Understanding overflow flag for ADC on the 6502

[Disch]

i guess the clearest way to set or reset the V flag in ADC and SBC instructiones is by testing if the result is major than 0x7f. it works in both instructions.

That actually does not work for either instruction.

Consider the following:

Code: Select all

LDA #$D0
CLC
ADC #2  ; this should CLEAR V
   ; your code would set V (wrong)

Your SBC code fails for the same reason

[ehguacho]

i guess the clearest way to set or reset the V flag in ADC and SBC instructiones is by testing if the result is major than 0x7f. it works in both instructions.

That actually does not work for either instruction.

Consider the following:

Code: Select all

LDA #$D0
CLC
ADC #2  ; this should CLEAR V
   ; your code would set V (wrong)

Your SBC code fails for the same reason

why should it be failing? $D2 + 2 = $D2 > 127
maybe i'm wrong, but why?

[tokumaru]

why should it be failing? $D2 + 2 = $D2 > 127
maybe i'm wrong, but why?

$D0, as a signed number, is -48. Add 2 and it becomes -46 ($D2). There is no overflow.