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

Toby Thain toby at
Sun Mar 1 17:05:54 CST 2015

On 01/03/15 6:42 AM, Johnny Billquist wrote:
> On 2015-03-01 06:14, Jon Elson wrote:
>> On 02/28/2015 04:59 AM, Johnny Billquist wrote:
>>> Not that Unix was ever written in B anyway, but I believe that B did
>>>>> exist for the PDP-7.
>> I looked up the instruction set of the PDP-7, ghastly little machine,
>> basically
>> a PDP-8 extended to 18 bits. ...
> ...
> The ones and twos complement is the most interesting part. I suspect
> that is because they really had decided that twos complement was the
> future. However, the DEC 18-bit series really started with the PDP-1,
> and all follow on machines were somewhat backwards compatible, and the
> PDP-1 was actually a ones complement machine. The only one DEC built
> that way. So all followon 18-bitters would have to keep the capability,
> I guess.

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.


>      Johnny

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