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Indirect indexed addressing: Addresses are not wrapping within zeropage. #55

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mibi88 opened this issue Jul 5, 2024 · 0 comments · May be fixed by #56
Open

Indirect indexed addressing: Addresses are not wrapping within zeropage. #55

mibi88 opened this issue Jul 5, 2024 · 0 comments · May be fixed by #56

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@mibi88
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mibi88 commented Jul 5, 2024

Addresses are not wrapping within zeropage as they do on a real 6502.
When running LDA ($FF), Y with Y = 0, the 6502 simulator takes the low byte from $FF and the high byte from $0100.
It can be seen when running this code:

LDA #$00
STA $00

LDA #$02
STA $FF
LDA #$04
STA $0100

LDA #$AC
STA $0402

LDA #$c2
STA $02

LDY #$00
LDA ($FF), Y

After executing this code, A should be equal to C2 and not AC.
PinoBatch, on the Nesdev wiki figured out that it is due to getWord() which does not handle wrapping:

easy6502/simulator/assembler.js

Lines 587 to 593 in bb213ae

i11: function () {
var zp = popByte();
var value = memory.getWord(zp) + regY;
regA |= memory.get(value);
ORA();
},

beckadamtheinventor added a commit to beckadamtheinventor/easy6502 that referenced this issue Jul 7, 2024
@beckadamtheinventor
Fix skilldrick#55
9490c71
@beckadamtheinventor beckadamtheinventor linked a pull request Jul 7, 2024 that will close this issue
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Fix #55 beckadamtheinventor/easy6502
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@mibi88