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NNTP-Posting-Date: Fri, 09 Mar 2012 17:52:08 -0600
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Date: Fri, 09 Mar 2012 18:52:08 -0500
From: pete <pfiland@mindspring.com>
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Subject: Re: difference between ~0U and ~0
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Xref: csiph.com comp.lang.c:18421

Tim Rentsch wrote:
> 
> pete <pfiland@mindspring.com> writes:
> 
> > 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.
> 
> The first method turns x into -x;  the second turns -x into x.

I'm not going to dignify that with a response.

Wait, I told that wrong ...

-- 
pete