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

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist