> On Jun 11, 2026, at 11:19 AM, r.stricklin via cctalk <[email protected]>
> wrote:
>
>
>> On Jun 11, 2026, at 8:09 AM, Paul Koning via cctalk <[email protected]>
>> wrote:
>>
>> A lot of systems used one's complement through the 1960s. I'm not really
>> sure why. Not just for integers but floating point also (as on the CDC
>> mainframes, and around the same time the Electrologica X8 with a slightly
>> different approach).
>
> My understanding of it is, it makes the logic simpler. But the cost is
> software complexity.
It doesn't really make the software more complex, just different.
I can see the logic thing, more or less. You end up with just complement both
for logic and arithmetic operations (it doubles as negate). The end-around
carry seems to complicate things but it doesn't really because you have carry
anyway, so you just end up with an ALU that looks like a circle. The only
additional logic I can see is that a test for zero requires matching both +0
and -0.
Using one's complement style float notation means a bunch of the operations
(like negate/complement) apply to floating point operands as well, no separate
fneg operation needed. And with the right exponent notation, an integer not
exceeding the mantissa size doubles as a valid floating point value for that
same number.
CDC made things a little easier for programmers to understand because they used
a subtractor circuit, so you end up with +0 for all operations that involve
non-zero or +0 operands. Others tended to use adders, so you end up with -0
most of the time. Still no problem so long as you remember it.
paul
