Six-bit character code: Difference between revisions

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{{Short description|Computer encoding of characters}}

{{Use dmy dates|date=May 2019|cs1-dates=y}}

A '''six-bit character code''' is a [[character encoding]] designed for use on computers with [[word length]]s 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. The [[IBM 7 track|7-track]] [[Magnetic -tape data storage|magnetic tape]] format was developed to store data in such codes, along with an additional [[parity bit]].

==Types of six-bit codes==

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The TTS code had two pairs of shift codes allowing a total of four shift states. The first operated much like a keyboard's shift key and selected between a lower-case and digits repertoire, and an upper-case and symbols one. A second pair of Linotype-specific "lower rail" and "upper rail" shift codes would select an alternate (usually italic) font.

===BCD six-bit codecodes===

Six-bit [[BCD (6-bitcharacter encoding)|BCD]] code wascodes thewere adaptationadaptations of the [[punched card code]] to [[binary code]]. [[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-[[IBM 700/7000 series#Decimal architecture (7070/7072/7074)|decimal architecture]] members of the [[IBM 700/7000 series]].

===COBOL databases six-bit code===

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===DEC SIXBIT code<span class="anchor" id="DEC six-bit code"></span>===

A popular six-bit code was [[Digital Equipment Corporation|DEC]] SIXBIT. This is simply the ASCII character codes from 32 to 95 coded as 0 to 63 by subtracting 32 (i.e., columns 2, 3, 4, and 5 of the ASCII table (16 characters to a column), shifted to columns 0 through 3, by subtracting 2 from the high bits); it includes the space, punctuation characters, numbers, and capital 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 [[filename]]s and [[assembly language|assembler]] [[identifier|symbol]]s could be stored in a single [[36-bit]] word of the [[PDP-10]], and three characters fit in each word of the [[PDP-1]] and two characters fit in each word of the [[PDP-8]]. See [[#ASCII-variants|table 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]] (1965).

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==={{anchor|ECMA-1}}ECMA and ISO six-bit code===

A six-bit code similar to DEC's, but replacing a few punctuation characters with the most useful control characters&mdash;including [[Shift Out and Shift In characters|SO/SI]], allowing code extension&mdash;was specified as [[Ecma International|ECMA]]-[https://ecma-international.org/publications-and-standards/standards/ecma-1/ 1] in 1963 (see [[#ASCII-variants|below]]). Four years later, ISO Recommendation R 646-1967 (which later evolved into [[ISO/IEC_646 | ISO Standard 646]]) included an almost identical six-bit code, differing only in some of the alternative options permitted for a few characters. ECMA-1 was eventually withdrawn, and ISO 646-1973 explicitly removed the six-bit code, standardizing only its 7-bit code.

{|{{chset-table-header1|ECMA-1 and ISO/R 646:1967}}

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===AIS SixBit ASCII===

{|{{chset-table-header1|SixBit ASCII (used byThe [[Automaticautomatic identification system|AIS]] (AIS) uses this code.<ref name='Raymond'>{{cite web |url=https://gpsd.gitlab.io/gpsd/AIVDM.html#_ais_payload_data_types |title=AIVDM/AIVDO protocol decoding |at=AIS Payload Data Types |access-date=2024-03-14 |author-last=Raymond |author-first=Eric S. |date=2023-06-24}}</ref>}}

{|{{chset-table-header1|AIS SixBit ASCII}}

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|{{chset-left1|0x}}