Barrons Publishing Dictionary of Computer and Internet Terms 10th

to prevent copying, even when the copying itself is legal. “No person shall circumvent a technological measure that effectively controls access to a work protected under this title” (17 USC 1201). The law also makes it illegal to distribute circumvention technology but this is hard to enforce.

Critics argue that the DMCA is badly flawed. It prohibits people from experimenting to learn exactly how certain software works even when that software is running on their own computers, and unlike all earlier copyright laws, the DMCA enables copyright owners to block people from reading and viewing material that they possess, not just copying from and redistributing it. See also COMPUTER LAW; DECSS.

DNA

1.(deoxyribonucleic acid) the molecule that carries inherited information in all living things. DNA encodes the sequence of amino acids that form proteins. The operation of DNA is somewhat like a computer that uses three-digit base-4 numbers. In this manner, a set of DNA molecules contains the entire plans for building a plant, animal, or human being. Like computer programs, DNA contains “go to” instructions that skip over some of the material.

2.(Windows Distributed Internet Architecture) Microsoft’s model for developing distributed computing applications with Windows.

DNS

1. (domain name server) a server responsible for translating domain addresses, such as www.example.com, into IP (Internet Protocol) numbers such as 127.192.92.95. Domain name servers are interconnected so that if the nearest one cannot look up a name, it will query several other servers at various locations. Normally, when a computer is attached to the Internet, the IP address of a DNS has to be given to it as part of the setup information. However, computers that receive IP addresses through DHCP are also given DNS information automatically. See also

IANA; TLD.

2. (Do Not Set) a copyediting abbreviation used to mark marginal notations or special typesetting instructions on a manuscript.

do the keyword, in C and related languages, to execute over and over until a condition becomes false, checking the condition at the end rather than at the beginning, so that the loop is guaranteed to execute at least once. Here is an example:

/* This is Java; C, C++, C# are very similar */ int i=5;

do

{

System.out.print(i + ” ... ”); i—;

}

while(i>0);

System.out.println(”Finished!”);

See WHILE for explanation of how this works; the only difference is that even if the condition of the do loop were false at the beginning, the loop would execute once, because the test is not performed until the end.

Compare REPEAT.

Dock

1.to anchor; to fix into position. Many application programs allow the user to move subordinate windows, such as toolboxes, from their original position to a new location at the top, bottom, or side of the main window, where they snap into place. This is called docking the toolbox.

The Windows taskbar (with “Start” and icons for open applications) is normally docked at the bottom of the screen, but by dragging it with the mouse, you can dock it at the top or either side instead.

2.(in Mac OS X) a panel of animated icons at the bottom of the screen that shows the current active (running) programs, and provides a place for aliases of frequently used files and the Trash. The Dock assumes some of the functions of the Application Menu and FINDER from earlier versions of Mac OS. It is similar to the TASKBAR in Windows.

docking station an accessory that gives a laptop computer additional capabilities when it is used at a fixed location. A typical docking station will include a charger for the laptop’s battery, connection to a larger monitor, and possibly additional disk drives or other peripherals.

doctor the version of ELIZA (a simulated psychotherapist) built into the Emacs editor. See ELIZA.

document a file containing a text to be printed (e.g., a letter, term paper, or book chapter) or a drawing or other piece of work that a human being is editing with the aid of the computer.

document mode the normal way of typing documents that are to be printed. The word processor includes codes that indicate hyphenation, page breaks, and the like, thereby producing a special word processing file rather than a text file. See also NONDOCUMENT MODE; TEXT FILE.

documentation written descriptions of computer programs. Documentation falls into several categories:

1.Internal documentation, consisting of comments within the program. (See COMMENT.) Internal documentation is addressed mostly to future programmers who may have to make corrections or other modifications.

2.Online documentation, information that is displayed as the program runs or that can be called up with a command such as help. The user should be able to control the amount of information displayed (more for beginners, and less as the user’s experience increases). Also, help commands should be sensitive to the context in which they are invoked; for instance, typing help within an editor should call up information about the editor, not the whole operating system.

3Reference cards, containing easily forgotten details for quick reference. A reference card assumes that the user is already familiar with the general principles of the program. Reference cards printed on paper are becoming obsolete, but the same kind of documentation is often made available online.

4Reference manuals, setting out complete instructions for the program in a systematic way. Related information should be grouped together, and a good index should be provided.

5Tutorials, serving as introductions for new users. Unlike a reference manual, a tutorial gives the information in the order in which the user will want to learn it; items are grouped by importance rather than by function or logical category.

Documents and Settings the directory which, in Windows 2000 and later, normally contains a directory for each user account, containing that user’s “My Documents” folder and other data. In Windows Vista, it was renamed Users.

dodge/burn tool a paint program tool that simulates the effects of traditional dodging and burning methods in the photographic darkroom: the burn tool gradually increases the darkness of the area you pass the tool over; likewise, the dodge tool lightens the area.

DOM (Document Object Model) a system in which a web page is viewed as a collection of objects that can be manipulated by an object-oriented scripting language such as JAVASCRIPT. See also DYNAMIC HTML.

domain

1. a portion of the Internet distinguished by a particular final part of the name. For instance, www.covingtoninnovations.com and ftp.covingtoninnovations.com are two servers in the domain covingtoninnovations.com, which is a subdomain of .com, its top-level domain (TLD).

2. in Windows NT and its successors, a group of networked computers that share a server and a set of user accounts.

domain address an Internet address in conveniently readable form, such as jones.com, as opposed to the IP ADDRESS, which consists of numbers. See

INTERNET.

domain name hijacking see DOMAIN NAME POACHING.

domain name hoarding the practice of registering multiple domain names, some of which will remain unused, simply to prevent a competitor from using them.

domain name poaching the practice of registering an Internet domain name with the intention of reselling the domain name rights to a corporation or individual. For example, you might want to register www.ford.com for yourself in the hope that Ford Motor Company would then buy it from you. Also called DOMAIN NAME HIJACKING and CYBERSQUATTING. See also UDRP.

dongle a device that attaches to a computer, typically on a USB port, and must be present in order to run a particular piece of software, but has no other purpose. Dongles are used to prevent unauthorized copying. See

COPY PROTECTION.

DoS abbreviation for “Denial of Service.” See DENIAL-OF-SERVICE ATTACK.

DOS (Disk Operating System) the name of various operating systems produced by various computer manufacturers, including an early operating system for the IBM 360; the disk operating system for the Apple II (Apple DOS); MS-DOS, developed by Microsoft for 16-bit microcomputers; PC-DOS, a version of MS-DOS commonly sold with the IBM PC; and Caldera DOS (formerly DR-DOS), a third-party substitute for MS-DOS.

Since 1983 the name DOS has almost always referred to MS-DOS or equivalent operating systems, as it does throughout this book. See MS-DOS.

dot

1.the character . (period), often used in filenames and Internet addresses.

2.the decimal point. For example, “nineteen dot two” means 19.2 thousand bytes per second, a standard data transmission rate.

dot-bomb (humorous) an Internet business that failed, usually in a spectacularly quick manner.

dot-com an Internet address ending in .com; more generally, a business that operates on the Internet. See COM.

dot-compost (humorous) derisive term describing the residue of failed dotcom business ventures. See also DOT-BOMB; DOT-GONE.

dot-con (humorous) fraud committed on the Internet. See also HOAX; PYRAMID SCHEME.

dot-gone (humorous) a failed Internet business, the opposite of a DOT-COM.

See also DOT-BOMB, DOT-COMPOST.

dot-matrix printer a printer that forms characters as patterns of dots made by pressing pins onto a ribbon. These printers were widely used in the 1980s, but inkjet printers give comparable or higher quality and speed without making as much noise. See also INKJET PRINTER; LASER PRINTER.

dots per inch see DPI.

double in C and related programming languages, a keyword for declaring floating-point numbers.

double buffering

1. in graphics, the practice of computing the next frame of an animation in memory while the previous frame is being displayed, and then copying the new frame to the screen quickly. That way, the process of drawing each frame on the screen does not produce flicker.

2. more generally, the use of two buffers (memory areas) to hold data being sent to an output device. One buffer can continue to accept data while the other buffer is being copied from memory to the device.

double-byte font a FONT that uses two bytes (16 bits) to represent each character, thereby allowing more than the 256 characters that could fit into a single-byte font. See UNICODE. Contrast ASCII, ANSI, and EBCDIC, which are single-byte character sets.

double-click to depress the button of a mouse twice very rapidly (if the mouse has more than one button, use the leftmost one). This is usually the shortcut to open or launch the selected file. If you find it difficult to double-click quickly enough, you can adjust the mouse’s double-click speed. You’ll find the mouse adjustments in the Control Panel (Macintosh and Microsoft Windows).

FIGURE 84. Double-click

double dagger the character ‡, which is sometimes used to mark footnotes. See FOOTNOTE for more information.

double precision a way of representing floating-point numbers with about twice the number of significant digits used in earlier implementations. Double-precision data types were first implemented in FORTRAN and later in BASIC and C. Today, “double” is in fact the normal floatingpoint type in C and its derivatives on most CPUs. See also ROUNDING ERROR.

down not available for use. A computer is said to be down when it malfunctions or when it is being tested or maintained.

downlevel pertaining to an earlier version of a product. For example, Windows Server 2003 supports downlevel clients; that is, client computers attached to the server can run earlier versions of Windows.

Contrast UPLEVEL.

download to transmit a file or program from a central computer to a smaller computer or a computer at a remote site. See KERMIT; FTP.

Contrast UPLOAD.

downstream (describing data transmission) in a direction from the server to the client, or from the main computer to the peripheral. Compare

DOWNLOAD. Contrast UPSTREAM.

downward compatibility the ability to work with older equipment and/or software than that for which a computer program or accessory was designed. Contrast UPWARD COMPATIBILITY.

dpi (dots per inch) the number of pixels or printer dots per linear inch. The first generation of laser printers could print with a resolution of 300 dots per inch. 600and 1200-dpi laser printers are now available. See RESOLUTION for further details.

draft quality a printout in low resolution, unsuitable for CAMERA-READY COPY, but adequate for proofing the copy and checking the placement and alignment of graphics.

drag to move an object by using a mouse. To do this, move the mouse pointer to the object, and then hold down a mouse button (usually the leftmost button if there is more than one), and move the mouse. The selected object will move with the mouse pointer, as if the pointer were dragging the object. When you are finished, drop the object by releasing the button.

FIGURE 85. Drag

drag and drop

1.the ability to move text or graphics by dragging it to a new location with the mouse.

2.a method of opening an application program. Simply pick up a file icon and drag it to the icon of an application program that can open the file.

drain one of the three regions in a field-effect transistor.

DRAM (Dynamic Random-Access Memory, pronounced “D-ram”) a computer memory that requires a refresh signal to be sent to it periodically. Almost all computers use DRAM chips for memory. See EDO; MEMORY;

RAM; RDRAM; SDRAM. Contrast SRAM.

draw program a graphics program that operates in terms of lines and shapes. Unlike a paint program, a draw program treats the picture as a collection of objects, each of which will be printed as sharply as the printer can print. Thus, the sharpness of the picture is not limited by the resolution of the screen. Also, individual circles, lines, rectangles, and other shapes can be moved around without affecting other objects they overlap. However, individual pixels cannot be edited. Draw programs are sometimes called vector graphics or object-oriented graphics programs.

Draw programs are preferred for drawing diagrams, while paint programs are superior for pictorial artwork or correcting photographs. Some

popular draw programs include Adobe Illustrator, CorelDraw, and Macromedia Freehand. Contrast PAINT PROGRAM.

FIGURE 86. Draw program

drill down (informal) to follow a series of menus or otherwise reach a piece of information through a series of steps.

drive see DISK DRIVE.

drive bay a space in a computer enclosure that can hold a disk drive.

driver see DEVICE DRIVER.

drizzle an algorithm to combine digital images while resampling them to a higher resolution, invented by Andrew Fruchter and Richard Hook for processing Hubble Space Telescope data.

The drizzle algorithm gives sharper and smoother results than ordinary resampling. It is called “drizzle” because each input pixel is treated like a drop of water that is to be divided into smaller droplets. Its content is put into output pixels in proportion to the part of its area that falls on each of them.

The key step, however, is that before this is done, each drop is shrunk so that it does not cover the entire original pixel, but only its central portion. In effect, the drizzle algorithm resamples from spots rather than squares. As a result, the larger pixel size of the original images has less of a degrading effect on the output. See RESAMPLE.

DRM (Digital Rights Management) a system for restricting the use and copying of digital versions of intellectual property (such as music and movies). Several different systems have been used, but determined copyright infringers can find ways around them (since any digital con-

tent is subject to being copied by the right hardware). Also, DRM systems may be too restrictive by preventing content purchasers from exercising rights that are allowed under copyright law. Libraries and museums are concerned that in the future, they may not be able to play today’s media, even though suitable equipment and software are available, if no one is still in business who can continue to verify the licenses. On the other hand, artists’ livelihoods are at risk when their works can be freely copied. See COPYRIGHT; DIGITAL MUSIC.

drop cap (drop capital) a large capital letter that occupies more than one line at the beginning of a chapter in a book.

FIGURE 87. Drop caps

drop-down menu a menu that appears when a particular item in a menu bar is selected (see Figure 88). Also called a pop-up menu. See also

MENU BAR; PULL-DOWN MENU.

FIGURE 88. Drop-down menu

drum in a LASER PRINTER, the large photosensitive cylinder that receives the image and then transfers it to paper.

To print a page, the drum is first given an electric charge of several thousand volts from the CORONA WIRE to make it repel toner. Then it is scanned with a laser; the points hit by the laser conduct electricity and lose their charge. Finally, fine particles of TONER are applied to the drum; they stick only in the places discharged by the effect of the laser.

Then the drum rolls the image onto the transfer roller, which transfers it to the paper, and the paper is heated by the FUSER to melt the toner particles and make them stick.

Damage to the drum of a laser printer causes streaks and spots that recur on every page. Some printers include a new drum in every toner

cartridge; a recycled cartridge may contain a previously used drum. Because the drum is photosensitive, it should not be exposed to bright light.

DSA (Digital Signature Algorithm) a U.S. federal standard for digital signatures using public key encryption, developed in 1991 by the National Institute of Standards and Technology. See

ENCRYPTION.

DSL (Digital Subscriber Line) any of several ways of transmitting highspeed digital signals over existing telephone lines (see also HDSL;

SDSL; VDSL).

The DSL service usually offered to homes and small businesses is known as ADSL Lite or G.lite (see ADSL) and provides maximum data rates of 1.5 Mbps upstream and 0.5 Mbps downstream and is what the rest of this article will describe.

DSL uses radio-frequency signals in wires that were designed for ordinary audio. This makes it possible to provide “always-on,” constant network connections, and at the same time, provide ordinary telephone service on the same line.

The strength of the radio-frequency signals falls off rapidly with distance from the DSLAM (DSL Access Multiplexer) at the telephone company’s central office (CO). At greater distances, or where signals are weak for other reasons, lower data rates are used, down to 384 kbps downstream. Beyond 15,000 feet from the DSLAM, DSL service is usually not available. However, as demand for DSL increases, telephone companies are quickly installing DSLAMs in wiring boxes along major roads.

The incoming phone line connects to a special DSL modem, which, in turn, is connected to a combined ROUTER and FIREWALL and then to the local area network. Ordinary telephones connect to the same line through one or more filters (see DSL FILTER).

Compare T1 LINE; T3 LINE; ISDN.

DSL filter a device that blocks high-frequency DSL signals from entering ordinary telephone equipment, where they might cause noise on the telephone, or, worse, be absorbed and weakened, interfering with DSL performance elsewhere in the building.

The best place for a DSL filter is where the phone line enters the house. The line should split there; one branch should go through a DSL filter to the rest of the ordinary telephone wiring, and a separate branch should go through high-quality Cat 5 or better cable directly to the DSL modem.

DSL filters can also be installed on individual telephones, preferably at the wall outlet.

DSLAM (DSL Access Multiplexer) a device that joins a high-speed computer network to a set of ordinary telephone lines in a telephone company central office. In order to offer DSL service, a telephone company has to install a DSLAM and give it a fast connection to the Internet, which will be shared by the DSL subscribers.