Six-bit character code: Difference between revisions

Revision as of 17:18, 18 June 2020

Revision as of 20:42, 18 June 2020

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An early six-bit binary code was used for [[Braille]], the reading system for the blind that was developed in the 1820s.

The earliest computers dealt with numeric data only, and made no provision for character data. [[Six-bit BCD]] was used by [[IBM]] on early computers such as the [[IBM 704]] in 1954.<ref>{{cite book |author=IBM Corporation |title=704 electronic data-processing machine: manual of operation |date=1954 |url=http://www.bitsavers.org/pdf/ibm/704/24-6661-2_704_Manual_1955.pdf}}</ref>{{rp|p.35}} This encoding was replaced by the 8-bit [[EBCDIC]] code when [[System/360]] standardized on 8-bit [[byte]]s. There are some variants of this type of code (see [[#Examples of BCD six-bit codesvariants|below]]).

Six-bit character codes generally succeeded the five-bit [[Baudot code]] and preceded seven-bit [[ASCII]]. One popular variant 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 [[PDP-10]], and three characters fit in each word of the [[PDP-1]] and two characters fit in each word of the [[PDP-8]].

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==={{anchor|ECMA-1}}DEC six-bit code===

A DEC SIXBIT code including a few control characters&mdash;along with [[Shift Out and Shift In characters|SO/SI]], allowing code extension&mdash;was specified as [[Ecma International|ECMA]]-[http://www.ecma-international.org/publications/files/ECMA-ST-WITHDRAWN/ECMA-1,%201st%20Edition,%20March%201963.pdf 1] in 1963 (see [[#Example of six-bit ASCII -variants|below]]).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).

===FIELDATA six-bit code===

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A number of schemes exist to pack 8-bit data into text-only representations which can pass through text mail systems, to be decoded at the destination. Examples of 6-bit character subsets used for packing binary data include [[Uuencode]] and [[Base64]]. These sets contain no control characters (only printable numbers, letters, some punctuation, and maybe space) and allow data to be transmitted over any medium which is also able to transmit human-readable text.

=={{anchor|BCD-variants}}Examples of BCD six-bit codes==

Characters are shown with their [[Unicode]] equivalents.

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=={{anchor|ASCII-variants}}Examples of six-bit ASCII variants==

{| {{chset-tableformat}}

{{chset-table-header|DEC SIXBIT}}

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|{{chset-color-punct}}|{{chset-cell|003F|[[question mark|&#x3F;]]|63}}

|}

==GOST 6-bit code==