> That seems to work. I don't need to clear carry or anything.
> (Oh. I see there is no "clear minus" instruction anyway.
> I guess the LDA will always set that, but not the carry.
> I guess a sharper 6502 programmer would know that pretty
> much instantly...)
Yes, any LoaD instruction (including register to register transfers like
TYA, though maybe not PLA) will update the N & Z flags.
A CoMPare instruction sets the flags just like
register + ~memory + 1 (Carry flag ignored)
So if A = $00 then CMP #$00 is the same as
$00 + ~$00 + 1 = $00 + $FF + 1 = $100
So Carry = 1 because the result is more than 8 bits, Zero = 1 because the 8
bit result is $00, and Negative (aka Sign) = 0 because the result AND $80 is
$00.
(Note: CoMPare does not change the oVerflow flag; which is changed by only
ADC, SBC and BIT.)
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