Ben Larson wrote:
> OK I am seriously confused as to the how the compare
> and branch instructions work. Ok first of all,
> correct me if I am wrong, but when you do a BPL
> doesn't that check to see if the value you are
> comparing is greater than the the value in the
> accumulator?
A compare works like a subtraction (like SBC, but without the carry
influence):
- N-flag: set, when the subtracted signed value is bigger, so the result
is negative (signed compare)
- C-flag: set, when the subtracted unsigned value is bigger (unsigned
compare)
- V-flag: set, when there is an signed overflow in subtraction
(i.E. -128 - 1 and +127 - -1) (signed compare)
> Well if that's so, then is the processor
> comparing the two numbers as unsigned 8-bit integers
> or signed? In other words, what would be the outcome
> of, say...this:
> LDA #100
> CMP #-20 ; -20 signed = 236 unsigned
> BPL AIsLess ; in a 2's comp. representation
- 100 is bigger than -20 (or: the result is 120 = $78, that's
positive), so the N-flag is clear.
- 236 is bigger than 100, so the C-flag is set,
- there was no signed overflow (100- -20 = 120), so the V-flag is clear.
Have fun!
Thomas
_______________________________________________________
Thomas Jentzsch | *** Every bit is sacred ! ***
tjentzsch at web dot de |
-
Archives (includes files) at http://www.biglist.com/lists/stella/archives/
Unsub & more at http://www.biglist.com/lists/stella/
