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Date: Tue, 18 Jul 2006 11:41:37 -0700
From: glen herrmannsfeldt <gah@ugcs.caltech.edu>
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Subject: Re: ONES COMPLEMENT
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Date: 2006-07-18T11:41:37-07:00
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robin wrote:
> glen herrmannsfeldt wrote in message ...

(snip)

>>Well, you want unsigned arithmetic to do multiple precision,
>>which is harder if a machine only supplies ones complement.

> In what way?
> The arithmetic is done using fewer bits than word.
> You have to do that with twos complement also.

The bits resulting in an unsigned addition or subtraction operation are 
the same as for a twos complement operation, but the detection of 
overflow (or carry/borrow) is different.  If you can properly detect
carry/borrow you can do it with all the bits in the word.

> The arithmetic required is addition; and all values
> manipulated are positive.  Results are the same in
> machines supporting ones or twos complement
> representation for negative values.

Hmm.  If you can detect the end around carry you can generate
the correct unsigned result, and so still use all the bits.
This is not usually easy in an high-level language, but should
be provided to the assembly programmer.

-- glen