2’s Complement

Mike G

I’m sure there is a quicker way to calculate 2’s complement than my 3 lines of Propeller assembly code?

neg     chkSum,t1
sub     chkSum,#1
and     chkSum,#$FF

deSilva

Yes, it is generally done by

XOR chkSum, cTRUE
...
cTRUE LONG TRUE

However it seems you want the least 8 bits only, then it would be

XOR chkSum, #$FF
AND chkSum, #$FF

You will not need the AND if the original bits 8..31 are already cleared.

Phil Pilgrim (PhiPi)

deSilva,

Your code does a 1's complement. For 2's complement, you need to add a ADD chkSum,#1 between the XOR and AND. But this would be quicker:

    NEG chkSum, chkSum
    AND chkSum, #$FF

-Phil

deSilva

Phil, you might be right according to the book, but I did exactly what the OP requested So let's call it 1's complement

Mike G

Your right my mistake. I need 1's complement. Thanks guys

Phil Pilgrim (PhiPi)

Well, that'll teach me about reading the title and just skimming the original code!

-Phil