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

[Chuck Guzis]

On 03/01/2015 03:05 PM, Toby Thain wrote:

> Studying the CDC 6000 instruction set (through the Grishman book) really
> gives me the impression one's complement would be quite a headache to
> use correctly.

It wasn't, trust me.  Because the long add unit on the 6600 used a 
subtractive algoritm, negative zero was pretty uncommon as a result of 
arithmetic operations.

Either value registered as zero for the conditionals--remember, no 
condition codes.  The big difference is that negative zero registered as 
minus.  Remember that on X-register tests, you could, with a single 
instruction, only branch on zero/nonzero or minus/plus.

--Chuck