two's complement, was Re: Now OT

[Paul Koning]

> On Jun 23, 2016, at 12:11 PM, Paul Koning <paulkoning at comcast.net> wrote:
> 
> 
>> On Jun 23, 2016, at 12:07 PM, Al Kossow <aek at bitsavers.org> wrote:
>> ...
>> I have also heard that 2s compliment was popular in shorter word length
>> machines because 1s compliment multiple precision arithmetic is a PITA
>> to implement.
> 
> That's true.  It certainly can be done and has been. But since one's complement arithmetic uses end around carry, when you have multiple word you have to defeat the word carry and instead do the carry around the whole number.

Something to look into.... in the Electrologica machines (Dutch computers from the late 1950s to mid 1960s), double-length values are encoded with the sign bit replicated in each word.  I wonder if that makes this problem go away (entirely or mostly).

	paul