Marcel Kilgus wrote:
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
As I said, it was due to my learning MC on a 6502 where the C *HAS* to be
set before a SBC and the result with a C still set means no carry was
required; the 650x series didn't have a subtractor, only an adder [AFAICS],
and so it used the add negated method...I was working an example with my
650x manual open. Sawree
