Dictionary of Computer and Internet Terms phần 2 pps

The advantage of BCD shows up when a number has a fractional part, such as 0.1. There is no way to convert 0.1 into binary exactly; it would require an infinite number of digits, just like converting into decimal (see ROUNDING ERROR). But in BCD, 0.1 is represented by the code for 1 immediately after the point, and no accuracy is lost. BCD arithmetic is considerably slower and takes more memory than binary arithmetic. It is used primarily in financial work and other situa- tions where rounding errors are intolerable. Pocket calculators use BCD. FIGURE 31. Binary addition: half adder FIGURE 32. Binary addition: full adder binary file a file containing bits or bytes that do not necessarily represent printable text. The term binary file usually denotes any file that is not a text file, such as executable machine language code. Crucially, special software is required to print a binary file or view it on the screen. Contrast TEXT FILE. binary multiplication a basic operation in computer arithmetic. For single- digit numbers, the binary multiplication table is very simple and is the same as the Boolean AND operation (see AND GATE): 0 × 0 = 0 0 × 1 = 0 1 × 0 = 0 1 × 1 = 1 For numbers with more than one digit, the computer does something very similar to what we do to decimal numbers in pencil-and-paper arithmetic. To find 13 × 21 in decimal, we proceed like this: 1 3 51 binary multiplication 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 51 21 × 13 63 21 273 First, find 3 × 21. Then, find 10 × 21, and add these two results together to get the final product. Note that the product has more digits than either of the two original numbers. You can follow the same procedure to multiply two binary numbers: 10101 × 01101 10101 00000 10101 10101 00000 100010001 Notice that each of the partial products is either zero or a copy of 10101 shifted leftward some number of digits. The partial products that are zero can, of course, be skipped. Accordingly, in order to multiply in binary, the computer simply starts with 0 in the accumulator and works through the second number to be multiplied (01101 in the example), checking whether each digit of it is 1 or 0. Where it finds a 0, it does nothing; where it finds a 1, it adds to the accumulator a copy of the first number, shifted leftward the appropriate number of places. binary number a number expressed in binary (base-2) notation, a system that uses only two digits, 0 and 1. Binary numbers are well suited for use by computers, since many electrical devices have two distinct states: on and off. Writing numbers in binary requires more digits than writing numbers in decimal, so binary numbers are cumbersome for people to use. Each digit of a binary number represents a power of 2. The right- most digit is the 1’s digit, the next digit leftward is the 2’s digit, then the 4’s digit, and so on: Decimal Binary 2 0 = 1 1 2 1 = 2 10 2 2 = 4 100 2 3 = 8 1000 2 4 = 16 10000 Table 4 shows examples of numbers written in binary and decimal form. See also DECIMAL NUMBER; HEXADECIMAL NUMBER; OCTAL. binary number 52 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 52 TABLE 4 DECIMAL-BINARY EQUIVALENTS Decimal Binary Decimal Binary 0 0 11 1011 1 1 12 1100 2 10 13 1101 3 11 14 1110 4 100 15 1111 5 101 16 10000 6 110 17 10001 7 111 18 10010 8 1000 19 10011 9 1001 20 10100 10 1010 21 10101 binary search a method for locating a particular item from a list of items in alphabetical or numerical order. Suppose you need to find the location of a particular word in a list of alphabetized words. To execute a binary search, look first at the word that is at the exact middle of the list. If the word you’re looking for comes before the midpoint word, you know that it must be in the first half of the list (if it is in the list at all). Otherwise, it must be in the second half. Once you have determined which half of the list to search, use the same method to determine which quarter, then which eighth, and so on. At most, a binary search will take about N steps if the list contains about 2 N items. binary subtraction a basic operation in computer arithmetic. The easiest way to subtract two binary numbers is to make one of the numbers neg- ative and then add them. Circuits for doing binary addition are readily constructed with logic gates (see BINARY ADDITION). The negative coun- terpart of a binary number is called its 2-complement. Suppose that we have a number x, represented as a binary number with k digits. The 2-complement of x (written as x) is x = 2 k – x Then, to find the difference a – x we can compute a – x = a + x – 2 k This is easier than it looks, for two reasons. First, subtracting 2 k is triv- ial, because 2 k is a binary number of the form 1000, 100000, and so on, with k +1 digits. So all we have to do is discard the leftmost digit to get our k-digit answer. Second, finding the 2-complement of x is easy: just invert all the dig- its of x (changing 0’s to 1’s and 1’s to 0’s) and then add 1. See INVERTER. 53 binary subtraction 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 53 Suppose we want to compute 5 – 2 using 4-digit binary representa- tions. That is, we want to compute: 0101 – 0010 First, change the second number to its complement, change the minus to a plus, and subtract 2 k : 0101 + 0010 – 10000 To actually compute the complement, invert the digits of 0010 and add 1, so the whole computation becomes: 0101 + (1101 + 1) – 10000 Evaluate this expression by performing the two additions 0101 + 1101 + 1 = 10011 and then throwing away the leftmost digit, giving 0011 (= 3), which is the answer. This method for handling subtraction suggests a way to represent neg- ative numbers. Suppose we want to represent –3. Positive 3 is binary 011. Negative 3 can be represented by the 2-complement of 3, which is the binary representation of 5: 101. However, we need an extra bit to indicate that 101 indicates –3 instead of 5. The bit indicating the sign will be included as the first digit of the number, with 1 indicating nega- tive and 0 indicating positive. The range of numbers that can be represented is different than before. Without the sign bit, 4 binary digits can hold numbers from 0 to 15; with the sign bit, the numbers range from –8 to 7. The table shows how. Positive Numbers Negative Numbers Decimal Binary Decimal Binary 0 0 0 0 0 1 0 0 0 1 –1 1 1 1 1 2 0 0 1 0 –2 1 1 1 0 3 0 0 1 1 –3 1 1 0 1 4 0 1 0 0 –4 1 1 0 0 5 0 1 0 1 –5 1 0 1 1 6 0 1 1 0 –6 1 0 1 0 7 0 1 1 1 –7 1 0 0 1 –8 1 0 0 0 On real computers it is typical to use 16 bits (2 bytes) to store integer values. Since one of these bits is the sign bit, this means that the largest positive integer that can be represented is 2 15 – 1 = 32,767, and the most negative number that can be represented is –(2 15 )= –32,768. Some pro- gramming languages also provide an “unsigned integer” data type that ranges from 0 to 65,535. binary subtraction 54 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 54 bind to associate symbols with data, or to associate one piece of data with another, in several different ways, among them: 1. to give a variable a value; to INITIALIZE it. 2. to associate a network protocol with a particular Ethernet port or the like. See PROTOCOL. 3. to map an XML document onto a set of variables or objects in Java or another programming language. 4. to put together the pages of a book. binding see BIND (all definitions). biometrics measurable physical characteristics of the human body, used to identify an individual for security purposes. They include fingerprints, the distinctive appearance of faces and eyes, and the distinctive sound quality of one’s voice. There are computer input devices to read these characteristics. BIOS (Basic Input Output System) a set of procedures stored on a ROM chip inside PC-compatible computers. These routines handle all input- output functions, including screen graphics, so that programs do not have to manipulate the hardware directly. This is important because if the hardware is changed (e.g., by installing a newer kind of video adapter), the BIOS can be changed to match it, and there is no need to change the application programs. The BIOS is not re-entrant and is therefore not easily usable by mul- titasking programs. Windows programs do not call the BIOS; instead, they use procedures provided by the operating system. BIOS enumerator the BIOS routine that tells a PLUG AND PLAY system what hardware is installed. bipolar transistor a semiconductor device formed by sandwiching a thin layer of P- or N-type semiconductor between two layers of the opposite type of semiconductor. (See TRANSISTOR.) The other general type of tran- sistor is the field-effect transistor (FET). bis Latin for “a second time,” used to denote revised CCITT and ITU-T standards. See CCITT; ITU-T. BIST (built-in self test) a feature included in newer integrated circuits and other electronic equipment. An electronic device that has BIST can test itself thoroughly whenever it is turned on. See INTEGRATED CIRCUIT. bit a shorthand term for binary digit. There are only two possible binary digits: 0 and 1. (See BINARY NUMBER.) Bits are represented in computers by two-state devices, such as flip-flops. A computer memory is a collec- tion of devices that can store bits. A byte is the number of bits (usually 8) that stand for one character. Memory is usually measured in units of kilobytes or megabytes. See MEMORY. 55 bit 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 55 One important measure of the capability of a microprocessor is the number of bits that each internal register can contain. For example, the classic Z80 microprocessor had 8-bit registers. The Intel 8088, used in the original IBM PC, had 16-bit registers but only an 8-bit bus, leading to some confusion as to whether it should really have been called a 16- bit processor. Newer microprocessors have 32 or 64 bits per register. In general, a processor with a greater number of bits per instruction can process data more quickly (although there are other factors to consider that also determine a computer’s speed). See also MICROPROCESSOR. The number of colors that can be displayed is sometimes given by listing the number of bits used to represent a color. For example, a 24- bit color system uses 8 bits for red, 8 for green, and 8 for blue, so it can display 2 8 = 256 different levels of each of the three primary colors, or 2 24 = 16,777,216 different mixtures of colors. See COLOR. The term bit is also used to indicate the quality of digitized sound, as in 8 bit or 16 bit. See SAMPLING RATE. bit bucket (slang) a place where data is lost. For example, under UNIX, the filename /dev/null can be used as a bit bucket; anything written to it will be ignored, but the program will think it is successfully writing to a file. bit depth in graphics, the number of bits that are used to record the inten- sity and color of each pixel. For example, 1-bit graphics can distinguish only black and white; 8-bit graphics can distinguish 256 shades of gray or 256 colors; and 24-bit graphics can distinguish more than 16 million colors. Sometimes bit depth denotes the number of levels of each color; for example, an image in which each pixel has 8 bits each for red, green, and blue might be called either a 24-bit image or an 8-bit RGB image. bitblt (bit-block transfer, pronounced “bitblit”) the rapid copying of a block of memory or a portion of an image from one place to another. Compare BLIT. bitlocker a security feature of Vista that encrypts data on a hard drive, using an encryption key contained in a separate microchip in the com- puter, or provided on a flash drive. bitmap a graphical image represented as an array of brightness values. For example, if 0 represents white and 1 represents black, then 00000000 01111110 01000010 01000010 01111110 00000000 is a bitmap of a black rectangle on a white background. Each point for which there is a value is called a PIXEL. See also DIGITAL IMAGE PROCESSING. bit bucket 56 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 56 Bitmaps can be imported into other application programs such as word processors and page layout programs, but you will not be able to edit bitmaps in those environments. You must use a PAINT PROGRAM to change bitmaps. Contrast VECTOR GRAPHICS. See also DRAW PROGRAM; PAINT PROGRAM. bitmap graphics a method of displaying pictures on a computer. The pic- ture is treated as a large array of pixels (see PIXEL), each of which is stored in a specific memory location. The picture is drawn by specifying the color of each pixel. Contrast VECTOR GRAPHICS. See also BITMAP; DRAW PROGRAM; PAINT PROGRAM. bitness (slang) the property of using a specific number of bits. For exam- ple, a single-precision integer and a double-precision integer differ in bitness. BITNET a wide-area network linking university computer centers all over the world. It originated in the northeastern United States in the early 1980s and was later combined with the Internet. Its most common use was to transmit electronic mail among scholars who were working together. BitTorrent a peer-to-peer file sharing system that reduces dependency on the original host (or the SEED) by having everyone who downloads the file also offer it for anonymous upload to others. The more people who download the file (and therefore host the pieces they already have), the faster the file is downloaded. This format is especially useful for large files such as rich media (movies, music, etc.). See www.bittorrent.com. .biz a suffix indicating that a web or e-mail address belongs to a business (in any country). Contrast .COM. See also ICANN; TLD. black hat someone who attempts to break into computers maliciously; a villain (like the characters in old Western movies who wore black hats). Contrast WHITE HAT. BlackBerry a wireless device produced by Research In Motion, Inc., which is a combination cellular telephone, PDA, and web browser. Web address: www.blackberry.com. See also PDA. Blackberry thumb an informal name for painful repetitive stress injuries caused by excessive typing on small keyboards like the ones on Blackberries or cellular phones. See CARPAL TUNNEL SYNDROME. Blackcomb Microsoft’s code name for the version of Windows that will succeed Windows Vista. Blackcomb is better known as Windows 7. blacklist a list of senders or sites from which messages will not be accepted. Synonyms: IGNORE LIST; KILL FILE. 57 blacklist 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 57 blend 1. a drawing program command that computes the intermediate shapes between two selected objects. You would use the blend command to make the smooth highlights on a rendering of a three-dimensional object. In many ways, the blend command is like the morphing special effects we see on television commercials. You could make the letter C turn into a cat, for example. However, blend has practical applications as well as the playful ones. You can use it to create equally spaced objects, such as lines for a business form. Align two identical objects, and then set the intermediate blend steps to the desired number. 2. a paint program filter that smooths colors and removes texture over a selected area. 3. A piece of digital art where several images have been combined seam- lessly into a visually interesting whole. Figures and objects are often lay- ered so that it takes several seconds to identify what you are seeing. FIGURE 33. Blend (in a draw program) blind copies see BCC. blit block image transfer, the rapid copying of a portion of an image (or, sometimes, any type of memory contents) from one place to another. Compare BITBLT. blittable capable of being copied rapidly by BLIT. bloatware (slang) bloated software; inefficient, slow software that requires unreasonable amounts of disk space, memory, or CPU speed. Too many added features can make bloatware difficult to use (see CREEP- ING FEATURISM) and prone to crashes. Many critics claim that much mod- ern software is designed to sell computers larger and faster than are actually needed to do the computations efficiently. block move the operation of moving a section of a file from one place to another within the file. See EDITOR. block protect to mark a block of text so that it will not be split across pages when printed out. This is useful to prevent a table or formula from being broken up. blog a “web log”; a type of personal column posted on the Internet. Most blogs consist of small, plentiful entries. Some blogs are like an individ- blend 58 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 58 ual’s diary while others have a focused topic, such as recipes or political news. Blogger a web site (www.blogger.com) providing one of the most popular and oldest web log services. Anyone can maintain a BLOG there and update it from any computer with an Internet connection. Blogger has been owned by GOOGLE since 2003. Compare LIVEJOURNAL; WORDPRESS; XANGA. blogosphere The world of BLOGs; the very loosely-knit community of blog writers and their audiences. The blogosphere provides important forums for political discussion and news reporting separate from the established news media. Blu-Ray disc an optical disc similar to a DVD and the same size, but read and written with a blue or violet laser, whose shorter wavelength makes a higher data density possible. Blu-Ray discs can hold 25 GB (single layer) or 50 GB (double layer). Contrast HD DVD. Blue Screen of Death (slang) (sometimes written BSOD) in Windows, a seri- ous error message displayed in white type on a blue screen, without any use of windows or graphics (see Figure 34). It usually means that the entire operating system has become inoperative. The memory addresses and file- names it displays are sometimes explained on www.microsoft.com, but they are usually meaningful to only the authors of Windows. After experiencing a “Blue Screen of Death,” one should always restart the computer in order to load a fresh copy of the FIGURE 34. Blue Screen of Death operating system into memory. Windows Vista usually reboots after a serious error of this type, often bypassing the blue screen. Bluetooth a standard for wireless networking of relatively slow devices in the same room; for details see www.bluetooth.org. The name alludes to a medieval Danish king. Contrast 802.11. 59 Bluetooth 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 59 blur a paint program filter that throws the image slightly out of focus. Blur can be repeated until the desired effect is achieved. See also MOTION BLUR. FIGURE 35. Blur filter .bmp the filename extension for files in Microsoft Windows that contain bitmap representations of images. See BITMAP. BNC connector a push-and-twist connector (see Figure 36) used to join coaxial cables in thinwire Ethernet networks and in some types of video equipment. See 10BASE-2; COAXIAL CABLE; ETHERNET. Contrast RCA PLUG. FIGURE 36. BNC connectors board 1. a printed circuit board for a computer, the MOTHERBOARD, or an add- on board, sometimes also called a card. Most computers contain expan- sion slots where you can add additional boards to enhance the capabilities of the machine. 2. a bulletin board system (BBS) or similar discussion forum. See BBS. boat anchor (slang) obsolete, useless machine. BODY tag used in HTML to indicate the main part of the material for a web page, as opposed to the HEAD. For an example see HTML. BOF (birds of a feather) an informal meeting of a group of computer pro- fessionals with an interest in common, held as part of a larger convention. blur 60 7_4105_DO_CompInternetTerms_B 12/29/08 10:18 AM Page 60 [...]... cascading style sheet a set of HTML rules governing the appearance of a set of pages at a web site on the World Wide Web Cascading style sheets use precedence rules to decide which of two commands should take effect in case of a conflict See STYLE SHEET CASE (computer- aided software engineering) the use of computers to help with the process of designing software case 1 the property of being capitalized (uppercase,... to which copies of the message should be sent See also BCC 7_4105_DO_CompInternetTerms_C 12/ 29/08 10 :21 AM CCD image sensor Page 82 82 FIGURE 51 CCD image sensor (in webcam with lens removed) CCD image sensor (charge-coupled device) the electronic image sensor most often used in digital cameras, video cameras, and scanners A lens forms an image on an array of cells (Figure 52) , each of which contains... READER 7_4105_DO_CompInternetTerms_C 12/ 29/08 10 :21 AM Page 79 79 carrier 2 a printed-circuit board, especially one designed to be added to a microcomputer to provide additional functions (see Figure 49) 3 see PUNCHED CARD FIGURE 49 Card (definition 2) card reader 1 a device that enables a computer to read FLASH MEMORY (SMARTMEDIA, COMPACTFLASH, and the like) 2 a device that enabled a computer to read... frequency and phase of the carrier 7_4105_DO_CompInternetTerms_C 12/ 29/08 10 :21 AM Page 80 80 cartridge encode the binary data DSL networks use a radio-frequency carrier to transmit data over phone lines cartridge a self-contained, removable part of a computer, usually small and contained in a plastic case For example, laser printers often take toner cartridges (containing toner, i.e., ink) Game machines often... 555- 121 2 as *70W555- 121 2 or *70,,555- 121 2 callout the line and caption marking the specific parts of a labeled illustration For examples, see Figure 29 8 at WINDOW on page 529 -cam abbreviation for camera, especially a digital or video camera whose images are made available by computer network For instance, a camera connected to the World Wide Web is a webcam; a camera mounted on a tower is a towercam; and. .. because of the wooden cases in which printers’ type was stored in the 1800s 2 a statement in Pascal that directs a program to choose one action from a list of alternatives, depending on the value of a given variable Here is an example of a case statement: CASE place OF 1 : writeln(’First place !!!’); 2 : writeln(’Second place’); 3 : writeln(’Third place’) END; 7_4105_DO_CompInternetTerms_C 12/ 29/08 10 :21 ... strapped to the back of a horse might be called a horsecam CAM (Computer- Aided Manufacturing) the use of computers in a manufacturing process For example, a computer could store a three-dimensional representation of an object and then control the manufacture of the object by automated machinery Some of the principles of CAM are the same as with computer- aided design (see CAD), and sometimes a system... HTML; VRML; FIREFOX; INTERNET EXPLORER; OPERA 7_4105_DO_CompInternetTerms_B 12/ 29/08 10:18 AM BSD Page 66 66 FIGURE 41 Browser displaying a web page BSD a version of UNIX that was developed at the University of California at Berkeley (UCB) (See UNIX.) BSD UNIX introduced the vi full-screen editor and a number of other enhancements SunOS (Solaris) and System V are combinations of BSD UNIX with the... keep track of your chatting buddies but show whether or not they are presently logged in 7_4105_DO_CompInternetTerms_B 12/ 29/08 10:18 AM 67 Page 67 buffer class bubblesort { /* This Java program performs a bubble sort */ /* Array to be sorted and number of items in it Element a[0], which contains 0 here, is ignored */ static int a[] = {0 ,29 ,18,7,56,64,33, 128 ,70,78,81, 12, 5}; static int n= 12; public... communication avenue in a computer For a diagram, see COMPUTER ARCHITECTURE The bus consists of a set of parallel wires or lines to which the CPU, the memory, and all input-output devices are connected The bus contains one line for each bit needed to give the address of a device or a location in memory, plus one line for each bit of data to be transmitted 7_4105_DO_CompInternetTerms_B 12/ 29/08 10:18 AM 69 . in pencil -and- paper arithmetic. To find 13 × 21 in decimal, we proceed like this: 1 3 51 binary multiplication 7_4105_DO_CompInternetTerms_B 12/ 29/08 10:18 AM Page 51 21 × 13 63 21 27 3 First,. the 2- complement of x is easy: just invert all the dig- its of x (changing 0’s to 1’s and 1’s to 0’s) and then add 1. See INVERTER. 53 binary subtraction 7_4105_DO_CompInternetTerms_B 12/ 29/08. bits for red, 8 for green, and 8 for blue, so it can display 2 8 = 25 6 different levels of each of the three primary colors, or 2 24 = 16,777 ,21 6 different mixtures of colors. See COLOR. The