Barrons Publishing Dictionary of Computer and Internet Terms 10th

dictionary attack an attempt to guess a computer password by trying every word in a large dictionary and, often, every combination of letters that would make a pronounceable word.

die (plural dice or die) the individual piece of silicon containing a transistor or integrated circuit. A memory cache located “on die” is one on the same piece of silicon as the CPU.

digerati (slang, plural) people knowledgeable about the Web and other digital communications media. (From digital and Italian litterati.)

digital representing information as electrical “on” and “off” signals that correspond to binary digits and can be stored in computer memory. Digital electronics contrasts with analog electronics, which represents information with signals that vary within a predefined range. Digital signals have two advantages: they can be copied exactly, without even the slightest loss of quality, and they can be further processed by computer.

See also DIGITAL COMPUTER.

digital audio sound represented in digital form. See DIGITIZED MUSIC; MP3;

WAVE FILE.

digital camera a camera that takes pictures with a CCD IMAGE SENSOR or CMOS image sensor and transmits them directly to a computer or records them on a memory card without using film.

The most important specification of a digital camera is the number of PIXELs in the image. To show as much detail as a good 35-mm slide, a digital image requires about 2000 × 3000 pixels. A much lower resolution is sufficient for snapshots. The image on a TV screen is equivalent to about 400 × 600 pixels. See MEGAPIXEL (table). See also CCD IMAGE SENSOR;

CMOS IMAGE SENSOR; FOCAL LENGTH; FOVEON; F-RATIO; IMAGE PROCESSING.

digital computer a computer that represents information in discrete form, as opposed to an analog computer, which allows representation to vary along a continuum. For example, the temperature of a room might be any value between 0° and 100°F. An analog computer could represent this as a continuously varying voltage between 0.00 and 1.00 volts. In contrast, a digital computer would have to represent it as a decimal or binary number with a specific number of digits (e.g., 68.80 or 68.81).

All modern, general-purpose computers are digital. Analog computer circuits are, however, frequently used in industrial control equipment.

A digital computer is more accurate than an analog computer because it only needs to sense the difference between clearly distinguishable states. For example, a slight voltage fluctuation would affect the result in an analog computer, but a slight voltage fluctuation would not affect a digital computer because the computer could still easily distinguish the 0 state from the 1 state of any circuit element. For the same reason, digital music reproduction (e.g., a compact disc) is more accurate than analog reproductions (e.g., a traditional record).

digital divide the division of the world into those that have access to computers and the Internet and those that are too poor to afford that access.

Digital Equipment Corporation (DEC) a company whose products included the PDP-8, PDP-11, and VAX minicomputers, the VT-100 terminal, and the Alpha microprocessor. In 1998, Digital Equipment Corporation was aquired by Compaq, now part of Hewlett-Packard.

digital film the memory cards or other storage devices used in digital cameras. Unlike real photographic film, the brand of digital film does not affect the quality of the picture or the amount of light required to make it. The digital memory device simply records whatever the camera gets.

digital image processing see IMAGE PROCESSING.

Digital Millennium Copyright Act see DMCA.

digital music music signals that have been converted to numbers that are stored electronically. A musical sound wave is continuous, but it can be represented by numbers that give the amplitude of the wave at each moment in time, as long as the sound is sampled often enough to seem continuous. See SAMPLING RATE. A music CD stores sound digitally (as opposed to a traditional vinyl record, which stored sound in analog format by the variations in the groove.) Music files can also be stored on a computer hard disk, but the file representing a song is too large to do this conveniently unless it has been compressed. The development of MP3 compression led to a boom in digital music as people stored music on computer hard disks and digital players such as the IPOD. However, rampant file sharing through services such as NAPSTER and GROKSTER led to court cases that restricted the use of these services. See also P2P; AAC;

WAVE FILE; MIDI; WMA; OGG VORBIS; ITUNES.

digital photo frame a device that displays one or more digital pictures in succession, over and over, without being constantly connected to a computer. The pictures are delivered to it on a memory card or by temporarily connecting to a computer using USB or network connections.

digital picture frame see DIGITAL PHOTO FRAME.

digital signal processing (DSP) the use of computers to process signals such as sound or video. Applications of DSP include decoding modulated signals (e.g., in modems), removing noise, and converting video to different formats.

digital signature a way of authenticating that an electronic message really came from the person it claims to have come from. A digital signature can be encrypted with your PRIVATE KEY. The recipient can decrypt the message with your PUBLIC KEY to verify that it is really you. Nobody who does not know your private key could create a signature that would correctly decrypt with your public key. However, the digital signature cannot be sent by itself, because then someone could simply copy the encrypted version. Instead, the signature needs to be sent as part of an abstract (or manifest or digest) of a particular message. The recipient can check to make sure that the hash function given in the manifest matches the hash function calculated from the message. If someone tampered

dimmed not available for selection. If a menu option appears in light gray rather than black type, it cannot be chosen. For example, if you wish to align two objects, the align command will be dimmed until you have two or more objects selected. If you click on a dimmed command, nothing happens. See Figure 78.

FIGURE 78. Dimmed (disabled) selections on menu

DIN paper sizes see PAPER SIZES (ISO).

dingbats special characters that are neither letters nor mathematical symbols, such as ‡ ♠ ♥ ◊ ♣ ↔. See also FLEURON.

diode a semiconductor device that allows electric current to pass in one direction, but not in the other. Diodes are formed by joining two types of doped semiconductors: P type, with a deficiency of free electrons (and an excess of holes), and N type, with a surplus of free electrons. (See SEMICONDUCTOR.) The place where the two regions are joined is called the junction.

Electrons can flow from N-type to P-type material, but not the other way. The diode is said to be forward-biased when the voltage across it is in the right direction to make it conduct, and reverse-biased when the voltage is applied in the opposite direction. The diode is forward biased when a negative voltage is applied to the N region and a positive voltage to the P region; then free electrons in the N region are driven to the junction where they combine with holes from the P region. If a positive voltage is applied to the N region, electrons are pulled away from the junction and no current can flow (the diode is reverse biased).

See also ELECTRONIC CIRCUIT DIAGRAM SYMBOLS; LED; TRANSISTOR.

DIP (dual in-line package) a plastic case with two rows of pins (Fig. 79), the way INTEGRATED CIRCUITS (ICs) were packaged in the early days of personal computers. Most ICs now come in smaller packages which mount on the surface of a printed-circuit board rather than by placing pins into holes.

DIP switch a miniature switch or set of switches in the same size package as a DIP integrated circuit. See DIP.

FIGURE 79. DIP (dual in-line package) integrated circuits

dir the command, under Windows command line, that makes the computer display all the files in a particular directory. For example,

dir

by itself lists the files in the current directory of the current disk. Here are some other examples:

To see a directory of all of the files on the current drive that have names ending in .bas, type

dir *.bas

The asterisk acts as a “wild card,” matching all filenames.

direct-to-press a method of production where a machine similar to a laser printer takes data directly from a computer to produce film or plates for a printing press without the intermediate step of printing out cameraready copy.

direct(ly) attached storage (DAS) disk drives or other storage devices that are connected directly to the computer that uses them, rather than connected through a network. Contrast

WORK ATTACHED STORAGE.

directory an area on a disk where the names and locations of files are stored. A disk can, and usually does, contain more than one directory; directories can contain other directories.

On the Macintosh and in Windows, directories are called folders. Directories are pictured as tree structures or boxes within boxes (Figure 80).

Directories are a way of classifying files; they do not divide the disk itself into sections. Any file can use as much space as needed, anywhere on the disk, regardless of what directory it is in.

FIGURE 80. Directories (shown graphically)

DirectX an optional add-on animation and sound subsystem for Microsoft Windows, required by certain games and other software that needs higher graphics performance than can be obtained through normal operating system services. DirectX is available as a free download from Microsoft and is updated frequently. See also DXDIAG.

dirty (slang) needing to be written to disk or needing to be updated in some way.

disassembler a program that converts machine instructions into assembly language so that a human being can read them.

disc a compact disc (CD) or digital video disc (DVD). See DISK and usage note there.

disclaimer a statement absolving someone (or even oneself) of responsibility. The so-called “usual disclaimer” on e-mail messages is roughly, “These are not the opinions of my employer or the organization that transmits this message for me.” Many people abbreviate this to “usual disclaimer applies.” The disclaimer is unnecessary unless there is something about the e-mail message that would specifically cause people to misunderstand it.

discrete speech speech that is spoken with pauses between words to make it easier for a computer to recognize. Contrast CONTINUOUS SPEECH. See

SPEECH RECOGNITION.

discretionary hyphen a hyphen that is used only when the word falls near the end of a line; sometimes called a SOFT HYPHEN. By specifying where you want the word to be broken, you override the word processor’s automatic hyphenation. Contrast REQUIRED HYPHEN; HARD HYPHEN.

disk a round, flat device for storing computer data. There are three main kinds: DISKETTEs, HARD DISKs, and OPTICAL DISKs (of which CD-ROMs are the most common kind).

Diskettes are made of flexible plastic coated with iron oxide. Information is recorded on them by magnetizing the iron oxide coating

in specific places. They were the main storage medium during the first couple of decades of microcomputer use but they are less common now. There were two sizes: 51⁄4 inches and 31⁄2 inches.

Hard disks are like diskettes except that the iron oxide is coated on stiff aluminum disks. There are several layers in a single disk pack. Microcomputers come with a hard disk permanently mounted within the computer, and external hard disks can be used to back up the internal hard disk.

Hard drive capacities have increased dramatically since their introduction. Early drives only held five megabytes of data. Modern hard drives typically store hundreds of gigabytes.

The iron oxide on the disk consists of microscopically small needles, each of which acts like a tiny bar magnet. Information is stored by magnetizing these needles. The read-write head, which skims the surface of the disk, can either generate a magnetic field to magnetize the needles or detect the magnetic field of needles that are already magnetized. The binary digits 0 and 1 are represented by changes in the direction of magnetization.

Data on disks is stored in many concentric circles, each of which is called a track. Each track is divided into sectors, which are the smallest units that the computer can read into memory in a single step. On a dou- ble-sided or multilayer disk pack, the set of tracks in corresponding positions on different layers is known as a cylinder.

The directory of a disk is a special area in which the computer records the names and locations of all the files on the disk. The user can create many directories on a single disk.

Optical disks store information by etching a transparent plastic medium with a laser beam. See CD-ROM; CD-R; CD-RW; DVD.

Usage note: This word is often spelled disc when referring to compact discs (CDs), DVDs, and laser discs, but is always spelled disk when referring to magnetic disks and diskettes. The difference in spelling probably reflects the European origin of the CD.

disk drive a device that enables a computer to read (and, in most cases, write) data on disks. Microcomputers typically contain one hard disk drive and one or more CD or DVD drives. See DISC and DISK.

disk farm (slang) a room full of disks; a large set of disk drives used by a single computer or network.

disk server a computer that performs DISK SHARING. Contrast FILE SERVER.

disk sharing the use of networking to allow a computer to use a disk drive that is located in another computer, and to format and control it as if it were locally attached, without relying on the server to own and manage files. Disk sharing is essentially the same thing as a storage area network (SAN). Contrast FILE SHARING, NAS.

diskette a removable flexible magnetic disk on which computer programs and data can be stored. 51⁄4-inch diskettes were often called floppy disks because the entire diskette, including cover, was flexible.

Usage note: The alternative term floppy disk is going out of fashion because present-day diskettes have rigid shells. See also DISK.

diskless workstation a computer that has no disk drive of its own, downloading the operating system and all software and data files through a network.

display panel a small panel that displays information on a piece of equipment that does not have a screen. For example, the HP LaserJet 4M printer has a display panel that can display the status of print jobs.

dissolve see TRANSITION EFFECT.

Distort a set of Photoshop filters that twist an image. Glass, Ocean Ripple, Pinch, and Wave are examples of effects that can be applied. See Figure 81 for an example.

FIGURE 81. Distort

distribute a drawing program command that places objects evenly over a defined area.

FIGURE 82. Distribute

distributed spread over more than one computer. For example, the WORLD is a distributed library of information.

distro (slang) a distribution (a prepared, ready-to-install or ready-to-use copy) of a piece of free software such as Linux or TEX. Some free software packages are so complex that different people have prepared different ready-to-use distributions of the same software. See FREE SOFTWARE; LINUX; TEX.

dithering the representation of an intermediate color by mixing dots of two other colors. (See Figure 83.) Hardware limitations make it impossible to print or display all possible colors. Dithering is used to represent

shades of gray or colors on a printer or screen that cannot produce them directly.

FIGURE 83. Dithering patterns

DL (describing a DVD or similar disc) double-layer; recorded in two layers from the same side, giving twice as much capacity as a single layer.

DLL (Dynamic Link Library) a file containing a library of machine-lan- guage procedures that can be linked to programs as needed at run time. See LINK, definition 3.

DLLs are used in Microsoft Windows. Their practical benefit is that programs don’t need to include code to perform common functions because that code is available in the DLL. The program is smaller, and changes can be made once to the DLL routine instead of separately to each program.

DLL Hell (slang) in Windows (especially before 2000), the confusing situation that results when more than one DLL file has the same name. If the DLLs are installed in a system directory, then the most recently installed file replaces earlier ones. Since nothing prevents two programmers from choosing the same file name, the conflicting DLLs can be completely unrelated. More often, the conflicting DLLs are different versions of the same file, one newer than the other; in that case, software relying on the file will start up normally, but there will be subtle malfunctions.

Windows 2000, XP, and Vista take various measures to separate DLLs that belong to different programs, and to undo any modifications made to system DLLs. The .NET framework eliminates DLL Hell by tying each application explicitly to its DLLs and allowing multiple versions to coexist.

DLP (Digital Light Processing), a system developed by Texas Instruments for projectors that reflect light off an array of microscopic mirrors. By controlling the fraction of the time that a particular mirror reflects, the brightness of each pixel can be controlled. See www.dlp.com.

DMA (Direct Memory Addressing) the ability of a peripheral device, such as a disk controller, to access the memory of a computer directly, without going through the CPU. DMA makes it possible to transfer information to or from external devices much more quickly than would be possible if the CPU had to handle every byte of information during the transfer.

DMCA (Digital Millennium Copyright Act) a controversial law passed by the U.S. Congress in 1998 designed to secure copyright protection of digital works.

Unlike earlier copyright laws, the DMCA prohibits not only copying, but also the use of technology to get around technical measures designed