A simple way to get the two's complement is to get the one's complement
(invert the bits), then add one. The old PDP-8 called this instruction
CIA for complement and increment accumulator.
        Another interesting two's complement idea I saw written years and years
ago is to consider the "weighting" of the bits the same as for a pure
binary number, but reverse the sign of the MSB.  So, for example,

01101110 = 2+4+8+32+64=110 decimal

10010001 is 1's complement (inverting bits)

10010010 is 2's complement (adding 1 to 1's complement)

Looking at weighting, we get

10010010 = 2+16-128= -110 decimal (note the MSB has a weight of -128
instead of +128, as it would in pure binary).


Harold



On Tue, 7 Mar 2000 10:06:10 +0200 Werner Soekoe <wsoekoe@GLOBAL.CO.ZA>
writes:
> Hi
>
> I'm looking for the way to calculate what the Two's
> Complement of a value will be. Can anyone help me?
> For instance:
>
> 8-bit value:
> 01101110
> 10010001 (Inverted)
>
> But this is not two's complement yet!? What Next?
>
> Can somebody maybe also solve this one:
> 16-Bit value:
> 01011100 11110110
> 10100011 00001001 (Inverted)
>
> Thanks!!!
>
> Werner Soekoe
> IS Manager
> Free State Legislature
> wsoekoe@global.co.za
> Tel. +27 (51) 407 1109
> Fax. +27 (51) 407 1137
> Cell. +27 82 376 8383

FCC Rules Online at http://hallikainen.com/FccRules
Lighting control for theatre and television at http://www.dovesystems.com

________________________________________________________________
YOU'RE PAYING TOO MUCH FOR THE INTERNET!
Juno now offers FREE Internet Access!
Try it today - there's no risk!  For your FREE software, visit:
http://dl.www.juno.com/get/tagj.