Binary-coded decimal: Difference between revisions

Revision as of 19:07, 11 November 2004

Revision as of 01:13, 12 November 2004

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[[IBM]] used the terms '''binary-coded decimal''' and '''BCD''' for six-bit ''alphameric'' codes that represented numbers, upper-case letters and special characters. Some variation of BCD was used in most early IBM computers, including the [[IBM 1620]], [[IBM 1400 series]] and non-decimal members of the [[IBM 700/7000 series]]. With the introduction of [[System/360]], IBM replaced BCD with 8-bit [[EBCDIC]].

Bit positions in BCD were usually labledlabeled ''B, A, 8, 4, 2'' and ''1.'' For encoding digits, ''B'' and ''A'' were zero. The letter '''A''' was encoded ''(B,A,1).''

In the 1620 BCD ''alphamerics'' were encoded using digit pairs, with the "zone" in the even digit and the "digit" in the odd digit. Input/Output translation hardware converted between the internal digit pairs and the external standard six-bit BCD codes.