operating system. ISO (International Standards Organization)
9660 has two levels. Level one provides for DOS file system
compatibility, while Level two allows file names of up to 32
characters.
ITU International Telecommunications
Union. Formerly called CCITT. An international committee
organized by the United Nations to set international
communications recommendations, which frequently are adopted
as standards, and to develop interface, modem, and data
network recommendations. The Bell 212A standard for 1,200 bps
communication in North America, for example, is observed
internationally as CCITT V.22. For 2,400 bps communication,
most U.S. manufacturers observe V.22bis, while V.32, V.32bis,
V34, and V34+ are standards for 9,600, 14,400, 28,800, and
33,600 bps, respectively. Work is now under way to define a
new standard for 56Kbps.
Java An object-oriented programming
language and environment similar to C or C++. Java was
developed by Sun Microsystems and is used to create
network-based applications.
JEDEC Joint Electron Devices Engineering
Council. A group that establishes standards for the
electronics industry.
J-lead J-shaped leads on chip carriers
that can be surface-mounted on a PC board or plugged into a
socket that then is mounted on a PC board, usually on
.050-inch centers.
JPEG Joint Photographic Experts Group.
The international consortium of hardware, software, and
publishing interests that, under the auspices of the ISO, has
defined a universal standard for digital compression and
decompression of still images for use in computer systems.
JPEG compresses at about a 20:1 ratio before visible image
degradation occurs. It is lossy data compression standard that
was originally designed for still images but can also compress
real-time video (30 frames per second) and animation. Lossy
compression permanently discards unnecessary data, resulting
in some loss of precision.
jumper A small, plastic-covered, metal
clip that slips over two pins protruding from a circuit board.
Sometimes also called a shunt. When in place, the
jumper connects the pins electrically and closes the circuit.
By doing so, it connects the two terminals of a switch,
turning it "on."
Kermit A protocol designed for
transferring files between microcomputers and mainframes.
Developed by Frank DaCruz and Bill Catchings at Columbia
University (and named after the talking frog on The Muppet
Show), Kermit is widely accepted in the academic world.
The complete Kermit protocol manual and the source for various
versions is available from Kermit Distribution at the Columbia
University Center for Computing Activities.
key disk In software copy protection, a
distribution floppy disk that must be present in a floppy disk
drive for an application program to run.
keyboard macro A series of keystrokes
that automatically input when a single key is pressed.
kilo A multiplier indicating one thousand
(1,000) of some unit. Abbreviated as k or K. When used to
indicate a number of bytes of memory storage, the multiplier
definition changes to 1,024. One kilobit, for example, equals
1,000 bits, and one kilobyte equals 1,024 bytes.
kilobyte (K) A unit of information
storage equal to 1,024 bytes.
local area network (LAN) The connection
of two or more computers, usually via a network adapter card
or NIC.
landing zone An unused track on a disk
surface on which the read/write heads can land when power is
shut off. The place that a parking program or a drive with an
autopark mechanism parks the heads.
LAPM Link-Access Procedure for Modems. An
error-control protocol incorporated in CCITT Recommendation
V.42. Like the MNP and HST protocols, uses cyclic redundancy
checking (CRC) and retransmission of corrupted data (ARQ) to
ensure data reliability.
laptop computer A computer system smaller
than a briefcase but larger than a notebook that usually has a
clamshell design in which the keyboard and display are on
separate halves of the system, which are hinged together.
These systems normally run on battery power.
laser printer A type of printer that is a
combination of an electrostatic copying machine and a computer
printer. The output data from the computer is converted by an
interface into a raster feed, similar to the impulses that a
TV picture tube receives. The impulses cause the laser beam to
scan a small drum that carries a positive electrical charge.
Where the laser hits, the drum is discharged. A toner that
also carries a positive charge is then applied to the drum.
This toner, a fine black powder, sticks only to the areas of
the drum that have been discharged electrically. As it
rotates, the drum deposits the toner on a negatively charged
sheet of paper. Another roller then heats and bonds the toner
to the page.
latency The amount of time required for a
disk drive to rotate half of a revolution. Represents the
average amount of time to locate a specific sector after the
heads have arrived at a specific track. Latency is part of the
average access time for a drive.
LBA Logical Block Addressing. A method
used with SCSI and IDE drives to translate the cylinder, head,
and sector specifications of the drive to those usable by an
enhanced BIOS. LBA is used with drives that are larger than
528M and causes the BIOS to translate the drive's logical
parameters to those usable by the system BIOS.
LCC Leadless Chip Carrier. A type of
integrated circuit package that has input and output pads
rather than leads on its perimeter.
LCD Liquid Crystal Display. A display
that uses liquid crystal sealed between two pieces of
polarized glass. The polarity of the liquid crystal is changed
by an electric current to vary the amount of light that can
pass through. Because LCD displays do not generate light, they
depend on either the reflection of ambient light or
backlighting the screen. The best type of LCD, the
active-matrix or thin-film transistor (TFT) LCD, offers fast
screen updates and true color capability.
LED Light-Emitting Diode. A semiconductor
diode that emits light when a current is passed through
it.
LIF Low Insertion Force. A type of socket
that requires only a minimum of force to insert a chip
carrier.
light pen A hand-held input device with a
light-sensitive probe or stylus connected to the computer's
graphics adapter board by a cable. Used for writing or
sketching on-screen or as a pointing device for making
selections. Unlike mice, not widely supported by software
applications.
local bus A generic term used to describe
a bus that is directly attached to a processor that operates
at the processor's speed and data transfer width.
local echo A modem feature that enables
the modem to send copies of keyboard commands and transmitted
data to the screen. When the modem is in command mode (not
online to another system), the local echo normally is invoked
through an ATE1 command, which causes the modem to
display your typed commands. When the modem is online to
another system, the local echo is invoked by an ATF0
command, which causes the modem to display the data it
transmits to the remote system.
logical drive A drive as named by a DOS
drive specifier, such as C: or D:. Under DOS 3.3 or later, a
single physical drive can act as several logical drives, each
with its own specifier.
logical unit number See LUN.
lossless compression A compression
technique that preserves all the original information in an
image or other data structures.
lossy compression A compression technique
that achieves optimal data reduction by discarding redundant
and unnecessary information in an image.
lost clusters Clusters that have been
marked accidentally as "unavailable" in the FAT even though
they don't belong to any file listed in a directory.
low-level formatting Formatting that
divides tracks into sectors on the platter surfaces. Places
sector-identifying information before and after each sector
and fills each sector with null data (usually hex F6).
Specifies the sector interleave and marks defective tracks by
placing invalid checksum figures in each sector on a defective
track.
LUN Logical Unit Number. A number given
to a device (a logical unit) attached to a SCSI physical unit
and not directly to the SCSI bus. Although as many as eight
logical units can be attached to a single physical unit,
normally a single logical unit is a built-in part of a single
physical unit. A SCSI hard disk, for example, has a built-in
SCSI bus adapter that is assigned a physical unit number or
SCSI ID, and the controller and drive portions of the hard
disk are assigned a LUN (usually 0). Also see PUN.
LZW Lempel Zev Welch. A compression
scheme used in the GIF graphic format.
magnetic domain A tiny segment of a track
just large enough to hold one of the magnetic flux reversals
that encode data on a disk surface.
magneto-optical recording An erasable
optical disk recording technique that uses a laser beam to
heat pits on the disk surface to the point at which a magnet
can make flux changes.
master partition boot sector On hard
disks, a one-sector record that gives essential information
about the disk and tells the starting locations of the various
partitions. Always the first physical sector of the disk.
MCA Micro-Channel Architecture. Developed
by IBM for the PS/2 line of computers and introduced on April
2, 1987. Features include a 16- or 32-bit bus width and
multiple master control. By allowing several processors to
arbitrate for resources on a single bus, the MCA is optimized
for multitasking, multiprocessor systems. Offers switchless
configuration of adapters, which eliminates one of the biggest
headaches of installing older adapters.
MCGA MultiColor Graphics Array. A type of
PC video display circuit introduced by IBM on April 2, 1987,
that supports text and graphics. Text is supported at a
maximum resolution of 80x25 characters in 16 colors with a
character box of 8x16 pixels. Graphics are supported at a
maximum resolution of 320x200 pixels in 256 (from a palette of
262,144) colors or 640x480 pixels in two colors. The MCGA
outputs an analog signal with a horizontal scanning frequency
of 31.5KHz and supports analog color or analog monochrome
displays.
MCI Media Control Interface. A
device-independent specification for controlling multimedia
devices and files. MCI is a part of the multimedia extensions
and offers a standard interface set of device control
commands, making it easy to program multimedia applications.
MCI commands are used for audio recording and playback and
animation playback. Videodisk players and other optional
devices are controlled by MCI. Device types include CD audio,
digital audio tape players, scanners, MIDI sequencers,
videotape players or recorders, and audio devices that play
digitized waveform files. MCI classifies compound and simple
device drivers. Compound drivers require a device element
(usually a file and a path) during operation. Simple devices
do not require a device element for playback.
MDA Monochrome Display Adapter. A type of
PC video display adapter introduced by IBM on August 12, 1981,
that supports text only. Text is supported at a maximum
resolution of 80x25 characters in four colors with a character
box of 9x14 pixels. Colors, in this case, indicates black,
white, bright white, and underlined. Graphics modes are not
supported. The MDA outputs a digital signal with a horizontal
scanning frequency of 18.432KHz and supports TTL monochrome
displays. The IBM MDA also included a parallel printer
port.
mean time between failure See
MTBF.
mean time to repair See MTTR.
medium The magnetic coating or plating
that covers a disk or tape.
mega A multiplier indicating 1 million
(1,000,000) of some unit. Abbreviated as m or M. When used to
indicate a number of bytes of memory storage, the multiplier
definition changes to 1,048,576. One megabit, for example,
equals 1,000,000 bits, and one megabyte equals 1,048,576
bytes.
megabyte (M) A unit of information
storage equal to 1,048,576 bytes.
memory caching A service provided by
extremely fast memory chips that keeps copies of the most
recent memory accesses. When the CPU makes a subsequent
access, the value is supplied by the fast memory rather than
by relatively slow system memory.
memory Any component in a computer system
that stores information for future use.
memory-resident program A program that
remains in memory after it has been loaded, consuming memory
that otherwise might be used by application software.
menu software Utility software that makes
a computer easier to use by replacing DOS commands with a
series of menu selections.
MFM Modified Frequency Modulation
encoding. A method of encoding data on the surface of a disk.
The coding of a bit of data varies by the coding of the
preceding bit to preserve clocking information.
MHz An abbreviation for megahertz, a unit
of measurement indicating the frequency of one million cycles
per second. One hertz (Hz) is equal to one cycle per second.
Named after Heinrich R. Hertz, a German physicist who first
detected electromagnetic waves in 1883.
MI/MIC Mode Indicate/Mode Indicate
Common. Also called forced or manual originate.
Provided for installations in which equipment other than the
modem does the dialing. In such installations, the modem
operates in dumb mode (no auto-dial capability), yet must go
off-hook in originate mode to connect with answering
modems.
micro (µ) A prefix indicating one
millionth (1/1,000,000 or .000001) of some unit.
microprocessor A solid-state central
processing unit much like a computer on a chip. An integrated
circuit that accepts coded instructions for execution.
microsecond (µs) A unit of time equal to
one millionth (1/1,000,000 or .000001) of a second.
MIDI Musical Instrument Digital
Interface. An interface and file format standard for
connecting a musical instrument to a microcomputer and storing
musical instrument data. Multiple musical instruments can be
daisy-chained and played simultaneously with the help of the
computer and related software. The various operations of the
instruments can be captured, saved, edited, and played back. A
MIDI file contains note information, timing (how long a note
is held), volume, and instrument type for as many as 16
channels. Sequencer programs are used to control MIDI
functions such as recording, playback, and editing. MIDI files
store only note instructions and not actual sound data.
milli (m) A prefix indicating one
thousandth (1/1,000 or .001) of some unit.
millisecond (ms) A unit of time equal to
one thousandth (1/1,000 or .001) of a second.
MIPS Million Instructions Per Second.
Refers to the average number of machine- language instructions
a computer can perform or execute in one second. Because
different processors can perform different functions in a
single instruction, MIPS should be used only as a general
measure of performance among different types of computers.
mnemonic An abbreviated name for
something, which is used in a manner similar to an acronym.
Computer processor instructions are often abbreviated with a
mnemonic such as JMP (Jump), CLR (Clear), STO (Store), INIT
(Initialize). A mnemonic name for an instruction or an
operation makes it easy to remember and convenient to use.
MNP Microcom Networking Protocol.
Asynchronous error-control and data- compression protocols
developed by Microcom, Inc. and now in the public domain.
Ensure error-free transmission through error detection (CRC)
and retransmission of errored frames. MNP Levels 1 through 4
cover error control and have been incorporated into CCITT
Recommendation V.42. MNP Level 5 includes data compression but
is eclipsed in superiority by V.42bis, an international
standard that is more efficient. Most high-speed modems will
connect with MNP Level 5 if V.42bis is unavailable.
MO Magneto-Optical. MO drives use both
magnetic and optical storage properties. MO technology is
erasable and recordable, as opposed to CD-ROM (Read Only) and
WORM (Write Once) drives. MO uses laser and magnetic field
technology to record and erase data.
modem Modulator-demodulator. A device
that converts electrical signals from a computer into an audio
form transmittable over telephone lines or vice versa.
Modulates or transforms digital signals from a computer into
the analog form that can be carried successfully on a phone
line; also demodulates signals received from the phone line
back to digital signals before passing them to the receiving
computer.
module An assembly that contains a
complete circuit or subcircuit.
Monochrome Display Adapter See
MDA.
morphing Slang term for metamorphosis,
the transformation of one object into another. Morphing is
performed by software that analyzes two images and creates
several in-between images so that one image appears to become
the other. Originally requiring expensive, high-powered
computer hardware, morphing can now be done on PC systems with
sophisticated software now available.
MOS Metal-Oxide Semiconductor. Refers to
the three layers used in forming the gate structure of a
Field-Effect Transistor (FET). MOS circuits offer low power
dissipation and enable transistors to be jammed close together
before a critical heat problem arises. PMOS, the oldest type
of MOS circuit, is a silicon-gate P-channel MOS process that
uses currents made up of positive charges. NMOS is a
silicon-gate N-channel MOS process that uses currents made up
of negative charges and is at least twice as fast as PMOS.
CMOS, Complementary MOS, is nearly immune to noise, runs off
almost any power supply, and is an extremely low-power circuit
technique.
motherboard The main circuit board in the
computer. Also called planar, system board, or
backplane.
MPC A trademarked abbreviation for
Multimedia Personal Computer. The original MPC specification
was developed by Tandy Corporation and Microsoft as the
minimum platform capable of running multimedia software. In
the summer of 1995, the MPC Marketing Council introduced an
upgraded MPC 3 standard. The MPC 1 Specification defines the
following minimum standard requirements: a 386SX or 486 CPU;
2M RAM; 30M hard disk; VGA video display; 8-bit digital audio
subsystem; CD-ROM drive; and systems software compatible with
the Applications Programming Interfaces (APIs) of Microsoft
Windows version 3.1 or higher. The MPC 2 Specification defines
the following minimum standard requirements: a 25MHz 486SX
with 4M RAM; 160M hard disk; 16-bit sound card; 65,536 color
video display; double-speed CD-ROM drive; and systems software
compatible with the APIs of Microsoft Windows version 3.1 or
higher. The MPC 3 Specification defines the following minimum
standard requirements: a 75MHz Pentium with 8M RAM; 540M hard
disk; 16-bit sound card; 65,536 color video display; quad
speed CD-ROM drive; OM-1 compliant MPEG-1 video, and systems
software compatible with the APIs of Microsoft Windows version
3.1 and DOS 6.0 or higher.
MPEG (Motion Picture Experts Group) A
working committee which, under the auspices of the ISO, has
defined standards for lossy digital compression and
decompression of motion video/audio for use in computer
systems. See also lossy. These standards consist of
MPEG-1 and MPEG-2. The MPEG-1 standard delivers decompression
data at 1.2 to 1.5M/sec, allowing CD players to play
full-motion color movies at 30 frames per second. MPEG-1
compresses at about a 50:1 ratio before image degradation
occurs, but compression ratios as high as 200:1 are
attainable. Building on the MPEG-1 standard is MPEG-2, which
extends to the higher data rates (2 to 15Mbps) needed for
signals delivered from remote sources (such as broadcast,
cable, or satellite). MPEG-2 is designed to support a range of
picture aspect ratios, including 4:3 and 16:9. MPEG
compression produces about a 50 percent volume reduction in
file size.
MSDOS.SYS One of the DOS system files
required to boot the machine. Contains the primary DOS
routines. Loaded by IO.SYS, it in turn loads COMMAND.COM.
MTBF Mean Time Between Failure. A
statistically derived measure of the probable time a device
will continue to operate before a hardware failure occurs,
usually given in hours. Because no standard technique exists
for measuring MTBF, a device from one manufacturer can be
significantly more or significantly less reliable than a
device with the same MTBF rating from another
manufacturer.
MTTR Mean Time To Repair. A measure of
the probable time it takes a technician to service or repair a
specific device, usually given in hours.
MultiColor Graphics Array See
MCGA.
multimedia The integration of sound,
graphic images, animation, motion video, and text in one
environment on a computer. It is a set of hardware and
software technologies that is rapidly changing and enhancing
the computer environment.
multitask To run several programs
simultaneously.
multithread To concurrently process more
than one message by an application program. OS/2, Windows 95,
and Windows NT are examples of multithreaded operating
systems. Each program can start two or more threads, which
carry out various interrelated tasks with less overhead than
two separate programs would require.
multiuser system A system in which
several computer terminals share the same central processing
unit (CPU).
nano (n) A prefix indicating one
billionth (1/1,000,000,000 or .000000001) of some unit.
nanosecond (ns) A unit of time equal to
one billionth (1/1,000,000,000 or .000000001) of a second.
network A system in which a number of
independent computers are linked in order to share data and
peripherals, such as hard disks and printers.
nonvolatile memory (NVRAM) Random-access
memory whose data is retained when power is turned off.
Sometimes NVRAM is retained without any power whatsoever, as
in EEPROM or flash memory devices. In other cases, the memory
is maintained by a small battery. NVRAM that is battery
maintained is sometimes called CMOS memory. CMOS
NVRAM is used in IBM-compatible systems to store
configuration information. True NVRAM often is used in
intelligent modems to store a user-defined default
configuration loaded into normal modem RAM at power-up.
nonvolatile RAM disk A RAM disk powered
by a battery supply so that it continues to hold its data
during a power outage.
notebook computer A very small personal
computer approximately the size of a notebook.
NTSC The National Television Standards
Committee, which governs the standard for television and video
playback and recording in the United States. The NTSC was
originally organized in 1941 when TV broadcasting first began
on a wide scale in black and white, and the format was revised
in 1953 for color. The original standard it created was called
RS-170A, which is now simply referred to as NTSC. The NTSC
format has 525 scan lines, a field frequency of 60Hz, a
broadcast bandwidth of 4MHz, line frequency of 15.75KHz, frame
frequency of 1/30 of a second, and a color subcarrier
frequency of 3.58MHz. It is an interlaced signal, which means
that it scans every other line each time the screen is
refreshed. The signal is generated as a composite of red,
green, and blue signals for color and includes an FM frequency
for audio and a signal for stereo. Twenty years later, higher
standards were adopted in Europe with the PAL and SECAM
systems, both incompatible with the NTSC standard of North
America. NTSC is also called composite video.
null modem A serial cable wired so that
two Data Terminal Equipment (DTE) devices, such as personal
computers, or two Data Communication Equipment (DCE) devices,
such as modems or mice, can be connected. Also sometimes
called a modem-eliminator. To make a null-modem cable
with DB-25 connectors, you wire these pins together: 1-1, 2-3,
3-2, 4-5, 5-4, 6-8-20, 20-8-6, and 7-7.
object hierarchy Occurs in a graphical
program when two or more objects are linked and one object's
movement is dependent on the other object. This is known as a
parent-child hierarchy. In an example using a human
figure, the fingers are child objects to the hand, which is a
child object to the arm, which is a child to the shoulder, and
so on. Object hierarchy provides much control for an animator
in moving complex figures.
OCR Optical Character Recognition. An
information-processing technology that converts human-readable
text into computer data. Usually a scanner is used to read the
text on a page, and OCR software converts the images to
characters.
ODI Open Data-link Interface. A device
driver standard from Novell that enables you to run multiple
protocols on the same network adapter card. The ODI adds
functionality to Novell's NetWare and network computing
environments by supporting multiple protocols and drivers.
OEM Original Equipment Manufacturer. Any
manufacturer that sells its product to a reseller. Usually
refers to the original manufacturer of a particular device or
component. Most Compaq hard disks, for example, are made by
Conner Peripherals, which is considered the OEM.
OLE Object Linking and Embedding. An
enhancement to the original Dynamic Data Exchange (DDE)
protocol that allows you to embed or link data created in one
application into a document created in another application and
subsequently edit that data directly from the final
document.
online fallback A feature that enables
high-speed error-control modems to monitor line quality and
fall back to the next lower speed if line quality degrades.
Some modems fall forward as line quality improves.
operating system (OS) A collection of
programs for operating the computer. Operating systems perform
housekeeping tasks such as input and output between the
computer and peripherals as well as accepting and interpreting
information from the keyboard. DOS and OS/2 are examples of
popular OSes.
optical disk A disk that encodes data as
a series of reflective pits that are read (and sometimes
written) by a laser beam.
Orange Book The standard for recordable
compact discs (such as CD-ROM, but recordable instead of
read-only). Recordable compact discs are called CD-R
and are becoming popular with the widespread use of
multimedia. Part of the Orange Book standard defines
rewritable Magneto-Optical disks and another section defines
optical Write Once Read Many (WORM) disks.
originate mode A state in which the modem
transmits at the predefined low frequency of the
communications channel and receives at the high frequency. The
transmit/receive frequencies are the reverse of the called
modem, which is in answer mode.
OS/2 A universal operating system
developed through a joint effort by IBM and Microsoft
Corporation. The latest operating system from IBM for
microcomputers using the Intel 386 or better microprocessors.
OS/2 uses the protected mode operation of the processor to
expand memory from 1M to 4G and to support fast, efficient
multitasking. The OS/2 Workplace Shell, an integral part of
the system, is a graphical interface similar to Microsoft
Windows and the Apple Macintosh system. The latest version
runs DOS, Windows, and OS/2-specific software.
output Information processed by the
computer or the act of sending that information to a mass
storage device such as a video display, a printer, or a
modem.
OverDrive An Intel trademark name for its
line of upgrade processors.
overlay Part of a program that is loaded
into memory only when it is required.
overrun A situation where data moves from
one device faster than a second device can accept it.
overscanning A technique used in consumer
display products that extends the deflection of a CRT's
electron beam beyond the physical boundaries of the screen to
ensure that images will always fill the display area. See also
underscanning.
overwrite To write data on top of
existing data, thus erasing the existing data.
package A device that includes a chip
mounted on a carrier and sealed.
PAL Phase Alternating Line system.
Invented in 1961, a system of TV broadcasting used in England
and other European countries (except France). PAL's image
format is 4:3, 625 lines, 50Hz, and 4MHz video bandwidth with
a total 8MHz of video channel width. With its 625-line picture
delivered at 25 frames per second, PAL provides a better image
and an improved color transmission over the NTSC system used
in North America. PAL also can stand for Programmable Array
Logic, a type of chip that has logic gates specified by a
device programmer.
palmtop computer A computer system
smaller than a notebook that is designed so that it can be
held in one hand while being operated by the other. Many are
now called PDAs or Personal Digital
Assistants.
parallel A method of transferring data
characters in which the bits travel down parallel electrical
paths simultaneously--for example, eight paths for 8-bit
characters. Data is stored in computers in parallel form but
may be converted to serial form for certain operations.
parity A method of error checking in
which an extra bit is sent to the receiving device to indicate
whether an even or odd number of binary 1 bits were
transmitted. The receiving unit compares the received
information with this bit and can obtain a reasonable judgment
about the validity of the character. The same type of parity
(even or odd) must be used by two communicating computers, or
both may omit parity. When parity is used, a parity bit is
added to each transmitted character. The bit's value is 0 or
1, to make the total number of 1s in the character even or
odd, depending on which type of parity is used.
park program A program that executes a
seek to the highest cylinder or just past the highest cylinder
of a drive so that the potential of data loss is minimized if
the drive is moved.
partition A section of a hard disk
devoted to a particular operating system. Most hard disks have
only one partition devoted to DOS. A hard disk can have as
many as four partitions, each occupied by a different
operating system. DOS v3.3 or higher can occupy two of these
four partitions.
Pascal A high-level programming language
named for the French mathematician Blaise Pascal (1623-1662).
Developed in the early 1970s by Niklaus Wirth for teaching
programming and designed to support the concepts of structured
programming.
PC Card (PCMCIA) Personal Computer Memory
Card International Association. A credit card-sized expansion
adapter for notebook and laptop PCs. PC Card is the official
PCMCIA trademark; however, both PC Card and PCMCIA card are
used to refer to these standards. PCMCIA cards are removable
modules that can hold numerous types of devices including
memory, modems, fax/modems, radio transceivers, network
adapters, solid state disks, and hard disks.
Pentium An Intel microprocessor with
32-bit registers, a 64-bit data bus, and a 32-bit address bus.
The Pentium has a built-in Level 1 cache which is segmented
into a separate 8K cache for code and another 8K cache for
data. The Pentium includes an FPU (floating-point unit) or
math coprocessor. The Pentium is backward-compatible with the
486 and can operate in real, protected virtual, and virtual
real modes.
Pentium Pro An Intel microprocessor with
32-bit registers, a 64-bit data bus, and a 36-bit address bus.
The Pentium Pro has the same segmented Level 1 cache as the
Pentium, but also includes a 256K or 512K Level 2 cache on a
separate die in the same module. The Pentium Pro includes a
FPU (floating-point unit) or math coprocessor. The Pentium Pro
is backward-compatible with the Pentium and can operate in
real, protected virtual, and virtual real modes.
Pentium II An Intel Pentium Pro with MMX
capabilities, using Single Edge Contact (SEC) cartridge
packaging technology.
peripheral Any piece of equipment used in
computer systems that is an attachment to the computer. Disk
drives, terminals, and printers are all examples of
peripherals.
PGA Pin-Grid Array. A chip package that
has a large number of pins on the bottom designed for socket
mounting. Also can mean Professional Graphics Adapter, a
limited-production, high-resolution graphics card for XT and
AT systems from IBM.
Photo CD A technology developed by
Eastman Kodak and Philips that stores photographic images on a
CD-R recordable compact disc. Images stored on the Photo CD
may have resolutions as high as 2,048x3,072 pixels. Up to 100
true-color images (24-bit color) can be stored on one disk.
Photo CD images are created by scanning film and digitally
recording the images on compact discs (CDs). The digitized
images are indexed (given a four-digit code), and thumbnails
of each image on the disc are shown on the front of the case
along with its index number. Multi-session capability enables
several rolls of film to be added to a single disk on
different occasions.
physical drive A single disk drive. DOS
defines logical drives, which are given a specifier, such as
C: or D:. A single physical drive may be divided into multiple
logical drives. Conversely, special software can span a single
logical drive across two physical drives.
physical unit number See PUN.
PIF Program Information File. A file that
contains information about a non-Windows application
specifying optimum settings for running the program under
Windows 3.x. These are called property sheets in
Windows 95.
pixel A mnemonic term meaning picture
element. Any of the tiny elements that form a picture on a
video display screen. Also called a pel.
planar board A term equivalent to
motherboard, used by IBM in some of its literature.
plated media Hard disk platters plated
with a form of thin metal film media on which data is
recorded.
platter A disk contained in a hard disk
drive. Most drives have two or more platters, each with data
recorded on both sides.
PLCC Plastic Leaded-Chip Carrier. A
popular chip-carrier package with J-leads around the perimeter
of the package.
Plug and Play (PnP) A hardware and
software specification developed by Intel that enables a PnP
system and PnP adapter cards to automatically configure
themselves. PnP cards are free from switches and jumpers and
are configured via the PnP BIOS in the host system, or via
supplied programs for non-PnP systems.
port Plug or socket that enables an
external device such as a printer to be attached to the
adapter card in the computer. Also a logical address used by a
microprocessor for communications between itself and various
devices.
port address One of a system of addresses
used by the computer to access devices such as disk drives or
printer ports. You may need to specify an unused port address
when installing any adapter boards in a system unit.
portable computer A computer system
smaller than a transportable system but larger than a laptop
system. Most portable systems conform to the lunchbox style
popularized by Compaq or the briefcase style popularized by
IBM, each with a fold-down (removable) keyboard and built-in
display. These systems characteristically run on AC power and
not on batteries, include several expansion slots, and can be
as powerful as full-blown desktop systems.
POS Programmable Option Select. The
Micro-Channel Architecture's POS eliminates switches and
jumpers from the system board and adapters by replacing them
with programmable registers. Automatic configuration routines
store the POS data in a battery-powered CMOS memory for system
configuration and operations. The configuration utilities rely
on Adapter Description Files (ADF) that contain the setup data
for each card.
POST Power-On Self Test. A series of
tests run by the computer at power-on. Most computers scan and
test many of their circuits, sounding a beep from the internal
speaker if this initial test indicates proper system
performance.
PostScript A page-description language
developed primarily by John Warnock of Adobe Systems for
converting and moving data to the laser-printed page. Instead
of using the standard method of transmitting graphics or
character information to a printer and telling it where to
place dots one-by-one on a page, PostScript provides a way for
the laser printer to interpret mathematically a full page of
shapes and curves.
power supply An electrical/electronic
circuit that supplies all operating voltage and current to the
computer system.
PPP Point-to-Point Protocol. A protocol
that enables a computer to use the Internet with a standard
telephone line and a high-speed modem. PPP is a new standard
which replaces SLIP. PPP is less common than SLIP; however, it
is increasing in popularity.
primary partition An ordinary,
single-volume bootable partition. See also extended
partition.
processor speed The clock rate at which a
microprocessor processes data. A standard IBM PC, for example,
operates at 4.77MHz (4.77 million cycles per second).
program A set of instructions or steps
telling the computer how to handle a problem or task.
PROM Programmable Read-Only Memory. A
type of memory chip that can be programmed to store
information permanently--information that cannot be
erased.
proprietary Anything invented by a
company and not used by any other company. Especially applies
to cases in which the inventing company goes to great lengths
to hide the specifications of the new invention. The opposite
of standard.
protected mode A mode available in all
Intel 80286- or 80386-compatible processors. In this mode,
memory addressing is extended to 16 or 4,096M, and restricted
protection levels can be set to trap software crashes and
control the system.
protocol A system of rules and procedures
governing communications between two or more devices.
Protocols vary, but communicating devices must follow the same
protocol in order to exchange data. The data format, readiness
to receive or send, error detection, and error correction are
some of the operations that may be defined in protocols.
PUN Physical Unit Number. A term used to
describe a device attached directly to the SCSI bus. Also
known as a SCSI ID. As many as eight SCSI devices can
be attached to a single SCSI bus, and each must have a unique
PUN or ID assigned from 7 to 0. Normally the SCSI host adapter
is assigned the highest-priority ID, which is 7. A bootable
hard disk is assigned an ID of 0, and other nonbootable drives
are assigned higher priorities.
QAM Quadrature Amplitude Modulation. A
modulation technique used by high-speed modems that combines
both phase and amplitude modulation. This technique enables
multiple bits to be encoded in a single time interval. The
V.32bis standard-codes 6 data bits plus an additional trellis
coding bit for each signal change. An individual signal is
evaluated with respect to phase and amplitude compared to the
carrier wave. A plot of all possible QAM signal points is
referred to as the signal constellation pattern. The V.32bis
constellation pattern has 128 discrete signal points.
QIC Quarter-Inch Committee. An industry
association that sets hardware and software standards for
tape-backup units that use quarter-inch-wide tapes.
QWERTY keyboard The standard typewriter
or computer keyboard, with the characters Q, W, E, R, T, and Y
on the top row of alpha keys. Because of the haphazard
placement of characters, this keyboard can hinder fast
typing.
rails Plastic strips attached to the
sides of disk drives mounted in IBM ATs and compatibles so
that the drives can slide into place. These rails fit into
channels in the side of each disk drive bay position.
RAM Random-Access Memory. All memory
accessible at any instant (randomly) by a microprocessor.
RAM disk A "phantom disk drive" in which
a section of system memory (RAM) is set aside to hold data,
just as though it were a number of disk sectors. To DOS, a RAM
disk looks and functions like any other drive.
random-access file A file in which all
data elements (or records) are of equal length and written in
the file end to end, without delimiting characters between.
Any element (or record) in the file can be found directly by
calculating the record's offset in the file.
Random-Access Memory See RAM.
raster A pattern of horizontal scanning
lines normally on a TV screen. An electromagnetic field causes
the beam of the TV tube to illuminate the correct dots to
produce the required characters.
raster graphics A technique for
representing a picture image as a matrix of dots. It is the
digital counterpart of the analog method used in TV. There are
several raster graphics standards.
RCA jack Also called a phono
connector. A plug and socket for a two-wire coaxial cable
used to connect audio and video components. The plug is a
1/8-inch thick prong that sticks out 5/16-inch from the middle
of a cylinder.
read/write head A tiny magnet that reads
and writes data on a disk track.
read-only file A file whose attribute
setting in the file's directory entry tells DOS not to allow
software to write into or over the file.
read-only memory See ROM.
real mode A mode available in all Intel
8086-compatible processors that enables compatibility with the
original 8086. In this mode, memory addressing is limited to
1M.
real time The actual time in which a
program or event takes place. In computing, real time refers
to an operating mode under which data is received and
processed and the results returned so quickly that the process
appears instantaneous to the user. The term is also used to
describe the process of simultaneous digitization and
compression of audio and video information.
Red Book More commonly known as Compact
Disc Digital Audio (CD-DA) and is one of four compact disc
standards. Red Book got its name from the color of the manual
used to describe the CD Audio specifications. The Red Book
audio standard requires that digital audio is sampled at a
44.1KHz sample rate using 16 bits for each sample. This is the
standard used by audio CDs and many CD-ROMs.
refresh cycle A cycle in which the
computer accesses all memory locations stored by DRAM chips so
that the information remains intact. DRAM chips must be
accessed several times per second, or else the information
fades.
register Storage area in memory having a
specified storage capacity, such as a bit, a byte, or a
computer word, and intended for a special purpose.
remote digital loopback A test that
checks the phone link and a remote modem's transmitter and
receiver. Data entered from the keyboard is transmitted from
the initiating modem, received by the remote modem's receiver,
looped through its transmitter, and returned to the local
screen for verification.
remote echo A copy of the data received
by the remote system, returned to the sending system, and
displayed on-screen. A function of the remote system.
resolution A reference to the size of the
pixels used in graphics. In medium-resolution graphics, pixels
are large. In high-resolution graphics, pixels are small.
RFI Radio Frequency Interference. A high
frequency signal radiated by improperly shielded conductors,
particularly when signal path lengths are comparable to or
longer than the signal wavelengths. The FCC now regulates RFI
in computer equipment sold in the U.S. under FCC Regulations
Part 15, Subpart J.
RGB Red-Green-Blue. A type of computer
color-display output signal comprised of separately
controllable red, green, and blue signals, as opposed to
composite video, where signals are combined prior to
output. RGB monitors typically offer higher resolution than
composite monitors.
RISC An acronym for Reduced Instruction
Set Computer, as differentiated from CISC, Complex Instruction
Set Computer. RISC processors have simple instruction sets
requiring only one or a few execution cycles. These simple
instructions can be used more effectively than CISC systems
with appropriately designed software, resulting in faster
operations.
RLL Run-Length Limited. A type of
encoding that derives its name from the fact that the
techniques used limit the distance (run length) between
magnetic flux reversals on the disk platter. Several types of
RLL encoding techniques exist, although only two are commonly
used. (1,7)RLL encoding increases storage capacity by about 30
percent over MFM encoding and is most popular in the very
highest capacity drives due to a better window margin, while
(2,7)RLL encoding increases storage capacity by 50 percent
over MFM encoding and is used in the majority of RLL
implementations. Most IDE, ESDI, and SCSI hard disks use one
of these forms of RLL encoding.
RMA number Return-Merchandise
Authorization number. A number given to you by a vendor when
you arrange to return an item for repairs. Used to track the
item and the repair.
ROM Read-Only Memory. A type of memory
that has values permanently or semi-permanently burned in.
These locations are used to hold important programs or data
that must be available to the computer when the power
initially is turned on.
ROM BIOS Read Only Memory-Basic Input
Output System. A BIOS encoded in a form of read-only memory
for protection. Often applied to important start-up programs
that must be present in a system for it to operate.
root directory The main directory of any
hard or floppy disk. Has a fixed size and location for a
particular disk volume and cannot be resized dynamically the
way subdirectories can.
router Hardware that routes messages from
one local area network to another. It is used to internetwork
similar and dissimilar networks and can select the most
expedient route based on traffic load, line speeds, costs, and
network failures.
routine Set of frequently used
instructions. May be considered as a subdivision of a program
with two or more instructions that are related
functionally.
RS-232 An interface introduced in August
1969 by the Electronic Industries Association. The RS-232
interface standard provides an electrical description for
connecting peripheral devices to computers.
scan lines The parallel lines across a
video screen, along which the scanning spot travels in
painting the video information that makes up a monitor
picture. NTSC systems use 525 scan lines to a screen; PAL
systems use 625.
scratch disk A disk that contains no
useful information and can be used as a test disk. IBM has a
routine on the Advanced Diagnostics disks that creates a
specially formatted scratch disk to be used for testing floppy
drives.
SCSI Small Computer System Interface. A
standard originally developed by Shugart Associates (then
called SASI for Shugart Associates System Interface) and later
approved by ANSI in 1986.
SCSI-2 Approved in 1994 and is the
currently approved SCSI standard.
SCSI-3 Currently in the development
process. Normally uses a 50-pin connector and permits multiple
devices (up to eight including the host) to be connected in
daisy-chain fashion.
SDLC Synchronous Data Link Control. A
protocol developed by IBM for software applications and
communication devices operating in IBM's Systems Network
Architecture (SNA). Defines operations at the link level of
communications; for example, the format of data frames
exchanged between modems over a phone line.
SECAM Séquential Couleur à Mémoire
(Sequential Color within Memory). The French color TV system
also adopted in Russia. The basis of operation is the
sequential recording of primary colors in alternate lines. The
image format is 4:3, 625 lines, 50Hz, and 6MHz video bandwidth
with a total 8MHz of video channel width.
sector A section of one track defined
with identification markings and an identification number.
Most sectors hold 512 bytes of data.
security software Utility software that
uses a system of passwords and other devices to restrict an
individual's access to subdirectories and files.
seek time The amount of time required for
a disk drive to move the heads across one-third of the total
number of cylinders. Represents the average time it takes to
move the heads from one cylinder to another randomly selected
cylinder. Seek time is a part of the average access time for a
drive.
semiconductor A substance, such as
germanium or silicon, whose conductivity is poor at low
temperatures but is improved by minute additions of certain
substances or by the application of heat, light, or voltage.
Depending on the temperature and pressure, a semiconductor can
control the flow of electricity. Semiconductors are the basis
of modern electronic-circuit technology.
sequencer A software program that
controls MIDI file messages and keeps track of music timing.
Because MIDI files store note instructions instead of actual
sounds, a sequencer is needed to play, record, and edit MIDI
sounds. Sequencer programs allow for recording and playback of
MIDI files by storing the instrument, the note's pitch
(frequency), duration in real time that each note is held, and
loudness (amplitude) of each musical or sound effect note.
sequential file A file in which
varying-length data elements are recorded end-to-end, with
delimiting characters placed between each element. To find a
particular element, you must read the whole file up to that
element.
serial The transfer of data characters
one bit at a time, sequentially, using a single electrical
path.
servo data Magnetic markings written on
disk platters to guide the read/write heads in drives that use
voice-coil actuators.
session (single or multi-session) A term
used in CD-ROM recording to describe a recording event. In a
single session, data is recorded on a CD-ROM disc and an index
is created. If additional space is left on the disc, another
session can be used to record additional files along with
another index. The original index cannot be updated because
recordable CD-ROM drives are normally Write Once Read Many
(WORM) type drives. Many CD-ROM drives do not expect
additional recording sessions and therefore will be unable to
read the additional session data on the disk. The advent of
Kodak's Photo CD propelled the desire for multi-session CD-ROM
XA (extended architecture) drives. The first generation of XA
drives were capable of single-session reads only.
Multi-session CD-ROM XA drives will read all the indices
created when images are recorded many times on the same CD-ROM
XA drive.
settling time The time required for
read/write heads to stop vibrating after they have been moved
to a new track.
shadow ROM A copy of a system's slower
access ROM BIOS placed in faster access RAM, usually during
the start-up or boot procedure. This setup enables the system
to access BIOS code without the penalty of additional wait
states required by the slower ROM chips. Also called shadow
RAM.
shell The generic name of any
user-interface software. COMMAND.COM is the standard shell for
DOS. OS/2 comes with three shells: a DOS command shell, an
OS/2 command shell, and the OS/2 Presentation Manager, a
graphical shell.
shielded twisted pair
(STP) Unshielded Twisted Pair (UTP) network
cabling with a metal sheath or braid around it, usually used
in Token-Ring networks.
shock rating A rating (usually expressed
in G force units) of how much shock a disk drive can sustain
without damage. Usually two different specifications exist for
a drive powered on or off.
signal-to-noise (S/N) ratio The strength
of a video or audio signal in relation to interference
(noise). The higher the S/N ratio, the better the quality of
the signal.
SIMM Single Inline Memory Module. An
array of memory chips on a small PC board with a single row of
I/O contacts.
single-ended An electrical signaling
method where a single line is referenced to a ground path
common to other signals. In a single-ended bus intended for
moderately long distances, there is commonly one ground line
between groups of signal lines to provide some resistance to
signal crosstalk. Single-ended signals only require one driver
or receiver pin per signal, plus one ground pin per group of
signals. Single-ended signals are vulnerable to common mode
noise and crosstalk but are much less expensive than
differential signaling methods.
SIP Single Inline Package. A DIP-like
package with only one row of leads.
skinny dip Twenty-four- and 28-position
DIP devices with .300-inch row-to-row centerlines.
SLIP Serial Line Internet Protocol. An
Internet protocol that is used to run the Internet Protocol
(IP) over serial lines such as telephone circuits. IP enables
a packet to traverse multiple networks on the way to its final
destination.
SMPTE time code An 80-bit standardized
edit time code adopted by SMPTE, the Society of Motion Picture
and Television Engineers. The SMPTE time code is a standard
used to identify individual video frames in the video editing
process. SMPTE time code controls such functions as play,
record, rewind, and forward of video tapes. SMPTE time code
displays video in terms of hours, minutes, seconds, and frames
for accurate video editing.
soft error An error in reading or writing
data that occurs sporadically, usually because of a transient
problem such as a power fluctuation.
software A series of instructions loaded
in the computer's memory that instructs the computer how to
accomplish a problem or task.
SO-J Small Outline J-lead. A small DIP
package with J-shaped leads for surface mounting or
socketing.
spindle The central post on which a disk
drive's platters are mounted.
SQL Structured Query Language. A standard
relational database language used especially on midrange and
mainframe computers.
SRAM Static Random Access Memory. A form
of high speed memory. SRAM chips do not require a refresh
cycle like DRAM chips and can be made to operate at very high
access speeds. SRAM chips are very expensive because they
normally require six transistors per bit. This also makes the
chip larger than conventional DRAM chips. SRAM is volatile,
meaning it will lose data with no power.
ST-506/412 A hard disk interface invented
by Seagate Technology and introduced in 1980 with the ST-506
5M hard drive.
stair-stepping Jagged raster
representation of diagonals or curves; corrected by
anti-aliasing.
standby power supply A backup power
supply that quickly switches into operation during a power
outage.
start/stop bits The signaling bits
attached to a character before the character is transmitted
during asynchronous transmission.
starting cluster The number of the first
cluster occupied by a file. Listed in the directory entry of
every file.
stepper motor actuator An assembly that
moves disk drive read/write heads across platters by a
sequence of small partial turns of a stepper motor.
storage Device or medium on or in which
data can be entered or held and retrieved at a later time.
Synonymous with memory.
streaming In tape backup, a condition in
which data is transferred from the hard disk as quickly as the
tape drive can record the data so that the drive does not
start and stop or waste tape.
string A sequence of characters.
subdirectory A directory listed in
another directory. Subdirectories themselves exist as
files.
subroutine A segment of a program that
can be executed by a single call. Also called program
module.
surface mount Chip carriers and sockets
designed to mount to the surface of a PC board.
surge protector A device in the power
line that feeds the computer and provides protection against
voltage spikes and other transients.
S-VHS or Super VHS A higher-quality
extension of the VHS home videotape format, featuring higher
luminance and the capability to produce better copies.
S-Video (Y/C) Type of video signal used
in the Hi8 and S-VHS videotape formats in which the luminance
and chrominance (Y/C) components are kept separate, providing
greater control and quality of each image. S-video transmits
luminance and color portions separately, thus avoiding the
NTSC encoding process and its inevitable loss of picture
quality.
synchronous communication A form of
communication in which blocks of data are sent at strictly
timed intervals. Because the timing is uniform, no start or
stop bits are required. Compare with asynchronous
communication. Some mainframes support only synchronous
communications unless a synchronous adapter and appropriate
software have been installed.
system crash A situation in which the
computer freezes up and refuses to proceed without rebooting.
Usually caused by faulty software. Unlike a hard disk crash,
no permanent physical damage occurs.
system files The two hidden DOS files
IBMBIO.COM and IBMDOS.COM; they represent the interface
between the BIOS and DOS (IBMBIO) and the interface between
DOS and other applications (IBMDOS).
system integrator A computer consultant
or vendor who tests available products and combines them into
highly optimized systems.
target A device attached to a SCSI bus
that receives and processes commands sent from another device
(the initiator) on the SCSI bus. A SCSI hard disk is an
example of a target.
TCM Trellis-coded modulation. An
error-detection and correction technique employed by
high-speed modems to enable higher-speed transmissions that
are more resistant to line impairments. In TCM encoding, the
first 2 data bits of an encoded group are used to generate a
third TCM bit that is added to the group. For example, in
V.32bis, the first 2 bits of a 6-bit group are used to
generate the TCM bit, which then is placed as the first bit of
a new 7-bit group. By reversing the encoding at the other end,
the receiving modem can determine whether the received group
is valid.
TCP/IP Transmission Control
Protocol/Internet Protocol. A set of protocols developed by
the U.S. Department of Defense (DoD) to link dissimilar
computers across many kinds of networks. This is the primary
protocol used by the Internet. It was developed in the 1970s
by the DoD's Advanced Research Projects Agency (DARPA) as a
military standard protocol. TCP/IP is supported by many
manufacturers of minicomputers, personal computers,
mainframes, technical workstations, and data communications
equipment.
temporary backup A second copy of a work
file, usually having the extension BAK. Created by application
software so that you easily can return to a previous version
of your work.
temporary file A file temporarily (and
usually invisibly) created by a program for its own use.
tera A multiplier indicating 1 trillion
(1,000,000,000,000) of some unit. Abbreviated as t or T. When
used to indicate a number of bytes of memory storage, the
multiplier definition changes to 1,099,511,627,776. One
terabit, for example, equals 1,000,000,000,000 bits, and one
terabyte equals 1,099,511,627,776 bytes.
terabyte (T) A unit of information
storage equal to 1,099,511,627,776 bytes.
terminal A device whose keyboard and
display are used for sending and receiving data over a
communications link. Differs from a microcomputer in that it
has no internal processing capabilities. Used to enter data
into or retrieve processed data from a system or network.
terminal mode An operational mode
required for microcomputers to transmit data. In terminal
mode, the computer acts as though it were a standard terminal
such as a teletypewriter rather than a data processor.
Keyboard entries go directly to the modem, whether the entry
is a modem command or data to be transmitted over the phone
lines. Received data is transmitted directly to the screen.
The more popular communications software products control
terminal mode and enable more complex operations, including
file transmission and saving received files.
terminator A piece of hardware that must
be attached to both ends of an electrical bus. Functions to
prevent the reflection or echoing of signals that reach the
ends of the bus and to ensure that the correct impedance load
is placed in the driver circuits on the bus.
thin-film media Hard disk platters that
have a thin film (usually three-millionths of an inch) of
medium deposited on the aluminum substrate through a
sputtering or plating process.
through-hole Chip carriers and sockets
equipped with leads that extend through holes in a PC
board.
throughput The amount of user data
transmitted per second without the overhead of protocol
information such as start and stop bits or frame headers and
trailers.
TIFF Tagged Image File Format. A way of
storing and exchanging digital image data. Developed by Aldus
Corporation, Microsoft Corporation, and major scanner vendors
to help link scanned images with the popular desktop
publishing applications. Supports three main types of image
data: black-and-white data, halftones or dithered data, and
grayscale data.
time code A frame-by-frame address code,
time reference recorded on the spare track of a videotape or
inserted in the vertical blanking interval. The time code is
an eight-digit number encoding time in hours, minutes,
seconds, and video frames.
Token Ring A type of local area network
(LAN) in which the workstations relay a packet of data called
a token in a logical ring configuration. When a station
wants to transmit, it takes possession of the token, attaches
its data, and then frees the token after the data has made a
complete circuit of the electrical ring. Transmits at speeds
of 16 million bps. Because of the token-passing scheme, access
to the network is controlled, unlike the slower 10BaseX
Ethernet system where collisions of data can occur, which
waste time. The Token-Ring network uses shielded, twisted-pair
wiring, which is cheaper than the coaxial cable used by
10Base2 and 10Base5 Ethernet and ARCnet.
tower A personal computer that normally
sits on the floor, and which is mounted vertically rather than
horizontally.
TPI Tracks per inch. Used as a
measurement of magnetic track density. Standard 5 1/4-inch
360K floppy disks have a density of 48 TPI, and the 1.2M disks
have a 96 TPI density. All 3 1/2-inch disks have a 135.4667
TPI density, and hard disks can have densities greater than
3,000 TPI.
track One of the many concentric circles
that holds data on a disk surface. Consists of a single line
of magnetic flux changes and is divided into some number of
512-byte sectors.
track density Expressed as tracks per
inch (TPI); defines how many tracks are recorded in 1 inch of
space measured radially from the center of the disk. Sometimes
called radial density.
track-to-track seek time The time
required for read/write heads to move between adjacent
tracks.
transportable computer A computer system
larger than a portable system, and similar in size and shape
to a portable sewing machine. Most transportables conform to a
design similar to the original Compaq portable, with a
built-in CRT display. These systems are characteristically
very heavy, and run only on AC power. Because of advances
primarily in LCD and plasma-display technology, these systems
are largely obsolete and have been replaced by portable
systems.
troubleshooting The task of determining
the cause of a problem.
true-color images True-color images are
also called 24-bit color images because each pixel is
represented by 24 bits of data, allowing for 16.7 million
colors. The number of colors possible is based on the number
of bits used to represent the color. If 8 bits are used, there
are 256 possible color values (2 to the 8th power). To obtain
16.7 million colors, each of the primary colors (red, green,
and blue) is represented by 8 bits per pixel, which allows for
256 possible shades for each of the primary red, green, and
blue colors, or 256x256x256 = 16.7 million total colors.
TSR Terminate-and-Stay-Resident. A
program that remains in memory after being loaded. Because
they remain in memory, TSR programs can be reactivated by a
predefined keystroke sequence or other operation while another
program is active. Usually called resident
programs.
TTL Transistor-to-Transistor Logic.
Digital signals often are called TTL signals. A TTL display is
a monitor that accepts digital input at standardized signal
voltage levels.
tweens The name given to a series of
animation or video frames between the key frames. When one
object is transformed (morphed) into another, the initial
object and the final object are set on the computer. Tweens
are the frames that transpose the first object into the final
image.
twisted pair A type of wire in which two
small insulated copper wires are wrapped or twisted around
each other to minimize interference from other wires in the
cable. Two types of twisted-pair cables are available:
unshielded and shielded. Unshielded Twisted-Pair (UTP) wiring
commonly is used in telephone cables and provides little
protection against interference. Shielded Twisted-Pair (STP)
wiring is used in some networks or any application in which
immunity from electrical interference is more important.
Twisted-pair wire is much easier to work with than coaxial
cable and is cheaper as well.
UART Universal Asynchronous Receiver
Transmitter. A chip device that controls the RS-232 serial
port in a PC-compatible system. Originally developed by
National Semiconductor, several UART versions are in
PC-compatible systems: The 8250B is used in PC- or XT-class
systems, and the 16450 and 16550A are used in AT-class
systems.
unformatted capacity The total number of
bytes of data that can fit on a disk. The formatted capacity
is lower because space is lost defining the boundaries between
sectors.
uninterruptible power supply (UPS) A
device that supplies power to the computer from batteries so
that power will not stop, even momentarily, during a power
outage. The batteries are recharged constantly from a wall
socket.
Universal Asynchronous Receiver
Transmitter See UART.
UPC Universal Product Code. A 10-digit
computer-readable bar code used in labeling retail products.
The code in the form of vertical bars includes a five-digit
manufacturer's identification number and a five-digit product
code number.
update To modify information already
contained in a file or program with current information.
URL An acronym for Uniform Resource
Locator. The primary naming scheme used to identify a
particular site or file on the World Wide Web. URLs combine
information about the protocol being used, the address of the
site where the resource is located, the subdirectory location
at the site, and the name of the particular file (or page) in
question.
utility Programs that carry out routine
procedures to make computer use easier.
UTP Unshielded Twisted Pair. A type of
wire often used indoors to connect telephones or computer
devices. Comes with two or four wires twisted inside a
flexible plastic sheath or conduit and uses modular plugs and
phone jacks.
V.21 An ITU standard for modem
communications at 300 bps. Modems made in the U.S. or Canada
follow the Bell 103 standard but can be set to answer V.21
calls from overseas. The actual transmission rate is 300 baud
and employs FSK (Frequency Shift Keying) modulation, which
encodes a single bit per baud.
V.22 An ITU standard for modem
communications at 1,200 bps, with an optional fallback to 600
bps. V.22 is partially compatible with the Bell 212A standard
observed in the U.S. and Canada. The actual transmission rate
is 600 baud, using DPSK (Differential-Phase Shift Keying) to
encode as many as 2 bits per baud.
V.22bis An ITU standard for modem
communications at 2,400 bps. Includes an automatic
link-negotiation fallback to 1,200 bps and compatibility with
Bell 212A/V.22 modems. The actual transmission rate is 600
baud, using QAM (Quadrature Amplitude Modulation) to encode as
many as 4 bits per baud.
V.23 An ITU standard for modem
communications at 1,200 or 600 bps with a 75-bps back channel.
Used in the United Kingdom for some videotext systems.
V.25 An ITU standard for modem
communications that specifies an answer tone different from
the Bell answer tone used in the U.S. and Canada. Most
intelligent modems can be set with an ATB0 command so
that they use the V.25 2,100Hz tone when answering overseas
calls.
V.32 An ITU standard for modem
communications at 9,600 bps and 4,800 bps. V.32 modems fall
back to 4,800 bps when line quality is impaired and fall
forward again to 9,600 bps when line quality improves. The
actual transmission rate is 2,400 baud using QAM (Quadrature
Amplitude Modulation) and optional TCM (Trellis-Coded
Modulation) to encode as much as 4 data bits per baud.
V.32bis An ITU standard that extends the
standard V.32 connection range and supports 4,800; 7,200;
9,600; 12,000; and 14,400 bps transmission rates. The V.32bis
modems fall back to the next lower speed when line quality is
impaired, fall back further as necessary, and fall forward to
the next higher speed when line quality improves. The actual
transmission rate is 2,400 baud using QAM (Quadrature
Amplitude Modulation) and TCM (Trellis-Coded Modulation) to
encode as much as 6 data bits per baud.
V.32terbo A proprietary standard proposed
by several modem manufacturers that will be cheaper to
implement than the standard V.32 fast protocol, but which will
only support transmission speeds of up to 18,800 bps. Because
it is not an industry standard, it is not likely to have
widespread industry support.
V.34 An ITU standard that extends the
standard V.32bis connection range, supporting 28,800 bps
transmission rates as well as all the functions and rates of
V.32bis. This was called V.32fast or V.fast while under
development.
V.34+ An ITU standard that extends the
standard V.34 connection range, supporting 33,600 bps
transmission rates as well as all the functions and rates of
V.34.
V.42 An ITU standard for modem
communications that defines a two-stage process of detection
and negotiation for LAPM error control. Also supports MNP
error-control protocol, Levels 1 through 4.
V.42bis An extension of CCITT V.42 that
defines a specific data-compression scheme for use with V.42
and MNP error control.
vaccine A type of program used to locate
and eradicate virus code from infected programs or
systems.
VCPI Virtual Control Program Interface. A
386 and higher processor memory management standard created by
Phar Lap software in conjunction with other software
developers. VCPI provides an interface between applications
using DOS extenders and 386 memory managers.
Vertical Blanking Interval (VBI) Lines
1-21 of the video top field and lines 263-284 of the bottom
field, in which frame numbers, picture stops, chapter stops,
white flags, closed captions, and more may be encoded. These
lines do not appear on the display screen, but maintain image
stability and enhance image access.
VESA Video Electronics Standards
Association. Founded in the late 1980s by NEC Home Electronics
and eight other leading video board manufacturers with the
main goal of standardizing the electrical, timing, and
programming issues surrounding 800x600 resolution video
displays, commonly known as Super VGA. VESA has also
developed the Video Local Bus (VL-Bus) standard for connecting
high speed adapters directly to the local processor bus.
VGA Video Graphics Array. A type of PC
video display circuit (and adapter) first introduced by IBM on
April 2, 1987, that supports text and graphics. Text is
supported at a maximum resolution of 80x25 characters in 16
colors with a character box of 9x16 pixels. Graphics is
supported at a maximum resolution of 320x200 pixels in 256
(from a palette of 262,144) colors or 640x480 pixels in 16
colors. The VGA outputs an analog signal with a horizontal
scanning frequency of 31.5KHz, and supports analog color or
analog monochrome displays.
VHS Video Home System. A popular consumer
videotape format developed by Matsushita and JVC.
Video 8 or 8mm Video Video format based
on the 8mm videotapes popularized by camcorders.
video graphics array See VGA.
video A system of recording and
transmitting primarily visual information by translating
moving or still images into electrical signals. The term video
properly refers only to the picture, but as a generic term,
video usually embraces audio and other signals that are part
of a complete program. Video now includes not only broadcast
television but many non-broadcast applications such as
corporate communications, marketing, home entertainment,
games, teletext, security, and even the visual display units
of computer-based technology.
video-on-CD or video CD A full-motion
digital video format using MPEG video compression and
incorporating a variety of VCR-like control capabilities. See
also White Book.
virtual disk A RAM disk or "phantom disk
drive" in which a section of system memory (usually RAM) is
set aside to hold data, just as though it were a number of
disk sectors. To DOS, a virtual disk looks like and functions
like any other "real" drive.
virtual memory A technique by which
operating systems (including OS/2) load more programs and data
into memory than they can hold. Parts of the programs and data
are kept on disk and are constantly swapped back and forth
into system memory. The applications' software programs are
unaware of this setup and act as though a large amount of
memory is available.
virtual real mode A mode available in all
Intel 80386-compatible processors. In this mode, memory
addressing is limited to 4,096M, restricted protection levels
can be set to trap software crashes and control the system,
and individual real-mode compatible sessions can be set up and
maintained separately from one another.
virus A type of resident program designed
to replicate itself. Usually at some later time when the virus
is running, it causes an undesirable action to take place.
Visual Basic A high-level, graphically
oriented, fourth-generation programming language only used in
the Windows operating environment.
VL-Bus (VESA Local Bus) A standard 32-bit
expansion slot bus specification used in 486 PCs. Now replaced
by PCI bus.
VMM Virtual Memory Manager. A facility in
Windows enhanced mode that manages the task of swapping data
in and out of 386 and higher-processor virtual real-mode
memory space for multiple non-Windows applications running in
virtual real mode.
voice-coil actuator A device that moves
read/write heads across hard disk platters by magnetic
interaction between coils of wire and a magnet. Functions
somewhat as an audio speaker, from which the name
originated.
voltage regulator A device that smoothes
out voltage irregularities in the power that's fed to the
computer.
volume A portion of a disk signified by a
single drive specifier. Under DOS v3.3 and later, a single
hard disk can be partitioned into several volumes, each with
its own logical drive specifier (C:,D:,E:, and so on).
volume label An identifier or name of up
to 11 characters that names a disk.
VRAM Video Random-Access Memory. VRAM
chips are modified DRAMs on video boards that enable
simultaneous access by the host system's processor and the
processor on the video board. A large amount of information
thus can be transferred quickly between the video board and
the system processor. Sometimes also called dual-ported
RAM.
VxD Virtual Device Driver. A special type
of Windows driver for 386 Enhanced Mode. VxDs run at the most
privileged CPU mode (ring 0) and allows low-level interaction
with DOS and Windows programs running under Windows.
wait states Pause cycles during system
operation that require the processor to wait one or more clock
cycles until memory can respond to the processor's request.
Enables the microprocessor to synchronize with lower-cost,
slower memory. A system that runs with "zero wait states"
requires none of these cycles because of the use of faster
memory or a memory cache system.
wide area network (WAN) A LAN that
extends beyond the boundaries of a single building.
Web browser An application that locates a
document on the Internet using a URL (Uniform Resource
Locator), retrieves it, and formats the document for display.
Netscape Navigator, Spyglass Mosaic, and Microsoft Internet
Explorer are examples of Web browsers.
Web site An individual Web document
collection named by a unique URL.
Whetstone A benchmark program developed
in 1976 and designed to simulate arithmetic-intensive programs
used in scientific computing. Remains completely CPU-bound and
performs no I/O or system calls. Originally written in ALGOL,
although the C and Pascal versions became more popular by the
late 1980s. The speed at which a system performs
floating-point operations often is measured in units of
Whetstones.
White Book A standard specification
developed by Philips and JVC in 1993 for storing MPEG standard
video on CDs. An extension of the Red Book standard for
digital audio, Yellow Book standard for CD-ROM, Green Book
standard for CD-I, and Orange Book standard for CD Write
Once.
Whitney technology A term referring to a
magnetic disk design that usually has oxide or thin film
media, thin film read/write heads, low floating height
sliders, and low mass actuator arms that together allow higher
bit densities than the older Winchester technology. Whitney
technology was first introduced with the IBM 3370 disk drive,
circa 1979.
Winchester drive Any ordinary,
nonremovable (or fixed) hard disk drive. The name originates
from a particular IBM drive in the 1960s that had 30M of fixed
and 30M of removable storage. This 30-30 drive matched the
caliber figure for a popular series of rifles made by
Winchester, so the slang term Winchester was applied to
any fixed platter hard disk.
Winchester technology The term
Winchester is loosely applied to mean any disk with a
fixed or non-removable recording medium. More precisely, the
term applies to a ferrite read/write head and slider design
with oxide media that was first employed in the IBM 3340 disk
drive, circa 1973. Most drives today actually use Whitney
technology.
wire frames The most common technique
used to construct a three-dimensional object for animation. A
wire frame is given coordinates of length, height, and width.
Wire frames then are filled with textures, colors, and
movement. Transforming a wire frame into a textured object is
called rendering.
word length The number of bits in a data
character without parity, start, or stop bits.
World Wide Web Also called simply the
Web. A graphical information system based on hypertext that
enables a user to easily access documents located on the
Internet.
WORM Write Once, Read Many (or Multiple).
An optical mass-storage device capable of storing many
megabytes of information but that can be written to only once
on any given area of the disk. A WORM disk typically holds
more than 200M of data. Because a WORM drive cannot write over
an old version of a file, new copies of files are made and
stored on other parts of the disk whenever a file is revised.
WORM disks are used to store information when a history of
older versions must be maintained. Recording on a WORM disk is
performed by a laser writer that burns pits in a thin metallic
film (usually tellurium) embedded in the disk. This burning
process is called ablation. WORM drives are frequently
used for archiving data.
write precompensation A modification
applied to write data by a controller in order to alleviate
partially the problem of bit shift, which causes adjacent 1s
written on magnetic media to read as though they were farther
apart. When adjacent 1s are sensed by the controller,
precompensation is used to write them closer together on the
disk, thus enabling them to be read in the proper bit-cell
window. Drives with built-in controllers normally handle
precompensation automatically. Precompensation normally is
required for the inner cylinders of oxide media drives.
XGA eXtended Graphics Array. A type of PC
video display circuit (and adapter) first introduced by IBM on
October 30, 1990, that supports text and graphics. Text is
supported at a maximum resolution of 132x60 characters in 16
colors with a character box of 8x6 pixels. Graphics is
supported at a maximum resolution of 1024x768 pixels in 256
(from a palette of 262,144) colors or 640x480 pixels in 65,536
colors. The XGA outputs an analog signal with a horizontal
scanning frequency of 31.5 or 35.52KHz, and supports analog
color or analog monochrome displays.
XMM eXtended Memory Manager. A driver
that controls access to extended memory on 286 and higher
processor systems. HIMEM.SYS is an example of an XMM that
comes with DOS.
Xmodem A file-transfer protocol--with
error checking--developed by Ward Christ-ensen in the
mid-1970s and placed in the public domain. Designed to
transfer files between machines running the CP/M operating
system and using 300 or 1,200 bps modems. Until the late
1980s, because of its simplicity and public-domain status,
Xmodem remained the most widely used microcomputer
file-transfer protocol. In standard Xmodem, the transmitted
blocks are 128 bytes. 1K-Xmodem is an extension to Xmodem that
increases the block size to 1,024 bytes. Many newer
file-transfer protocols that are much faster and more accurate
than Xmodem have been developed, such as Ymodem and
Zmodem.
XMS eXtended Memory Specification. A
Microsoft developed standard that provides a way for real-mode
applications to access extended memory in a controlled
fashion. The XMS standard is available from Microsoft.
XON/XOFF Standard ASCII control
characters used to tell an intelligent device to stop or
resume transmitting data. In most systems, pressing Ctrl+S
sends the XOFF character. Most devices understand Ctrl+Q as
XON; others interpret the pressing of any key after Ctrl+S as
XON.
Y-connector A Y-shaped splitter cable
that divides a source input into two output signals.
Yellow Book standards See
CD-ROM.
Yellow Book The standard used by Compact
Disc Read Only Memory (CD-ROM). Multimedia applications most
commonly use the Yellow Book standard, which specifies how
digital information is to be stored on the CD-ROM and is to be
read by a computer. EXtended Architecture (XA) is currently an
extension of the Yellow Book that allows for the combination
of different data types (audio and video, for example) onto
one track in a CD-ROM. Without XA, a CD-ROM can only access
one data type at a time. Many CD-ROM drives are now XA
capable.
Ymodem A file-transfer protocol first
released as part of Chuck Forsberg's YAM (Yet Another Modem)
program. An extension to Xmodem designed to overcome some of
the limitations of the original. Enables information about the
transmitted file, such as the file name and length, to be sent
along with the file data and increases the size of a block
from 128 to 1,024 bytes. Ymodem-batch adds the capability to
transmit "batches" or groups of files without operator
interruption. YmodemG is a variation that sends the entire
file before waiting for an acknowledgment. If the receiving
side detects an error in midstream, the transfer is aborted.
YmodemG is designed for use with modems that have built-in
error-correcting capabilities.
ZIF Zero Insertion Force. Sockets that
require no force for the insertion of a chip carrier. Usually
accomplished through movable contacts and used primarily in
test devices in which chips will be inserted and removed many
times.
ZIP Zigzag Inline Package. A DIP package
that has all leads on one edge in a zigzag pattern and mounts
in a vertical plane.
Zip drive An external drive manufactured
by Iomega that supports 100M magnetic media on a 3 1/2-inch
removable drive.
Zmodem A file-transfer protocol
commissioned by Telnet and placed in the public domain. Like
Ymodem, it was designed by Chuck Forsberg and developed as an
extension to Xmodem to overcome the inherent latency when
using Send/Ack-based protocols such as Xmodem and Ymodem. It
is a streaming, sliding-window protocol.
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