BCD (character encoding)

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 represents the numbers 0-9 by a single punch, and uses multiple punches for upper-case letters and special characters. 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. 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, Bull, CDC, 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. The BCD code for this character is 328 in some BCD variants. The closest Unicode equivalent is , but that is not found in many fonts, so 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. The BCD code for this character is 778 in some BCD variants. The groupmark was proposed for Unicode standardization in 2015, and was assigned to value . 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: • 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. • 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. • The assignment of special characters. The characters assigned to codes beyond the basic alphanumeric set varied widely, even within one model of computer. For example, some computers had the percent and lozenge () at the same codes as left and right parentheses in other encodings. 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'. Tape style 48-character BCD code The first versions of BCDIC had 48 characters, as they were based on card punch patterns and the character sets of printers, neither of which encouraged having a power-of-two number of characters. This was based on a 40-character punched card code; the original 37 (10 digits, 26 letters, and blank), plus three commercially important characters added around 1932: Some of the characters in this code page are not in Unicode. At 0x1A is the record mark. At 0x3F is the group mark. PTTC/BCD code pages PTTC/BCD had 5 options. There were five code pages. They are shown below. The PTTC/BCD Standard Option was assigned as Code page 355, or CP355. The PTTC/BCD H Option was assigned as Code page 357, or CP357. The PTTC/BCD Correspondence Option was assigned as Code page 358, or CP358. The PTTC/BCD Monocase Option was assigned as Code page 359, or CP359. The PTTC/BCD Duocase Option was assigned as Code page 360, or CP360. IBM 704 storage style IBM 704 BCD code The IBM 704 reordered the BCDIC code to allow a normal alphabetic collating order internally, with 0 before 1 and A before Z. It could automatically translate between this internal form and the earlier BCDIC when reading and writing magnetic tapes. The following table shows the code assignments for the IBM 704 computer. Unassigned code positions appear as blanks. ( and were rarely used characters that corresponded to the punched-card convention of a digit 0 with an overpunched sign in rows 12 or 11.) The following table shows the code assignments for the type 716 printer used starting with the IBM 704 computer and through the 7094. The 704 interface sent virtual punched-card rows to this printer, two words (72 bits) at a time, so the mapping from 6-bit BCD characters was done by software, and was not built into the printer. This is a repertoire of 45 characters (not counting blank, which is handled specially by the printer), as the characters +, - and * are duplicated. Fortran character set There was some variation; IBM 704 Fortran had a different set of special characters (preserving only the duplicated minus sign and asterisk, period, comma, and dollar sign). A similar code was used for the IBM 709, 7090 and 7094 successors, but with some of the special characters reassigned: GBCD code Below is the table of GE/Honeywell's GBCD code, a variant of BCD. Burroughs B5500 BCD code The following table shows the code assignments for the Burroughs B5500 computer, sometimes referred to as BIC (Burroughs Interchange Code). ==See also==