Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!rochester!rutgers!husc6!bu-cs!m2c!necntc!ima!johnl 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!