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．
翻译：
计算机中的代码
尽管计算机存储单元的容量可变，但是每台计算机都是以代码形式存储数字、字母和其它字符的。存储器中的每个字符都用0、1（二进制编码系统中仅有的数字）数串表示。下面让我们看看如何只用两个数字表示任何字符。
1 ．BCD码
用BCD码，可以将十进制数的每一位转换成相等的二进制数，而不是将整个十进制数转换成纯二进制数的形式。所有十进制数对应的BCD码如图1-6所示。因为8和9需要4位二进制数表示，所以，所有十进制数均用4位BCD码表示。众所周知，用纯二进制数表示202等于
11001010，但是将202转换成BCD码，结果如下；
202（10）的BCD码=0010 0000 0010或001000000010
计算机设计者普遍采用6位7位或8位BCD字母数字型字符，代替只有16种可能字符的4位BI码。用6位二进制数，可以表示个不同字符（26） ,这就有足够的数对十进制数（10个）、大写字母（26个）及其它特殊字符和标点字符（28个）进行编码。其中一些字符对应的标准6位BCD码如图1-7(b)所示。
2. ASCI工字符代码
因为64种可能的位排列仍不能满足十进制数（10个）、小写字母（26个）、大写字母（26个）和大量的其它字符（26个以上）编码的需要，所以设计者将6位BCD码扩充到7位和8位。7位二进制数可以提供128种不I司的排列(2 ')；8位二进制数可以有256种排列（2”）。除了4位数字位之外，在7位编码中有3个标志位，而在8位编码中则有4个标志位。7位美国信息交换标准代码(ASCII)广泛地用于数据通信，而且是个人计算机内部数据表示中最为流行的代码ASCII码的格式及所选字符的编码如图1-8所示。
还有两种普遍使用的流行的8位编码。一种是扩充的二一十进制交换码(EBCDIC)。该编码用于IBM大型计算机及其它厂家生产的类似机器中。另一种8位编码是ASCII-8，是ASCII代码8位格式，常常用于一些厂商生产的较大机器中。'8位编码的格式及如何表示所列字符如图1-9所示。这两种编码的主要区别在于标志位模式的选择。
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.
