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NNTP-Posting-Date: Fri, 09 Mar 2012 16:53:55 -0600
Message-ID: <4F5A8A03.3657@mindspring.com>
Date: Fri, 09 Mar 2012 17:53:55 -0500
From: pete <pfiland@mindspring.com>
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Subject: Re: difference between ~0U and ~0
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James Piper wrote:
> 
> On Thursday, March 8, 2012 8:55:20 AM UTC-5, pete wrote:
> > somenath wrote:
> >
> > > According to my understanding ~0 is -1.
> > > So I will get all bit 1 as a
> > > result of ~0 in 2's complement system.
> >
> > Close, but backwards.
> >
> >     ~0 is all bit 1.
> >
> >     So you will get -1 as a
> >     result of ~0 in 2's complement system.
> That's true.
> 
> 1 in 8-bit format is: 0000 0001
> To get neg- 1:
> 1. Negate bits. 1111 1110
> 2. Add 1: 1111 11111
> 
> Same results regardless of the bit size.

Another way for negatising in two's complement is:
1. subtract 1. 0000 0000
2. Negate bits: 1111 11111

I've worked on assembly code 
that had it done either way in different parts of the code.
I don't know if it was done by the same person.

-- 
pete