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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: 2006-07-22T07:12:58-08:00
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"Shmuel (Seymour J.) Metz" <spamtrap@library.lspace.org.invalid> writes:
> mojaveg@mojaveg.lsan.sisna.com (Everett M. Greene) said:
> >"Shmuel (Seymour J.) Metz" <spamtrap@library.lspace.org.invalid>
> >> >A subtractive adder.
> >> 
> >> How so? 0+(-0) is a straight addition of 0...0 and 7...7, requiring
> >> no complementation.
> 
> >Context was deleted so I have no idea what you're trying to say
> >above.
> 
> Robin wrote "A properly designed 1s complement machine would not
> generate -0 in normal arithmetic operations."
> 
> I responded "0+(-0)?".
> 
> You responded "A subtractive adder."
> 
> I'm trying to say that there's nothing about 0+(-0) requiring a
> subtractive adder.

You are correct -- there's nothing /requiring/ a subtractive
adder.  But using such a design precludes certain cases of
-0 from being generated.

> >They test for +0 in one instruction.  If you want to reliably test
> >for zero, you have to add 0 first.
> 
> From JZ in UP-4053, UNIVAC 1108 PROCESSOR AND STORAGE:
> 
>      Program control is transferred to U if the (A) are all zeros or
> all ones.
> 
> I repeat, which UNIVAC mainframe are you talking about? It's clearly
> not the 1108.