One's complement - was Re: Rich kids are into COBOL

Chuck Guzis cclist at
Mon Mar 2 01:50:09 CST 2015

On 03/01/2015 04:08 PM, Al Kossow wrote:
>> one's complement would be quite a headache to use correctly.
> Extended precision arithmetic is a PITA on an 1's compliment machine
> I have heard the argument that one of the reasons for going to 2's
> compliment on short word length machines was because of that.

Again, it depends.  On the CDC 6000 series, integers for doing math were 
pretty much 48 bit as multiply and divide were done using the floating 
point unit.  The long add unit did 60 bit integer adds and subtracts. 
So, if you kept to 48 and 96 bit integers, it was no worse than on any 
other machine.

Even extended precision integer math on the PPUs was less of a hassle 
than you'd think--the memory word length was 12 bits, but the 
accumulator (A) register was 18 bits long.

Seymour Cray was no fool.


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