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From: johnl@ima.ISC.COM (John R. Levine)
Newsgroups: comp.unix.wizards,comp.arch
Subject: Re: *Why* do modern machines mostly have 8-bit bytes?
Message-ID: <624@ima.ISC.COM>
Date: Thu, 23-Jul-87 12:02:45 EDT
Article-I.D.: ima.624
Posted: Thu Jul 23 12:02:45 1987
Date-Received: Sat, 25-Jul-87 07:31:15 EDT
References: <142700010@tiger.UUCP> <2792@phri.UUCP> <8315@utzoo.UUCP> <2807@phri.UUCP> <6724@think.UUCP>
Reply-To: johnl@ima.UUCP (John R. Levine)
Organization: Javelin Software Corporation
Lines: 31
Summary: because the IBM 360 did
Xref: mnetor comp.unix.wizards:3377 comp.arch:1669

It seems to me that all of the eight-bit byte machines we have are following
the lead of the IBM 360.  In 1964, before the 360 came out, the most common
word size for binary machines was 36 bits and bytes, if the hardware supported
them at all were 6 bits.  Various six-bit codes were adequate for the upper
case alphabet, digits, and a smattering of punctuation.

The 360 was intended to replace both the word-addressed binary 7094 and
various character addressed BCD machines.  A six-bit character set was no
longer enough, because ASCII had recently been invented and there were an
increasing number of ASCII terminals in TWX service.  Seven bits was enough
for ASCII, but that's a fairly ugly byte size.  Eight bits has the added
advantage that you can put two BCD digits into an eight-bit byte, which the
360 and most subsequent machines do, so eight bits it was.

(I realize that you can encode two decimal digits in 7 bits, but in 1964
the logic needed to deal with such a format was too complicated.)

The 36 bit crowd tried to counterattack with either 9-bit bytes (the GE, later
Honeywell, 635 series and perhaps the Univac 110x) or any byte size you want
(the PDP-6, -10, and -20) but it was too late. Uniform character addressing
was a big win even on machines used for scientific computing (think of all
those Fortran compiles, after all) and the era of word addressed machines
was drawing to a close.  There are still some non-byte addressed architectures
out there, such as the Univac (subsequently Sperry, now Unisys) 1100 series,
the Burroughs (now Unisys) B 5000 and its many descendants, and the
GE 635 and its many Honeywell descendants.  But they seem to be fading out.
Even Seymour Cray has byte addressing in the Cray 2, doesn't he?
-- 
John R. Levine, Javelin Software Corp., Cambridge MA +1 617 494 1400
{ ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl, Levine@YALE.something
U.S. out of New Mexico!