Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!husc6!hao!ames!ptsfa!ihnp4!inuxc!iuvax!pur-ee!j.cc.purdue.edu!aa1 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) Distribution: world Organization: Purdue University Lines: 29 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.