計算機英語：計算機中的代碼

Computer Codes

　　Although the capacity of their storage locations can vary，every computer stores number, letters, and other characters in a coded form. Every character in storage is represented by a string of Os and Is-the only digits found in the binary numbering system. Let's see how it's possible to use just two digits to represent any character.

1 ．BCD Code
　　With BCD, it's possible to convert each decimal number into its binary equivalent rather than convert the entire decimal value into a pure binary form. The BCD equivalent of each possible symbol is shown in Figure 1-6.Since 8 and 9 require 4 bits, all decimal digits are presented in BCD by 4 bits. You've just seen that 202 is equal to 11001010 in pure binary form, converting 202 into BCD, however, produces the following result：
202 in BCD=0010 0000 0010 or 001000000010

　　Instead of using 4 bits with only 16 possible characters，computer designers commonly use 6, 7, 8bits to represent characters in alphanumeric versions of BCD. With 6 bits, it's possible to represent 64different characters(26)．This is a sufficient number to: ode the decimal digits(10), capital letters(26)，and other special characters and punctuation marks(28)．Figure 1-7(b)shows you how a few of the 64 possible characters are represented in a standard 6-bit BCDcode

2 ．ASCI Character Code
　　Since 64 possible bit combinations isn't sufficient to provide decimal numbers(10)，lowercase letters(26), capital letters (26), and a large numbers of other characters (28)，designers have extended the 6-bit BCD code to 7 and 8 bits．With 7 bits, it's possible to provide28 different arrangements(27)；with 8 bits，256 variations are possible (28).In addition to the four numeric place positions, there are three zone bit positions in a 7-bit code, and four zone bit positions in an 8-bit code: The 7-bit American Standard Code for Information Interchange (ASCII) is widely used in data communications work and is by far the most popular code used to represent data internally in personal computers．The ASCII format and the coding used to represent selected characters are shown in Figure 1-8.

　　There are also two popular 8-bit codes in common use. One is the Extended Binary Coded Decimal Interchange Code (EBCDIC)．This code is used in IBM mainframe models and in similar machines produced by other manufacturers. The other 8-bit code isASCII-8，an 8-bit version of ASCII that is frequently used in the larger machines produced by some vendors. Figure 1-9 presents the 8-bit format and show, selected characters are represented in these 8-bitcodes．The main difference is in the selection of bit patterns to use in the zone positions．

繙譯：

計算機中的代碼

　　盡琯計算機存儲單元的容量可變，但是每台計算機都是以代碼形式存儲數字、字母和其它字符的。存儲器中的每個字符都用0、1（二進制編碼系統中僅有的數字）數串表示。下麪讓我們看看如何衹用兩個數字表示任何字符。

1 ．BCD碼

　　用BCD碼，可以將十進制數的每一位轉換成相等的二進制數，而不是將整個十進制數轉換成純二進制數的形式。所有十進制數對應的BCD碼如圖1-6所示。因爲8和9需要4位二進制數表示，所以，所有十進制數均用4位BCD碼表示。衆所周知，用純二進制數表示202等於
11001010，但是將202轉換成BCD碼，結果如下；
　　202（10）的BCD碼=0010 0000 0010或001000000010

　　計算機設計者普遍採用6位7位或8位BCD字母數字型字符，代替衹有16種可能字符的4位BI碼。用6位二進制數，可以表示個不同字符（26） ,這就有足夠的數對十進制數（10個）、大寫字母（26個）及其它特殊字符和標點字符（28個）進行編碼。其中一些字符對應的標準6位BCD碼如圖1-7(b)所示。

2. ASCI工字符代碼

　　因爲64種可能的位排列仍不能滿足十進制數（10個）、小寫字母（26個）、大寫字母（26個）和大量的其它字符（26個以上）編碼的需要，所以設計者將6位BCD碼擴充到7位和8位。7位二進制數可以提供128種不I司的排列(2 ')；8位二進制數可以有256種排列（2”）。除了4位數字位之外，在7位編碼中有3個標志位，而在8位編碼中則有4個標志位。7位美國信息交換標準代碼(ASCII)廣泛地用於數據通信，而且是個人計算機內部數據表示中最爲流行的代碼ASCII碼的格式及所選字符的編碼如圖1-8所示。

　　還有兩種普遍使用的流行的8位編碼。一種是擴充的二一十進制交換碼(EBCDIC)。該編碼用於IBM大型計算機及其它廠家生産的類似機器中。另一種8位編碼是ASCII-8，是ASCII代碼8位格式，常常用於一些廠商生産的較大機器中。'8位編碼的格式及如何表示所列字符如圖1-9所示。這兩種編碼的主要區別在於標志位模式的選擇。

Binary Number System

　　In the binary number system, there are two dig-its：0 and I．The binary system is used for internal computer operations because only two signal levels are required, as opposed to decimal where ten signal levels would be necessary．Because a digit in the units position has a value of 0 or I，numbers greater than 1cause a carry to the next position, each position represents the base raised to a power. In base 10, the units position has a power of 100, the next position 10 l，and so on. Thus, a digit in any position other than the units position has a weight (value) depending on its positioning the number. A 4，for example, has a weight of 4 in the units position, 40 in the 10's position, 400 in the100's position and so on.