Barrons Publishing Dictionary of Computer and Internet Terms 10th

Often the port number and/or the filename is unnecessary. For example,

http://csam.uga.edu:80

means connect to port 80 of csam.uga.edu using Hypertext Transfer Protocol, and ftp://ai.uga.edu/pub/natural.language/Contents means to download a particular file from ai.uga.edu by File Transfer Protocol. A user name and password can be included thus:

ftp://username:password@www.somewhere.com/directoryname/filename

This is a handy way to access your web site on its server from your browser. See also FTP.

The URL for a World Wide Web site often includes www, as in http://www.spu.edu. Other possible protocols include gopher and telnet. See ABSOLUTE URL; BROWSER; GOPHER; HTML; HTTP; INTERNET;

PROTOCOL; RELATIVE URL; TELNET; TILDE; TLD; WORLD WIDE WEB.

.us suffix indicating that an e-mail or web address is registered in the United States. Although available to all types of organizations, .us addresses are used mostly by local governments. See TLD.

U.S. legal the size of paper used for legal documents in the United States, 81⁄2 × 14 inches. Contrast EXECUTIVE SIZE; U.S. LETTER. See also PAPER SIZES; illustration at Figure 151 on page 281.

U.S. letter the size of paper used for business letters in the United States, 81⁄2 × 11 inches. Elsewhere, ISO size A4 is the nearest equivalent.

Contrast EXECUTIVE SIZE; U.S. LEGAL. See also PAPER SIZES; illustration at Figure 151 on page 281.

USB (Universal Serial Bus) a standard way of connecting peripherals to computers, designed to replace serial, parallel, SCSI, and other kinds of ports. USB 1.0 provides a data rate of 12 million bits per second (Mbps), slower than its rival, Firewire (IEEE 1394), but cheaper to implement. USB 2.0 (“Hi-Speed USB”) achieves 480 Mbps and is fully compatible with the previous version. The maximum cable length is 5 meters (16 feet). Any USB port can be expanded into many ports by using a USB hub, and long cable runs can be achieved by inserting a hub every 5 meters (5.5 yards).

Unlike parallel, serial, and SCSI ports, USB ports allow devices to be plugged in and unplugged while the computer is running. Also, USB ports provide a limited amount of electric power to external devices, so that in many cases the peripherals do not need their own power connectors. USB ports are even used to power lights and speakers.

Figure 284 shows the USB “trident” logo, which represents a branching cable; the USB type A connector, which plugs into the computer; and the type B connector, which plugs into a peripheral device (although many peripherals use type A connectors instead).

USB was developed by Intel and other companies starting in 1997. For more information see www.usb.org.

FIGURE 284. USB: symbol, type A connector, type B connector

USB flash drive a small, keychain-sized FLASH MEMORY device with a USB interface (Fig. 285), treated by the computer as if it were a disk drive; also called a thumb drive or a jump drive.

USB flash drives have practically replaced diskettes as a handy way to transport data. They can be carried in one’s pocket and plugged into any computer for immediate access. The next step will be to build a CPU into the device so that it becomes a complete computer.

FIGURE 285. USB flash drive

USB hard disk a HARD DISK that connects to the USB port of a computer. USB hard disks are usually portable, with their own enclosures and power supplies, and are convenient backup devices since they can be attached to the computer when needed and stored in a safe place the rest of the time.

USB root hub the interface between a MOTHERBOARD and a set of USB ports.

USB storage device a data-storage device that connects to a USB port; a

USB FLASH DRIVE or USB HARD DISK.

USC (United States Code), the collection of laws of the United States of America, available online at uscode.house.gov. For example, 17 USC 1201 means Title 17, Section 1201.

use cases a description of events that occur as a user interacts with a system. Ivar Jacobson, one of the designers of UML, introduced the concept, although there are disagreements over whether developing use cases helps the process of system development. See www.pols.co.uk/usecasezone.

Usenet

1. a set of thousands of NEWSGROUPs (discussion forums) distributed via the Internet (formerly distributed through the Usenet wide-area network). Newsgroups have descriptive names such as sci.astro.amateur and are arranged into hierarchies (classifications), of which the main ones are:

Usenet has no headquarters; the messages are copied back and forth among numerous servers. However, in recent years, participation in this network has declined, and many users prefer to access the newsgroups through groups.google.com (Google Groups). This sometimes creates the false impression that Google runs the whole system.

2. a wide-area network for UNIX machines that formerly exchanged files by modem through the UUCP (“UNIX-to-UNIX copy”) command. Usenet addresses were of the form psuvax!ugacc!aisun1!mcovingt (which means “user mcovingt on machine aisun1, which can be reached through ugacc, which can be reached through psuvax”). Usenet has been supplanted by the INTERNET.

user-friendly easy for people to use.

In the days when computers were operated only by specialists, little attention was paid to making programs user-friendly. However, when computers became more popular, it became very important to write programs that could be used easily by nonspecialists.

The most important requirement for making a program user-friendly is that the program should fit the task—that is, it should be suitable for the job it is supposed to do, and should take advantage of what the user already knows about how to do the work. Also necessary are clear, understandable documentation and good online help.

Other features that can help make a program user-friendly are menus that clearly list the available choices and command names that are easy to remember. However, a program should not contain so many on-screen explanatory messages that it becomes cumbersome to use for people who have already learned how the program works. See also INTUITIVE.

user interface the way a computer program communicates with the person who is using it. There are three important types of user interfaces:

1.Command languages. This is a common way of giving instructions to operating systems; for instance, in DOS, OS/2, or VAX/VMS, the user can obtain a list of files by typing the command dir.

Command languages work well only if they are used constantly so that the user never forgets the commands.

2.Menus. The user chooses an item from a displayed list. Menus are ideal for software that is seldom used, but experienced users may find them too slow.

3.Graphical environments. The user performs operations by selecting icons (pictures) with a mouse. Environments of this type can be highly productive. For examples, see MACINTOSH; WINDOWS (MICROSOFT). A drawback is that there is no simple way to describe

how something is done; you almost have to see someone else do it. By contrast, commands in a command language can be written down on paper and even embedded in computer programs.

/usr in UNIX, a directory that formerly contained users’ files (hence the name) and now contains installed software and parts of the operating system. Users’ files are now usually stored in subdirectories of /home.

UTF-8, UTF-16 see UNICODE.

utility a program that assists in the operation of a computer but does not do the main work for which the computer was bought. For instance, programs that compress data or defragment disks are utilities (see DATA COMPRESSION; FRAGMENTATION; NORTON UTILITIES). By contrast, word processors, financial programs, engineering programs, and other programs that do actual work for the user are called application programs.

UTP (unshielded twisted pair) a cable consisting of pairs of wires twisted together and insulated but not provided with a metallic shield. See

TWISTED PAIR. Contrast COAXIAL CABLE.

UUCP (UNIX-to-UNIX copy) a command to transfer files from one computer to another. It has largely been superseded by FTP. See FTP. The Usenet network was sometimes called the UUCP network. See USENET.

uudecode see UUENCODE.

uudecode, uuencode see BASE64.

uuencode a UNIX utility program that makes it possible to send binary files through electronic mail systems that only accept text files. A uuencoded file consists of printable characters arranged in lines of reasonable length. The uudecode program regenerates the original binary file from it. See also BINARY FILE; TEXT FILE.

UV protector a clear filter that protects a camera lens from dust and dirt and excludes ultraviolet (UV) light, supposedly resulting in a slightly sharper picture. If the UV protector is of low optical quality, it can easily degrade the picture rather than improving it.

UXGA (Ultra eXtended Graphics Array) a monitor resolution of 1600 by 1200 pixels.

V

vacuum flourescent display a type of display used on various electronic devices, based on vacuum tube triode technology. They operate at lower temperatures than traditional vacuum tubes, and produce a bright display.

vacuum tube an electronic component consisting of electrodes placed inside an evacuated glass tube. Vacuum tubes work by using electric and magnetic fields to control the movements of electrons that have been emitted by one of the electrodes.

A CRT (cathode ray tube) television or monitor screen is one example of a vacuum tube. The vacuum tubes originally used in computers performed the same type of functions that semiconductor diodes and transistors perform now. A vacuum tube diode consists of two electrodes: a cathode, which emits electrons, and an anode, or plate, which collects electrons. The diode conducts electricity only when a positive voltage is applied to the cathode. A vacuum tube triode contains an electrode called the grid, located between the cathode and the plate. The flow of electrons from the cathode to the plate is controlled by the electric field of the grid. (Similarly, the current flow from the emitter to the collector in a transistor is controlled by the voltage applied to the base.) Vacuum tube triodes can be used for amplification and logic functions.

An early electronic digital computer, the ENIAC, contained 18,000 vacuum tubes. The disadvantages of vacuum tube computers are that they are very big, they consume a great deal of power, and they generate a lot of heat. Also, it is necessary to constantly replace burned-out tubes. See COMPUTERS, HISTORY OF.

value-added reseller someone who buys computers, improves them in some way (e.g., by installing ready-to-use software of a specific type), and then sells them as complete working systems. See also

vampire tap a cable-piercing connector used with thickwire Ethernet (10base-5) coaxial cables. See 10BASE-5; COAXIAL CABLE.

vanilla (slang) plain, without extra features; for example, one might speak of “vanilla” Windows XP as opposed to Windows XP with .NET Framework. This term comes from the widespread misconception that vanilla ice cream is unflavored, a mistake that can only be made if one judges ice cream by its color and not its taste. See also FLAVOR.

vaporware software that is announced by a vendor but never actually reaches the market.

var (in Pascal) the keyword that marks variable declarations. In the parameter list of a procedure or function, var means that an item should be passed by address, not by making a copy of it.

VAR see VALUE-ADDED RESELLER.

variable a symbol in a programming language that represents a data item, such as a numerical value or a character string, that can change its value during the execution of a program. Programming languages typically require that variable names start with letters. See also ANONYMOUS VARIABLE; DATA STRUCTURES; DATA TYPES; SINGLETON VARIABLE.

variable pitch varying width of characters in a typeface; the I, for instance, will be much narrower than the W. Also called See Figure 104, page 193, at FIXED-PITCH TYPE.

variant (also called union) a data type that can take on values corresponding to more than one other type.

VAX popular line of minicomputers produced by Digital Equipment Corporation in the 1970s and 1980s. VAX stood for Virtual Address Extension (i.e., hardware support for virtual memory). See

MENT CORPORATION; VAX/VMS.

VAX/VMS the most widely used operating system for Digital Equipment Corporation VAX computers (now obsolete).

An unusual feature of VAX/VMS was the way it kept previous versions of files. If you created a file called XXXX.YYY, it would be known as XXXX.YYY;1. If you then edited it, you would create XXX.YYY;2, and XXXX.YYY;1 would still exist. Whenever you did not specify a version number, you would get the latest version of the file.

VB.NET see VISUAL BASIC.

VBScript a Microsoft product for adding executable commands to a web page using a language based on Visual BASIC. For another example of a web page scripting language, see JAVASCRIPT.

VDSL (Very-high-bit-rate Digital Subscriber Line) a type of high-speed network connection provided through ordinary telephone lines. VDSL is very much like ordinary DSL (ADSL), but the communication rate is much faster, up to 51 Mbps at very short distances from the DSLAM. See DSL and cross-references there.

VDT (video display terminal) a computer screen. See MONITOR.

vector graphics a method of creating pictures on a computer by telling it to draw lines in particular positions. The lines may be displayed on a screen or plotted on paper. An advantage of vector graphics is that a picture can be enlarged or reduced without loss of sharpness, since the picture is not made of a fixed number of pixels. Contrast BITMAP GRAPHICS.

See also DRAW PROGRAM; PAINT PROGRAM.

vector processor a computer that can perform an operation on an entire array of numbers in a single step; also known as an array processor. Cray supercomputers are vector processors. They have machine instructions that can (for example) add each entry of array A to the corre-

virtual machine a computer that does not physically exist but is simulated by another computer. In IBM’s VM/ESA operating system, the computer simulates multiple copies of itself.

A Java applet is run by a Java virtual machine, which is part of a web browser that executes Java bytecode. See JVM.

virtual memory a way of extending the effective size of a computer’s memory by using a disk file to simulate additional memory space. Typically, the operating system keeps track of which memory addresses actually reside in memory and which ones must be brought in from disk when they are referred to. Virtual storage is divided into units called pages, and the operation of bringing in a page from disk is called a page fault.

virtual private network see VPN.

virtual reality the simulation of a person’s entire environment (sights, sounds, movements, etc.) by computer. Most commonly this is done by displaying a realistic picture (see THREE-DIMENSIONAL GRAPHICS) and allowing the user to manipulate objects by clicking on them with the mouse. See VRML.

Truly realistic virtual reality is more a goal than an existing technology, but some computer games achieve a limited kind of realism by having the user wear a special helmet and gloves and/or stand on a special platform.

virtual storage see VIRTUAL MEMORY.

virus a computer program that automatically copies itself, thereby “infecting” other disks or programs without the user knowing it, and then plays some kind of trick or disrupts the operation of the computer.

Viruses have existed as academic pranks since the 1960s, but 1987 saw the first malicious viruses, apparently the work of disgruntled programmers trying to sabotage their competition.

Nowadays, most virus infections result when the user of a computer is tricked into running a malicious program by opening it as a file. To avoid viruses, never open a file that arrives unexpectedly in e-mail, even if it’s from someone you know and trust (see WORM VIRUS). Always run antivirus software; keep your antivirus software up to date; and always install software from the manufacturer’s original disks rather than from copies made on a potentially infected machine.

Knowingly spreading a computer virus is a crime under common law and under specific laws in various states. See also ANTIVIRUS SOFTWARE;

MACRO VIRUS; SPYWARE; TROJAN HORSE; WORM VIRUS.

virus protection software a computer program that offers protection from VIRUSES by making additional checks of the integrity of the operating system. No vaccine can offer complete protection against all viruses. Also known as ANTIVIRUS SOFTWARE.

Visicalc the first computer spreadsheet program, developed for the Apple II in the late 1970s by Dan Bricklin and Bob Frankston. See SPREADSHEET.

Visio a widely used software package for creating business and technical diagrams. Unlike earlier graphics software, Visio recognizes the ways in which symbols interact with their environment. Each symbol, or “SmartShape,” can be programmed to behave in specific ways as it is moved or resized. For example, in an electronic circuit diagram, when you stretch the symbol for a resistor, the stretching affects only the connecting lines on the ends; the symbol itself remains the proper shape. Thus, drafting can be done quickly, and Visio helps keep a complex diagram properly connected together as parts of it are rearranged. In 2000, Visio Corporation was purchased by Microsoft. Their web address is www.microsoft.com/office/visio.

FIGURE 289. Visio graphics software

vision, computer the use of computers to recognize objects in images (not just store, retrieve, and manipulate the images themselves). Computer vision is surprisingly difficult to implement. Some of the main challenges are: edge detection (finding the boundaries of objects); perspective (interpreting a two-dimensional image as three-dimensional); color constancy (adapting to variations in the color of light, including light reflected from other nearby objects); and shape constancy (recognizing the same shape from different angles).

Computer vision has found some industrial applications where the number of objects to be recognized is small and the positioning and light