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From: aa1@j.cc.purdue.edu (Saul Rosen)
Newsgroups: comp.unix.wizards,comp.arch
Subject: Re: *Why* do modern machines mostly have 8-bit bytes?
Message-ID: <4823@j.cc.purdue.edu>
Date: Wed, 22-Jul-87 13:20:45 EDT
Article-I.D.: j.4823
Posted: Wed Jul 22 13:20:45 1987
Date-Received: Sat, 25-Jul-87 11:02:35 EDT
References: <142700010@tiger.UUCP> <2792@phri.UUCP> <8315@utzoo.UUCP> <2807@phri.UUCP>
Reply-To: aa1@j.cc.purdue.edu (Saul Rosen)
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Organization: Purdue University
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Summary: some historical comments on word length
Xref: mnetor comp.unix.wizards:3391 comp.arch:1673

>
>	First, why did older machines have all sorts of strange word
>lengths -- 12, 36, and 60 being sizes that I know of, but I'm sure there
>were others.
>
>	Second (sort of the inverse of question #1), why do modern machines
>have such a strong trend towards having power-of-2 word and byte lengths?

	The early vacuum tube machines were very expensive to build,
and the cost increased very markedly with word length.  Word length
was a compromise between cost and function.  The IBM 701 was the first
large scale scientific computer that was sold in fairly large numbers
(about 18).  It provided only fixed point arithmetic.  The designers
didn't feel that 32 bits provided enough precision, and 64 bits would
be extravagant.  Also 36 bits, a multiple of 6, went well with the 
6-bit character that could represent all of the codes produced on the
standard keypunch.  Once the 701 designers settled on 36 bits,
compatibility requirements kept the 704 and 709, and then all of
the transistorized 7090 and 7040 series using the  36 bit word.

	A number of competing systems, The Philco Transac S-2000
and the Control Data 1604, were 48-bit word machines.  They stuck
to a multiple of 6, with a word length long enough to provide
reasonable precision for floating point numbers.  The same reasoning
carried one level further ler to the 60-bit supercomputer of its 
day, the CDC 6600.  That was a spectacular computer, many years
ahead of its time.  We are still running one of them here at Purdue.
It would have been an even better computer if it had used a 
64 bit word, and thus conveniently accomodate the 8-bit byte.