[Simh] Why 36-bit computing?

[Michael Mondy]

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 computers.

[ ... ]

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. ]

-- Mike