nopsteam/6502

Indirect addressing mode needs to return entire page when lo(0xFF)

Opened this issue · 1 comments

lucascebertin commented

Following the 6502 spec, there is a bug in architecture when indirect addressing mode is used.
Not sure if this thing affects all the indirect addressing like indirect, x and indirect, y but probably yes.
We need to check if the lo(ADDRESS) is 0xFF and, then, consider a full page return, so:

LDX #$02
STX $3000

LDX #$04
STX $3100

LDX #$03
STX $30FF

JMP ($30FF)

the jump will consider the hi(0x3000) + lo(0x30FF) = 0x0203

NB: WE CAN'T USE EASY6502 TO VALIDATE THIS BECAUSE THEY ARE NOT RESPECTING THIS BEHAVIOUR AS WE CAN SEE IN THE PICTURE:
image

rafaeldelboni commented

So, if I understood correctly, to simulate this bug we need to do something like this In the indirect function?

If address & 0x00FF = 0xFF

Then:
Address & 0xFF00 = hi
Address = lo

Else:
Address + 1 = hi
Address = lo
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