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From: mojaveg@mojaveg.lsan.sisna.com (Everett M. Greene)
Subject: Re: ONES COMPLEMENT (was: Ada vs Fortran for scientific applications)
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Date: 2006-07-19T09:11:34-08:00
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"Shmuel (Seymour J.) Metz" <spamtrap@library.lspace.org.invalid> writes:
> mojaveg@mojaveg.lsan.sisna.com (Everett M. Greene) said:
> >"robin" <robin_v@bigpond.com> writes:
> 
> >> Some implementations of ones complement used two forms of
> >> zero 
> 
> FSVO "some" equal to "all".
> 
> >>which required two tests.
> 
> That  makes no sense.
> 
> >A properly designed 1s complement machine would not
> >generate -0 in normal arithmetic operations.
> 
> 0+(-0)?

A subtractive adder.

> >Thus, no need to perform a program check for -0.
> 
> I never saw a need to test for it regardless. Every 1s complement
> machine I know of lets you test for zero in a single instruction.

Try the Univac mainframes.

I once got bagged by one of a Univac-designed machine that
did 0 x n = -0 for any negative value of n.  This wasn't
the way it should work, but it worked that way to be
compatible with an earlier machine.  ["It's a mistake, but
we are consistently wrong."]

> >> How many machines now offer ones complement arithmetic?
> 
> Before the S/360 captured the market there were large numbers of
> machines in the lines started with the CDC 160, CDC 6660 and the
> UNIVAC 1107. The shift seemed to be a copycat issue more than a
> technological one.