ULV Ultra Low Voltage. UML (Unified Modeling Language) a language used for the visual repre- sentation of software systems (and other types of systems). Much of UML was developed at Rational Software, and it was adopted as a stan- dard by the Object Management Group (OMG) in 1997. UML includes standard notation for representing classes and their attributes and asso- ciations, and it includes state transition, interaction, component, and deployment diagrams. Figure 279 shows an example of a UML class diagram for a program that keeps track of the movement of boats. Each class is enclosed in a rectangle, with the name of the class at the top. The middle part of the rectangle lists attributes (or variables) for the members of that class. FIGURE 279. UML class diagram For example, boats have a name represented as a string, x and y posi- tion coordinates and vx and vy velocity components, all of which are double-precision numbers. The bottom part of the class lists the opera- tions (or methods) of that class. In this example, two specific types of boats (sailboats and motor- boats) inherit from the class Boat. Inheritance is indicated by an open arrow. Another class is the wind. The line connecting the wind and sail- boat class indicates an interaction between those two classes since the operation that sets the velocity of the sailboat needs to know the wind direction and speed. Figure 280 shows an example of a UML deployment diagram for a three-tier system where customers use a browser to connect over the Internet to the web server. The web server is, in turn, connected to the corporate mainframe database. UML is designed for the development of complicated systems, so a complete design will include many different UML diagrams. UML dia- grams can be created by hand or with a specialized software drawing tool. 499 UML 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 499 FIGURE 280. UML deployment diagram unboxing (in Microsoft .NET Framework) the opposite of BOXING; the automatic conversion of simple objects back into value types, such as numbers and structs, after an operation for which BOXING was needed. UNC (Uniform Naming Convention) the standard way of identifying shared resources (disks, directories, or printers) on Microsoft networks. UNC names have the form \\computer\resource or \\computer\resource\path For example, if computer zeta shares its drive C under the name cdrive, then file c:\MyDoc\letter.doc on that machine is accessible through the network as \\zeta\cdrive\MyDoc\letter.doc. Under Windows 2000 and later versions, URLs (Uniform Resource Locators) can be used as an alternative to UNC paths. The example just given would be file://zeta/cdrive/MyDoc/letter.doc in URL form. underflow in computation, a situation that arises when a number is too small to be represented in the chosen format; it may then be incorrectly represented as 0. For example, on an 8-digit fixed-point calculator, the computation 1÷1,000,000,000 would cause an underflow because the result, 0.000000001, could not be distinguished from zero. On most computers, that particular computation would not cause a problem, since computers normally use a binary form of EXPONENTIAL NOTATION, but there are limits on the size of the exponent; for example, a computer might not be able to represent 2 –1024 because the exponent would have too many binary digits. Compare OVERFLOW. underrun an unexpected lack of data. When recording a CD-R or DVD-R, the drive must run at a fixed speed throughout the recording process. If the computer cannot deliver data to it fast enough—because of a net- work problem or because the data is coming from a slower device—an underrun occurs, and the disc is not recorded successfully. The cure is to make sure no other software is running, record at a lower speed, and, if necessary, temporarily take the computer off the network. unboxing 500 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 500 501 Unicode undo a command that allows the computer user to reverse the effects of the most recent operation. If “Undo” is DIMMED (printed in gray), it is not possible for your program to undo a command (perhaps you have per- formed an intermediate step—or perhaps the most recent command was a complex operation that cannot be undone). For this reason, it is wise to save different versions of the file as you work so that you will have a recent version to REVERT to in case of disaster. See also REDO. undocumented not described in the literature (documentation) provided by the manufacturer of a product. Some computer programs rely on undocumented features of the machine on which they run. The risk of using undocumented features is that they may not be the same in future versions of the program or machine. Undocumented features exist for several reasons. Some are acciden- tal omissions from the documentation. Others represent incompletely tested features that will be made reliable and documented in future ver- sions. Still others may be kept secret in order to give the vendor a com- petitive advantage in making add-on products. See also DOCUMENTATION. unerase see RECOVERING ERASED FILES. ungroup (in vector graphics) to cause a grouped object to be broken down into its component objects. It is necessary to ungroup to change the attributes of a single object or to change its shape. unhandled exception an error condition, such as inability to write on a file, that was detected while running a program that provided no specific way of handling it. This is a common error message in the .NET Framework, and if the cause is not obvious, it reflects a programmer’s error. See EXCEPTION; .NET FRAMEWORK. Unicode a system for representing characters using up to 20 bits, allowing for 1,048,576 different characters, enough to represent all the written languages of the world, including Japanese and Chinese. This contrasts with the 256 characters possible in ASCII and similar systems. The Unicode standard is not yet complete. Originally, Unicode char- acters were 16 bits, as in the UTF-16 format described later, and only 65,536 characters were possible. Unicode version 3 goes beyond this limit and defines over 90,000 characters. Complete information is avail- able at www.unicode.org. The first 128 Unicode character codes are the same as ASCII, includ- ing end-of-line marks (see CR, LF). In various programming languages and editors, Unicode character codes are written as U+XXXX or \uXXXX, where XXXX stands for a series of hexadecimal digits; thus, the letter A, ASCII hexadecimal 41, is Unicode U+0041. Figure 281 shows exam- ples of Unicode characters. There are several kinds of Unicode text files. The most important are: • UTF-8—Same as ASCII for codes up to 127; thus, a UTF-8 file can also be an ASCII file. Higher-numbered codes are represented by sequences of up to 4 bytes. 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 501 • UTF-16 big-endian—Each character occupies 2 bytes (16 bits), high-order byte first. The file begins with hexadecimal FE FF or with any Unicode character. Codes higher than 16 bits are repre- sented by pairs of 16-bit sequences. • UTF-16 little-endian—Just like UTF-16 big-endian, except that each pair of bytes has the low-order byte first, and the file begins with hexadecimal FF FE (representing the value FEFF). This is the Unicode system normally used in Microsoft Windows. Unicode is used internally by the Java programming language and many newer software packages. However, the characters that you will actually see on your machine are limited by the fonts installed. Hint: When you open a UTF-16 file in an ASCII text editor on a PC, you generally see characters separated by blanks (“l i k e t h i s”—the blanks are really ASCII 0). The remedy is to use a Unicode editor, such as Windows Notepad, and save the file as ASCII. See also ANSI; ASCII; BIG-ENDIAN; CHARACTER SET; LITTLE-ENDIAN. Character ASCII Unicode $ hex 24 U+0024 A hex 41 U+0040 π none U+03C0 ℵ none U+05D0 FIGURE 281. Unicode characters uniform fill one solid color or tint that fills a graphical object. Contrast FOUNTAIN FILL. FIGURE 282. Uniform fill uninitialized variable a VARIABLE in a computer program that has not been given a value. It contains random data depending on its exact location in memory and the programs that have run previously. Commonly, a pro- gram containing this kind of error will run correctly most of the time (when the variable happens to contain a reasonable value) but will fail at unpredictable times. Some compilers automatically initialize all variables to zero or signal an error when an uninitialized variable is used. See INITIALIZE (definition 1). uninterruptible power supply (UPS) a power supply that uses batteries to continue providing electricity to a computer for a few minutes in the event of a power failure. This makes it possible to shut down the com- puter in an orderly way without losing data. uniform fill 502 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 502 A UPS is not the same as a SURGE PROTECTOR. The surge protector absorbs momentary (millisecond-long) spikes of excess voltage; the UPS protects against the absence of voltage for minutes or hours. Most UPSes include surge protection, but not vice versa. The batteries in a UPS deteriorate with age. A UPS that has been in use for two or three years generally has much less capacity than when it was new. Usage note: The spelling uninterruptable is also widely used. Both spellings, with -ible and -able, are consistent with the word’s Latin etymology. union a data item that can hold values of more than one type. See VARIANT. uniprocessor free see FREE. universal serial bus see USB. UNIX an operating system, or family of operating systems, developed at Bell Laboratories in the early 1970s as a replacement for an earlier sys- tem called Multics. UNIX is noteworthy because, although developed by a small team of about four people, it is in many ways superior to operat- ing systems developed by large teams at tremendous expense (e.g., OS/360). The current version of the Macintosh operating system (MAC OS X) is based on UNIX, as is LINUX. The main features of UNIX include the following: 1. Highly modular, structured design. The entire system is defined in terms of a relatively small number of subroutines; UNIX can be implemented on any machine by implementing those routines. Furthermore, the operation of the whole system is based on a rel- atively small number of principles carried through very consis- tently; there is little arbitrary detail for the user to learn. 2. Extensibility. In many operating systems, users can create com- mands of their own that execute sequences of ordinary commands stored on files. UNIX, however, provides a full-fledged program- ming language for this purpose (the “shell”), with if, while, and case statements similar to those of Pascal, as well as extensive facilities for string handling. 3. Input-output redirection. Any program invoked by a command can be told to take its input from, and/or write its output onto, a file rather than the user’s terminal. For example, sort <alpha >beta tells the sort program to take its input from alpha and write its out- put onto beta. UNIX also supports pipes, which allow one program to transmit its output directly into the input of another program. See PIPE. Many UNIX commands, called filters, read a file and output a copy of it that has undergone some simple transformation, such as removing repeated lines. Powerful operations can be accom- plished by stringing filters together through pipes. 503 UNIX 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 503 4. Tree-structured directories. In all earlier operating systems, a user’s files were kept in a single list called a DIRECTORY. In UNIX, however, directories are handled like files, and a user can have an unlimited number of them, each of which must be listed in a higher directory until the main (“root”) directory is reached. This makes it possible to arrange files into logical groups—for example, to put all files related to a particular project into a single directory. Tree- structured directories are a virtual necessity if one user is to keep track of more than a few dozen files. The following are some common UNIX commands: cc Compile a C Program. (See C.) cp Copy a file onto a file or into a directory. diff Display the differences between two text files. grep Search a file for lines matching a pattern. ls List contents of a directory. mkdir Create a directory. lpr Print a file. rm Remove (delete) a file. rmdir Delete a directory (which must be empty). cat Copy file from standard input to standard output. chmod Change file permissions. See also /DEV; /ETC; /HOME; /USR. unsharp masking a method of sharpening an image by subtracting from it a blurred copy of itself. This was once done with photographic plates but is more easily done digitally, and most paint programs provide an unsharp masking filter. See also IMAGE PROCESSING and cross-references there. FIGURE 283. Unsharp masking uP (more properly µP, where µ is the Greek letter mu) abbreviation for MICROPROCESSOR. unsharp masking 504 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 504 uplevel pertaining to a later version of a product. For example, many Windows 3.1 programs are compatible with uplevel versions of Windows, such as Windows Vista. Contrast DOWNLEVEL. uplink 1. a connection through which signals can be transmitted to a satellite. Compare UPLOAD. 2. a connection from one hub to another hub. See HUB. upload to transmit a file to a central computer from a smaller computer or a computer at a remote location. See FTP; KERMIT. Contrast DOWNLOAD. UPnP (Universal Plug and Play) technology for standardizing connections between different devices. For information see the UPnP forum at www.upnp.org. uppercase capital letters, such as A, B, C, as opposed to a, b, c (lowercase) or A, B, C (small caps). The term “ uppercase” goes back to the early days of letterpress printing. The metal type was kept in divided drawers called cases; the capital letters were traditionally kept in the upper case, and the small letters in the lower. UPS 1. See UNINTERRUPTIBLE POWER SUPPLY. 2. (United Parcel Service) a major American carrier of packages for mail-order and e-tail businesses. (See E-TAIL.) Their web address is www.ups.com. upstream (describing data transmission) in a direction from the client to the server, or from the peripheral to the main computer. Compare UPLOAD. Contrast DOWNSTREAM. upward compatibility the situation in which a computer program or acces- sory works not only on the machine for which it was designed but also on newer models. For instance, programs written for the IBM PC in 1981 will still run (considerably faster) on present-day Pentium machines. Thus we say that the Pentium is upward compatible with the processor in the PC. Contrast DOWNWARD COMPATIBILITY. urban legend a story that is told by highly educated people as if it were true, but cannot be confirmed. Typically, it happened to a “friend of a friend” (i.e., a person who is almost, but not quite, identifiable), and it involves some improbable but highly amusing misadventure. The term was coined by Jan Harold Brunvand, who has written several books on the subject. The Internet is one of many means by which urban legends circulate. See also HOAX. URL (Uniform Resource Locator, Universal Resource Locator) a way of specifying the location of publicly available information on the Internet, in the form protocol://machine:port number/filename 505 URL 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 505 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, 8 1 ⁄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, 8 1 ⁄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 connec- tors. USB ports are even used to power lights and speakers. Figure 284 shows the USB “trident” logo, which represents a branch- ing 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. .us 506 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 506 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. 507 use cases 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 507 Usenet 1. a set of thousands of NEWSGROUPs (discussion forums) distributed via the Internet (formerly distributed through the Usenet wide-area net- work). Newsgroups have descriptive names such as sci.astro.amateur and are arranged into hierarchies (classifications), of which the main ones are: news. for announcements about Usenet itself; comp. for computer science and technology; sci. for other academic topics (including humanities); soc. for cultural interest groups; rec. for hobbies and sports; talk. for wide-ranging discussions, often heated; misc. for a few topics that don’t fit elsewhere; alt. for trial newsgroups and “alternative” topics. 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 pro- grams 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. Usenet 508 7_4105_DO_CompInternetTerms_U 12/29/08 10:39 AM Page 508 [...]... Separately, Microsoft and IBM collaborated on a 32-bit multitasking operating system for the 80386, 486, and Pentium, but the collaboration broke up and produced two separate products, IBM OS/2 and Microsoft Windows NT, the direct ancestor of present-day Windows versions 7_ 4105 _DO_CompInternetTerms_W 12/29/08 Windows (Microsoft) 10: 41 AM Page 530 530 Internally, Windows NT resembles UNIX, and UNIX software... 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 7_ 4105 _DO_CompInternetTerms_V... R4x00, and PowerPC CPUs (See ALPHA.) Versions of Windows XP support AMD64 (x64) and Itanium (IA-64) CPUs Windows Vista is available for the x64 but not the Itanium See also COMMAND PROMPT; DEVICE DRIVER; MULTITASKING; OS/2; UNIX 7_ 4105 _DO_CompInternetTerms_W 531 12/29/08 10: 41 AM Page 531 Windows (Microsoft) FIGURE 300 Windows user interface FIGURE 301 Windows versions 7_ 4105 _DO_CompInternetTerms_W... necessary because all functions of Windows are accessible without it See WINDOWS FIGURE 297 Win key Win2K unofficial abbreviation for Windows 2000 Win32 the 32-bit core of Windows 95 and its successors Programs written for Win32 will run under all of these operating systems 7_ 4105 _DO_CompInternetTerms_W 529 12/29/08 10: 41 AM Page 529 Windows (Microsoft) Win32s a subset of Win32 that could be installed... 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... number of objects to be recognized is small and the positioning and light 7_ 4105 _DO_CompInternetTerms_V vision, human 12/29/08 10: 40 AM Page 516 516 are constant, such as identifying objects that pass by on an assembly line and sounding an alarm when one of them is missing or looks abnormal See also IMAGE PROCESSING vision, human the ability to see Computer displays rely on many special properties of human... distinctive characteristics of their voices See BIOMETRICS void in C and related programming languages, a keyword that means “no data type” and declares a function or method that returns no value or a pointer to an unknown type 7_ 4105 _DO_CompInternetTerms_V VoIP 12/29/08 10: 40 AM Page 518 518 VoIP (Voice over Internet Protocol) the transmission of voice telephone conversations through the Internet or through... connected to the Internet continuously, and it runs an HTTP server program 7_ 4105 _DO_CompInternetTerms_W 12/29/08 10: 41 AM 525 Page 525 weld such as Apache, Microsoft’s Internet Information Server, or Netscape’s Enterprise Server The web server may also need to run special software (such as a CGI script) in response to a client’s request See WEB PAGE; WORLD WIDE WEB web services programs running on computers... (heaviness) of a style of type For example, this is heavy-weight type and this is ordinary-weight type Some fonts provide several different weights (light, medium, demibold, bold, and extra bold) See Figure 296 FIGURE 296 Weight (of typefaces) weld (in vector graphics) to join two or more objects so that the resulting single object has the outline of the group of objects 7_ 4105 _DO_CompInternetTerms_W... monitor resolution of 1600 by 1200 pixels 7_ 4105 _DO_CompInternetTerms_V 12/29/08 10: 40 AM Page 510 510 vacuum flourescent display 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 . sure no other software is running, record at a lower speed, and, if necessary, temporarily take the computer off the network. unboxing 500 7_ 4105 _DO_CompInternetTerms_U 12/29/08 10: 39 AM Page. www.pols.co.uk/usecasezone. 507 use cases 7_ 4105 _DO_CompInternetTerms_U 12/29/08 10: 39 AM Page 507 Usenet 1. a set of thousands of NEWSGROUPs (discussion forums) distributed via the Internet (formerly distributed. each entry of array A to the corre- 511 vector processor 7_ 4105 _DO_CompInternetTerms_V 12/29/08 10: 40 AM Page 511 sponding entry of array B and store the result in the corresponding entry of array