Six-bit character codes were designed for use on computers with word lengths a multiple of 6. Six bits can only encode 64 distinct characters, so these codes generally include only the upper-case letters, the numerals, some punctuation characters, and sometimes control characters. Such codes with additional parity bit were a natural way of storing data on 7-track magnetic tape.
Types of sixbit codes
Six-bit BCD was used by IBM[when?]. This encoding was replaced by the 8-bit EBCDIC code when System/360 standardized on 8-bit bytes. There are some variants of this type of code (see below).
Some six-bit variants were derived from ASCII. One popular variant was DEC SIXBIT. This is simply the ASCII character codes from 32 to 95 coded as 0 to 63 by subtracting 32; it includes the space, punctuation characters, numbers, and uppercase letters, but no control characters. Since it included no control characters, not even end-of-line, it was not used for general text processing. However, six-character names such as filenames and assembler symbols could be stored in a single 36-bit word of PDP-10. It was also used in COBOL databases, where end-of-record information was stored separately. This code, with added odd parity bit, is used on Track 1 of magnetic stripe cards, as specified in ISO/IEC 7811-2. Uuencode utilizes this character set for text representation of arbitrary binary data.
A DEC SIXBIT code including a few control characters — along with SO/SI, allowing code extension — was specified as ECMA-1 (see below). Another, less common, variant is obtained by just stripping the high bit of an ASCII code in 32 - 95 range (codes 32 - 63 remain at their positions, higher values have 64 subtracted from them). Such variant was sometimes used on DEC's PDP-8.
Fieldata was a six-bit format used by UNIVAC's 1100-series computers. These systems used a 36-bit word (capable of storing 6 Fieldata characters).
Examples of BCD sixbit codes
CDC 1604 : Magnetic tape BCD codes
| .0 | .1 | .2 | .3 | .4 | .5 | .6 | .7 | .8 | .9 | .A | .B | .C | .D | .E | .F | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | # | @ | tape mark |
|||
| 1. | space | / | S | T | U | V | W | X | Y | Z | record mark |
, | % | |||
| 2. | − | J | K | L | M | N | O | P | Q | R | −0 | $ | * | |||
| 3. | & | A | B | C | D | E | F | G | H | I | +0 | . | ¤ | group mark |
CDC 1604 : Punched card codes
| .0 | .1 | .2 | .3 | .4 | .5 | .6 | .7 | .8 | .9 | .A | .B | .C | .D | .E | .F | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | = | − | ||||
| 1. | space | / | S | T | U | V | W | X | Y | Z | , | ( | ||||
| 2. | --- | J | K | L | M | N | O | P | Q | R | −0 | $ | * | |||
| 3. | + | A | B | C | D | E | F | G | H | I | +0 | . | ) |
CDC 1612 printer codes (business applications)
| .0 | .1 | .2 | .3 | .4 | .5 | .6 | .7 | .8 | .9 | .A | .B | .C | .D | .E | .F | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0. | : | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 0 | = | ≠ | ≤ | ! | [ |
| 1. | space | / | S | T | U | V | W | X | Y | Z | ] | , | ( | → | ≡ | ~ |
| 2. | − | J | K | L | M | N | O | P | Q | R | % | $ | * | ↑ | ↓ | > |
| 3. | + | A | B | C | D | E | F | G | H | I | < | . | ) | ≥ | ? | ; |
Example of sixbit ASCII variant
ECMA-1 : DEC SIXBIT with control characters
| .0 | .1 | .2 | .3 | .4 | .5 | .6 | .7 | .8 | .9 | .A | .B | .C | .D | .E | .F | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0. | space | HT | LF | VT | FF | CR | SO | SI | ( | ) | * | + | , | - | . | / |
| 1. | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | : | ; | < | = | > | ? |
| 2. | NULL | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O |
| 3. | P | Q | R | S | T | U | V | W | X | Y | Z | [ | \ | ] | ESC | DEL |
External links
- Control Data 1604-C Computer. Reference Manual.PDF (7.44 MB)
- SIXBIT Character Code Reference Digital Equipment Corporation SIXBIT character coding system
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