[TUHS] One's complement (was: reviving a bit of WWB)

[John Cowan]

On Thu, Sep 24, 2020 at 8:22 PM Warner Losh <imp at bsdimp.com> wrote:

Don't know about the others, but I'm pretty sure PDP-10 wasn't 1's
>> compliment / was 2's compliment..
>
>
Correct.  The PDP-1 (18 bits) was DEC's 1's complement machine.  Its direct
successors the 4/7/9/15 had both 1's and 2's complement arithmetic.  The
12-bit 5/8/12 machines had only 2's complement, but retained the PDP-4
mnemonic TAD (Two's-complement Add).  By the time the 36-bit 6/10/20 line
was designed, it was clear that 1's complement was history, and the
mnemonic was changed to ADD.

(The PDP-3 was a PDP-1 with a 36-bit data path, and only one ever went into
production; the PDP-2 was to be a 24-bit machine, perhaps a compromise
between 6-bit and 8-bit byte systems, but was never even designed.)



John Cowan          http://vrici.lojban.org/~cowan        cowan at ccil.org
I marvel at the creature: so secret and so sly as he is, to come sporting
in the pool before our very window.  Does he think that Men sleep without
watch all night?    --Faramir
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