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Date: Wed, 19 Jul 2006 10:14:25 -0700
From: glen herrmannsfeldt <gah@ugcs.caltech.edu>
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Subject: Re: ONES COMPLEMENT
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Date: 2006-07-19T10:14:25-07:00
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Peter Flass wrote:

> Shmuel (Seymour J.) Metz wrote:

(snip on negative zero)

>> 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.

> What happens on a test for negative?  Did the machines just test the 
> sign bit, making -0 a negative number?

Well, consider the 704, which is actually sign magnitude but has the
same problem.  (The easiest way to do sign magnitude arithmetic is
to convert to ones complement first, and convert back later.)

The 704 has a three way test instruction recently discussed in some
newsgroup, with branch destinations for negative, zero, and positive.
Any questions about the origin of the arithmetic IF statement?

-- glen