Content deleted Content added
m Reverted edits by 195.222.65.109 (talk) (HG) (3.4.10) |
→Types of six-bit codes: Rearranged to discuss DEC SIXBIT in only one section, and to clarify difference between DEC and ECMA codes |
||
Line 7:
The earliest computers dealt with numeric data only, and made no provision for character data. [[Six-bit BCD]], with several variants, was used by [[IBM]] on early computers such as the [[IBM 702]] in 1953 and 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}} Six-bit encodings were replaced by the 8-bit [[EBCDIC]] code starting in 1964, when [[System/360]] standardized on 8-bit [[byte]]s. There are some variants of this type of code (see [[#BCD-variants|below]]).
Six-bit character codes generally succeeded the five-bit [[Baudot code]] and preceded seven-bit [[ASCII]].
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]].▼
Six-bit codes could encode more than 64 characters by the use of [[Shift Out and Shift In characters]], essentially incorporating two distinct 62-character sets and switching between them. For example, the popular [[IBM 2741]] communications terminal supported a variety of character sets of up to 88 printing characters plus control characters.
Line 20:
A six-bit code, with added odd [[parity bit]], is used on Track 1 of [[magnetic stripe card]]s, as specified in [[ISO/IEC 7811]]-2.
===
▲
A DEC SIXBIT code including a few control characters—along with [[Shift Out and Shift In characters|SO/SI]], allowing code extension—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 [[#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).▼
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).
==={{anchor|ECMA-1}}ECMA six-bit code===
▲A
===FIELDATA six-bit code===
| |||