Robert Newson wrote:
> Looking at the 1/-1 column (closest, as I've got 2/-127), I see that N is
> set and so branch should be taken. However, N is in fact clear - N,Z are
> clear and C,V are set: using 2's complement and adding (I learnt MC
> programming on a 6502) you can see this:
> -127 = 1000 0001 -> 1000 0001
> +2 = 0000 0010 -> 1111 1101 [1's complement]
> 1 [extra [initial] 1 for 2's complement]
> -----------
> 1 0111 1111
> C = 1 : Obvious
Obviously it's not that obvious. ;-)
It is true that the result of an addition with NEG(op) equals an
subtraction with op, but not when it comes to the flags. If you do a
direct subtraction you see that at the end there is no carry involved.
-127 = 1000 0001 Destination
+2 = 0000 0010 Source
0 1111 1110 Carry bits
--------------
0111 1111
Marcel
