As I recall from way back, wasn't the 36 bit potentially split into 32-bit data and 4 bit offset to allow fast jump to the next "card" in the deck on a branch? Not that (m)any implemented this, but I seem to recall this from my early days back at ADP 30+ years ago. With the advent of fast memory and disk drives this effectively became unnecessary but mainframe architecture took a while to adjust.
Dave -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Ian King Sent: 19 March 2013 19:05 To: [email protected] Subject: Re: [Simh] Why 36-bit computing? > -----Original Message----- > From: [email protected] [mailto:simh-bounces@trailing- > edge.com] On Behalf Of Michael Mondy > Sent: Tuesday, March 19, 2013 7:37 AM > To: [email protected] > Subject: [Simh] Why 36-bit computing? > > On Tue, Mar 19, 2013 at 02:43:10PM +0100, Johnny Billquist wrote: > > [ ... ] > > > > It wasn't just DEC. Back in the day, most everyone used various word > > lengths that wasn't a power of two. I can't really make many > > comments on why other word lengths were more popular. I've seen > > mentioned that floating point formats was pretty nice to do with > > something like 60 or > > 72 bits. Reason being that you had large enough exponents for useful > > things, and enough precision for most calculations. > > So a word length that related to this made sense. > > > > Number of bits being a power of two started with IBM in the 60s, and > > became common with the PDP-11 in the 70s. (Or so I'd like to think.) > > > > Johnny > > Wikipedia has an article on 36-bit computing: > http://en.wikipedia.org/wiki/36-bit > > Snipped from the wikipedia article: > > [ ... ] > > Many early computers aimed at the scientific market had a 36-bit word > length. This word length was just long enough to represent positive > and negative integers to an accuracy of ten decimal digits (35 bits > would have been the minimum). It also allowed the storage of six > alphanumeric characters encoded in a six-bit character encoding. Prior > to the introduction of computers, the state of the art in precision > scientific and engineering calculation was the ten-digit, electrically > powered, mechanical calculator, such as those manufactured by Friden, > Marchant and Monroe. These calculators had a column of keys for each > digit and operators were trained to use all their fingers when > entering numbers, so while some specialized calculators had more > columns, ten was a practical limit. Computers, as the new competitor, > had to match that accuracy. Decimal computers sold in that era, such > as the IBM 650 and the IBM 7070, had a word length of ten digits, as did ENIAC, one of the earliest com puters. > > [ ... ] > > By the time IBM introduced System/360, scientific calculations had > shifted to floating point and mechanical calculators were no longer a > competitor. [...] [ At which point the advantages of using powers of > two became more important than feature parity with mechanical > calculators. ] > The following is from a biography of Fred Brooks, the project manager for the IBM 360, on UNC-Chapel Hill's Computer Science department website (http://www.cs.unc.edu/cms/our-people/faculty/frederick-p.-brooks-jr): "In 1957, Dr. Brooks and Dura Sweeney invented a Stretch interrupt system that introduced most features of today's interrupt systems. Dr. Brooks coined the term computer architecture. His system/360 team first achieved strict compatibility, upward and downward, in a computer family. His early concern for word processing led to his selection of the 8-bit byte and the lowercase alphabet for the System/360, engineering of many new 8-bit input/output devices, and providing a character-string datatype in PL/I." Keep in mind that the S/360 was not only targeted for scientific computation. It was intended to consolidate IBM's customer bases. -- Ian _______________________________________________ Simh mailing list [email protected] http://mailman.trailing-edge.com/mailman/listinfo/simh _______________________________________________ Simh mailing list [email protected] http://mailman.trailing-edge.com/mailman/listinfo/simh
