ALPHANUMERIC REPRESENTATION

Many applications of digital computers require the handling of data that consist not only of numbers, but also of the letters of the alphabet and certain special characters.An alphanumeric character set is a set of elements that includes the 10 decimal digits, the 26 letters of the alphabet and a number of special characters, such as $, + , and =. Such a set contains between 32 and 64 elements (if only uppercase letters are included) or between 64 and 128 (if both uppercase and lowercase letters are included). In the first case, the binary code will require six bits and in the second case, seven bits. The standard alphanumeric binary code is the ASCII (American Standard Code for Information Interchange), which uses seven bits to code 128 characters. The binary code for the uppercase letters, the decimal digits, and a few special characters is listed in Table 3-4. Note that the decimal digits in ASCII can be converted to BCD by removing the three high-order bits, 011. A complete list of ASCII characters is provided in Table 11-1.

Binary codes play an important part in digital computer operations. The codes must be in binary because registers can only hold binary information. One must realize that binary codes merely change the symbols, not the meaning of the discrete elements they represent. The operations specified for digital computers must take into consideration the meaning of the bits stored in registers so that operations are performed on operands of the same type. In inspecting the bits of a computer register at random, one is likely to find that it represents some type of coded information rather than a binary number.

presents some type of coded information rather than a binary number. Binary codes can be formulated for any set of discrete elements such as the musical notes and chess pieces and their positions on the chessboard. Binary codes are also used to formulate instructions that specify control infor· mation for the computer. This chapter is concerned with data representation. Instruction codes are discussed in Chap. 5.

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