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From: Richard Damon <richard@damon-family.org>
Newsgroups: comp.lang.c
Subject: Re: What is your opinion about unsigned int u = -2 ?
Date: Fri, 9 Aug 2024 21:16:19 -0400
Organization: i2pn2 (i2pn.org)
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On 8/9/24 2:19 PM, David Brown wrote:
> On 09/08/2024 20:08, Bonita Montero wrote:
>> Am 08.08.2024 um 19:47 schrieb David Brown:
>>
>>> Think about negating a value.  For two's complement, that means 
>>> inverting each bit and then adding 1.  For sign-magnitude, you
>>> invert  the sign bit. For ones' complement, you invert each bit.
>>
>> But with one's complement you have the same circuits for ading
>> and substracting like with unsigned values.
> 
> If you are trying to say that for two's complement, "a + b" and "a - b" 
> use the same circuits regardless of whether you are doing signed or 
> unsigned arithmetic, then that is correct.  It is one of the reasons why 
> two's complement became the dominant format.
> 

No, a two's complement subtractor needs to invert the second operand, 
and inject a carry into the bottom bit. A one's complement subtractor 
just need to invert the second operand.

The fact that you want that carry input in the logic anyway for 
add-with-carry to make bigger additions, says that this doesn't really 
cost anything.