[Simh] Why 36-bit computing?

[Ian King]

> -----Original Message-----
> From: simh-bounces at trailing-edge.com [mailto:simh-bounces at trailing-
> edge.com] On Behalf Of Michael Mondy
> Sent: Tuesday, March 19, 2013 7:37 AM
> To: simh at trailing-edge.com
> 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