----- Original Message -----
From: "Paul Gilmartin" <00000014e0e4a59b-dmarc-requ...@listserv.uga.edu>
To: <ASSEMBLER-LIST@LISTSERV.UGA.EDU>
Sent: Friday, June 17, 2022 7:20 AM
Subject: Re: When did logical instructions appear?
On Jun 16, 2022, at 10:43:36, Robin Vowels wrote:
Computers have had instructions for signed and unsigned binary
since at least 1951. When negative values are expressed using
twos complement notation, ordinary addition will give the same
result whether the operation is signed or unsigned.
It puzzles me that some of the oldest computers employed sign-
magnitude notation when 2's (1's, 10's) complement would have
needed fewer gates and fewer clock cycles.
For a serial machine (and most of them were in the early days),
twos complement was the simplest. It needed only one cycle
for add and subtract. Even subtract (complement and add one)
was done in a single cycle, "on the fly". It was unnecessary to
add the one; complement commencing after the first non-zero bit.
Ones complement was a PITA, because a carry out of the high end
required a 1 to be added in the next cycle. This was no good at
all for array machines such as Pilot ACE, DEUCE, and ACE,
because the word just summed would not have been around
to add the final "1" produced by the carry out -- the arithmetic
unit already working on summing (or subtracting) the next pair
of words.
Perhaps an accommodation to the engineers' habits.
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