On Friday, April 13, 2007 9:39 AM [GMT-3=CET],
Gerhard Fiedler  wrote:

> Harold Hallikainen wrote:
>
> > Something I read in BYTE magazine MANY years ago, that I have not seen
> > published elsewhere, is, I think, much simpler.
> >
> > When using twos complement, make the sign of the weight of the msb
> > negative. Then add up bits times weights, as usual.
> >
> > For example, in an 8 bit number, the msb weight becomes -128 instead of
> > 128. So, 10101010 is:
> >
> > 1*(-128)=-128
> > 0*64=0
> > [...]
> > and -128+32+8+2=-86.
>
> This is a derivative of the original rule of what two's complement means.
> It's simply "folding" the values around at 0, which is equal to
subtracting
> 256 if the value is over 127. This can be achieved by converting the
number
> into decimal as if it were unsigned, then subtracting 256 (or whatever
your
> range is) if it's outside the positive range:
>
> 0b10101010 = 2+8+32+128 = 170 (for 0b10101010 being unsigned)
> 0b10101010 = 170-256 = -86 (for 0b10101010 being two's complement)
>
> Gerhard

...or

   sublw 0    ;take the two's complement (256 range -128 +127)



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