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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.arch
Subject: Re: two's complement and Flags (Re: arithmetic and endian history)
Date: Fri, 02 Oct 2026 21:19:10 -0700
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John Levine <johnl@taugh.com> writes:

> According to Tim Rentsch  <tr.17687@z991.linuxsc.com>:
>
>> John Levine <johnl@taugh.com> writes:
>>
>>> I'm not aware of any pure two's complement machines before S/360.
>>
>> How about the PDP-4?
>
> It had both one's complement "add" and two's complement "tad".
>
> The 12-bit PDP-5 was two's complement in 1963 which was about a year
> before the S/360 announcement so I guess it qualifies.  Wikipedia
> says that English Electric DEUCE in 1955 was two's comp.
>
> The PDP-4 and -5 had no condition codes.  There was "link" bit that
> held carries from the accumulator, but the way you did conditional
> tests was instructions like skip if AC=0 or if AC<0.

The PDP-6 used two's complement.  The design of the 6 started (and
may even have finished) before System/360 was announced.  Delivery
of PDP-6 units happened well before the delivery of any model of
System/360.  (All stated information taken from Wikipedia pages.)