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 http://forums.parallax.com/images/smilies/smile.gif 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! http://forums.parallax.com/images/smilies/smile.gif

-Phil