From: Hohberger, Clive (CHohberger@zebra.com)
Date: Sat Apr 11 2009 - 22:12:11 CDT
Jonathan Rosenne wrote:
>> Actually, I was under the impression that ASCII was defined in terms
>> of 7-bit code units, whereas there are virtually no computers or
>> users today who think in terms of 7-bit code units.
>
> There weren't such computers then, it was a communication code and 7
> bits were used for communication.
Jukka Korpela wrote:
I'm not sure what you mean by "such" here, but in fact, even in the
1980s and early 1990s, DECsystem-10 and -20 (PDP-10 and -20) used a word
length of
36 bits, packing five 7-bit ASCII characters in one word (and using the
spare bit for special purposes).
ASCII was surely designed to allow implementations where 7 bits are used
for one character. Don't confuse this with the current situation where
such implementations are obsolete and "everyone" uses at least 8 bits
for a character, even when working with ASCII only.
----------
Jony and Jukka are both correct:
ASCII was never designed as a computer code: It was an outgrowth of
telegraphic codes, evolving from 5-bit Baudot developed by Emile Baudot
in 1870. The thing about a 7-bit code is that it contains enough
codepoints to support both the upper case and lower case American
English alphabet, numerics and punctuation. In other words, normal
American English text.
So, how did ASCII end up in computers? Its started first in Europe with
IBM's Bob Bremer's development of the ESC sequence and its proponent
Hugh Ross at Ferranti; and it made great sense to those of us designing
computers using the technologies of the 1960's. And as computer
applications started supporting text document input and output rather
than Teletype I/O, ASCII became essential as the I/O code.
"On March 11, 1968, U.S. President Lyndon B. Johnson mandated that all
computers purchased by the United States federal government support
ASCII, stating:
I have also approved recommendations of the Secretary of Commerce
regarding standards for recording the Standard Code for Information
Interchange on magnetic tapes and paper tapes when they are used in
computer operations. All computers and related equipment configurations
brought into the Federal Government inventory on and after July 1, 1969,
must have the capability to use the Standard Code for Information
Interchange and the formats prescribed by the magnetic tape and paper
tape standards when these media are used.[28]" from:
http://en.wikipedia.org/wiki/ASCII
I worked with 36 bit machines in the 60's (Univac 1107 & 1108) and the
DEC PDP-10 family in the 70's: 7-bit ASCII was regarded as very
efficient for storage back when RAM core memory cost >$1/bit (as
contrasted with last week, when I paid $7 for 1 GB of 667 MHz SDRAM!).
As a result, back in the late 60's we a bit appalled to see IBM move to
the System/360 packing only 4 8-bit characters into 1 32-bit word.
Little did we know how prescient that descision would be, with the
advent of integrated circuits and Moore's Law.
Clive P. Hohberger, PhD
Consultant to
Zebra Technologies Corporation
333 Corporate Woods Parkway
Vernon Hills, IL 60061-3109 USA
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