BCD (character encoding)
BCD ("Binary-Coded Decimal"), also called alphanumeric BCD, alphameric BCD, BCD Interchange Code, or BCDIC, is a family of representations of numerals, uppercase Latin letters, and some special and control characters as six-bit character codes.
Unlike later encodings such as ASCII, BCD codes were not standardized. Different computer manufacturers, and even different product lines of the same manufacturer, often had their own variants, and sometimes included unique characters. Other six-bit encodings with completely different mappings, such as Fieldata and Transcode are sometimes incorrectly termed BCD. Many versions of BCD encode the characters '0' through '9' as the corresponding binary values.
Technically, Binary-coded decimal describes the encoding of decimal numbers where each decimal digit is represented by a fixed number of bits, usually four.
With the introduction of the IBM card in 1928, IBM created a code capable of representing alphanumeric information, later adopted by other manufacturers. This code represented the numbers 0-9 by a single punch, and used multiple punches for upper-case letters and special characters. A letter had two punches (zone [12,11,0] + digit [1–9]); most special characters had two or three punches (zone [12,11,0,or none] + digit [2–7] + 8).
The BCD code was the adaptation of the punched card code to binary code by encoding the ten numeric rows of the card into four bits and using one bit each for the "eleven" and "twelve" rows. IBM applied the terms binary-coded decimal and BCD to the variations of BCD alphamerics used in most early IBM computers, including the IBM 1620, IBM 1400 series, and non-Decimal Architecture members of the IBM 700/7000 series.
IBM later created the 8-bit code EBCDIC (Extended Binary-coded Decimal Interchange Code) based on BCD.
BCD code variations
There are different versions of the six-bit BCD code. There are at least 4 versions with some different characters.
None of the variations (nor the original) have room for control characters. In "Spanish speaking countries", the character "Ñ" did not exist in the original system, therefore "@" was chosen by most manufacturers: Bull, NCR, and Control Data, but there was an inconsistency when merging databases to 7-bit ASCII code, for in that coding system the "/" character was chosen, resulting in two different codes for the same character.
Examples of BCD codes
The following charts show the numeric values of BCD characters in octal (base-8) notation. For example, the code for 'A", shown as '002001'B4 would be binary '01 0001'.
IBM 704 BCD code
The following table shows the code assignments for the IBM 704 computer. Unassigned code positions appear as blanks. "+0" and "-0" represent positive and negative zero respectively, which do not have unicode assignments.
Burroughs B5500 BCD code
Notes and references
- Charles E. Mackenzie (1980). Coded character sets: history and development. Addison-Wesley Pub. Co. ISBN 978-0-201-14460-4. Retrieved 2 December 2011.
- Pugh, Emerson W.; Heide, Lars. "STARS:Punched Card Equipment". IEEE Global History Network. Retrieved June 9, 2012.
- Pugh, Emerson W. (1995). Building IBM: Shaping and Industry and Its Technology. MIT Press. pp. 50–51. ISBN 978-0-262-16147-3.
- Jones, Douglas W. "Punched Card Codes". Retrieved Jan 1, 2014.
- Burroughs Corporation (1964). Burroughs B5500 Information Processing Systems: Reference Manual.
- Control Data Corporation (1965). Codes/Control Data 6600 Computer System.
- IBM Corporation (1954). 704 electronic data-processing machine manual of operation. p. 35.
- Values are in octal
- Section: Tables of characters in BULL computers
- Burroughs B 5500 Information Processing Systems Extended Algol Reference Manual. 1966. p. B-1.
- BULL GCOS8 Manual
- SIX-BITS CHARACTERS, J.M.Bonten
- Mackenzie, Charles E. (1980). Coded Character Sets, History and Development. Addison-Wesley. ISBN 0-201-14460-3.