The DECSystem20 (and the PDP-10 before) were 36 bit two's complement 
machines.  You had to go back to the PDP-1 if you want one's complement 
in the DEC line.    The CDCs and UNIVACs were the only ones that were 
still kicking around in my era.


>>
>>Just to confirm, I pulled out my PDP-10 Hardware Reference Manual; Vol 
>>I - CPU
>>(EK-10/20-HR-001), and it does indeed say (pg. 1-12): "The fixed-point
>>arithmetic instructions use 2's complement representations to do 
>>binary
>>arithmetic." Selah.
>
>Back in school, we had our machine organization course. When we learned 
>about 1's complement, the professor said "I've used a lot of machines 
>that had this. You will likely never see one with it. There are no 
>operational machines on campus with that."It stuck with me. We had a 
>TOPS-20 machine...  the odd turn of phrase was due to a professor that 
>had a board of unknown origin hanging on the wall that was a rumored to 
>be a CDC or similar... ah, the mid 80s...

Warner