[Simh] Why 36-bit computing?

[Dave L]

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: simh-bounces at trailing-edge.com [mailto:simh-bounces at trailing-edge.com]
On Behalf Of Ian King
Sent: 19 March 2013 19:05
To: simh at trailing-edge.com
Subject: Re: [Simh] Why 36-bit computing?

> -----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 

_______________________________________________
Simh mailing list
Simh at trailing-edge.com
http://mailman.trailing-edge.com/mailman/listinfo/simh