ASCII

ASCII, in computer science, acronym for American Standard Code for Information Interchange. In computing, a coding scheme that assigns numeric values to letters, numbers, punctuation marks, and certain other characters. By standardizing the values used for these characters, ASCII enables computers and computer programs to exchange information.

ASCII provides for 256 codes divided into two sets—standard and extended—of 128 each. These sets represent the total possible combinations of either 7 or 8 bits, the latter being the number of bits in 1 byte. The basic, or standard, ASCII set uses 7 bits for each code, yielding 128 character codes from 0 through 127 (hexadecimal 00H through 7FH). The extended ASCII set uses 8 bits for each code, yielding an additional 128 codes numbered 128 through 255 (hexadecimal 80H through FFH).

In the standard ASCII character set, the first 32 values are assigned to communication and printer control codes—nonprinting characters, such as backspace, carriage return, and tab—that are used to control the way information is transferred from one computer to another or from a computer to a printer. The remaining 96 codes are assigned to common punctuation marks, the digits 0 through 9, and the uppercase and lowercase letters of the Roman alphabet.

The extended ASCII codes, 128 through 255, are assigned to variable sets of characters by computer manufacturers and software developers. These codes are not as interchangeable among different programs and computers as are the standard ASCII characters. IBM, for example, uses a group of extended ASCII characters generally called the IBM extended character set for its personal computers; Apple Computer uses a similar but different group of extended ASCII characters for its Macintosh line of computers. Thus, whereas the standard ASCII character set is universal among microcomputer hardware and software, extended ASCII characters can be interpreted correctly only if a program, computer, or printer is designed to do so.