BCD (binary-coded decimal), also called alphanumeric BCD, alphameric BCD, BCD Interchange Code,^[1] or BCDIC,^[1] 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 from the same manufacturer, often had their own variants, and sometimes included unique characters. Other six-bit encodings with completely different mappings, such as some FIELDATA^[1] variants or Transcode, are sometimes incorrectly termed BCD.

Many variants of BCD encode the characters '0' through '9' as the corresponding binary values.

History

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^{[lower-alpha 1]} capable of representing alphanumeric information,^[2] later adopted by other manufacturers. This code represents the numbers 0-9 by a single punch, and uses multiple punches for upper-case letters and special characters.^[3] A letter has two punches (zone [12,11,0] + digit [1–9]); most special characters have two or three punches (zone [12,11,0,or none] + digit [2–7] + 8).

The BCD code is the adaptation of the punched card code to a six-bit binary code by encoding the digit rows (nine rows, plus unpunched) into the low four bits, and the zone rows (three rows, plus unpunched) into the high two bits.^[4] The digit zero (a single punch in row 0) is usually handled specially in some way, and the digit code was extended to values 10 through 15 by combining a digit in the range 2–7 with a punch in row 8. 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.

Among the vendors using BCD were Burroughs,^[5] Bull, CDC,^[6] IBM, General Electric (the computer division was purchased by Honeywell in 1969), NCR, Siemens, and Sperry-UNIVAC.

IBM announced the 8-bit Extended Binary Coded Decimal Interchange Code (EBCDIC), based on BCDIC, in 1964 with the introduction of its System/360 line.

Special characters

The Recordmark or Record mark character (represented as ‡) is a character used to mark the end of a record.^[7] The BCD code for this character is 32₈ in some BCD variants. The closest Unicode equivalent is U+29E7 ⧧ THERMODYNAMIC, but that is not found in many fonts, so U+2021 ‡ DOUBLE DAGGER is often used instead. Functionally this corresponds to the EBCDIC IRS character (ASCII RS), X'1E'.

The Groupmark or Group mark character (represented as ) is a character used to indicate the start or finish of a group of related fields.^[8] The BCD code for this character is 77₈ in some BCD variants. The groupmark was proposed for Unicode standardization in 2015,^[9] and was assigned to value U+2BD2 ⯒ GROUP MARK. Functionally this corresponds to the EBCDIC IGS character (ASCII GS), X'1D'. It is now in Unicode 10.0 at this position, but only the Symbola and Unifont fonts support it.

The Wordmark, by contrast, is not a BCD character. Rather, it is a flag bit used to mark the end of a word on some variable word length computers such as the IBM 1401.

BCD code variations

There are many different versions of the six-bit BCD code. There are three major categories of difference:

1. The mapping from zone punches to high-order bits. All codes translate no zone punches to a bit pattern of 00, but some encode the zone punches in 12-11-0 order, preserving alphabetical order, while others use 0-11-12 order, resulting in a partially reversed alphabet.
2. The handling of the digit 0. The straightforward translation from punched form would place the blank before digits 1–9, and encode 0 at the start of the line with 'S' in it. All codes have some special-case handling which either translates the digit 0 to the all-zero binary code (and moves the blank elsewhere), or gives it binary code 001010 (decimal 10) and moves the 8+2 punch elsewhere.
3. The assignment of special characters. The characters assigned to codes beyond the basic alphanumeric set varied widely, even within one model of computer.

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 hexadecimal (base-16) notation, as that most clearly reflects the structure of 4-bit binary coded decimal, plus two extra bits. For example, the code for 'A', in row 3x and column x1, is hexadecimal 31, or binary '11 0001'.

See also

• Binary-coded decimal
• Six-bit character code
• EBCDIC
• SQUOZE

Notes

References

Further reading

• Operating System - GCOS 8 Operating System - Programmer's Guide - Bull NovaScale 9000 Series Assembly Instructions - GCOS 8 (PDF). Bull S.A. CEDOC. November 2003. 67 A2 RJ78 REV00. Archived from the original (PDF) on 2015-02-12.
• Bonten, Jo H. M. (2009-03-08) [2007]. "Data formats in old 48- and 36-bits computers: Six-bits characters". Geldrop, Netherlands. Archived from the original on 2016-06-16. Retrieved 2016-06-16.