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 interface 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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