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emulate indirect mode bug in 6502

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This commit is contained in:
James Tauber 2011-08-06 23:02:52 -04:00
parent 84bdef67cd
commit 309f8277e1
1 changed files with 5 additions and 1 deletions
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applepy.py
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@ -367,7 +367,11 @@ class CPU:
return (self.zero_page_mode() + signed(self.y_index)) % 0x100 return (self.zero_page_mode() + signed(self.y_index)) % 0x100
def indirect_mode(self): def indirect_mode(self):
return self.memory.read_word(self.absolute_mode()) address = self.absolute_mode()
hi = address + 1
# emulate a known bug in 6502
hi = (address & 0xFF00) + (((address & 0xFF) + 1) & 0xFF)
return self.memory.read_byte(address) + (self.memory.read_byte(hi) << 8)
def indirect_x_mode(self): def indirect_x_mode(self):
return self.memory.read_word((self.read_pc_byte() + signed(self.x_index)) % 0x100) return self.memory.read_word((self.read_pc_byte() + signed(self.x_index)) % 0x100)