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Date: Tue, 18 Jul 2006 23:54:39 -0700
From: glen herrmannsfeldt <gah@ugcs.caltech.edu>
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Subject: Re: ONES COMPLEMENT
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Date: 2006-07-18T23:54:39-07:00
List-Id: <comp.lang.ada>

Everett M. Greene wrote:

(snip)

> A properly designed 1s complement machine would not
> generate -0 in normal arithmetic operations.  Thus,
> no need to perform a program check for -0.

As far as I know, they are designed either to not generate
-0 under normal conditions, or to compare -0 equal to +0,
or both.

One solution is to do subtraction as the basic operation,
such that subtracting two of the same number will generate +0,
and addition as subtraction of the complement.
-0 can still result if -0 is used as an operand, though.

For a parallel adder, as all machines have had for about 40 years now,
no extra hardware is needed, nor is more time needed.  The "add with 
carry" operation that some machines have is not available, though.

Twos complement machines might require extra hardware to handle the 
overflow on negate or absolute value, which can't overflow on ones 
complement machines.

For multiply and divide it might be that ones complement is a little 
easier, though the difference should be small.

-- glen