improved implementation of indirect bug across page boundaries including indexed indirects as well

applepy.py

@@ -39,6 +39,12 @@ class Memory:
     def read_word(self, address):
         return self.read_byte(address) + (self.read_byte(address + 1) << 8)
     
+    def read_word_bug(self, address):
+        if address % 0x100 == 0xFF:
+            return self.read_byte(address) + (self.read_byte(address & 0xFF00) << 8)
+        else:
+            return self.read_word(address)
+    
     def write_screen(self, address, value):
         base = address - 0x400
         hi, lo = divmod(base, 0x80)
@@ -366,17 +372,13 @@ class CPU:
         return (self.zero_page_mode() + signed(self.y_index)) % 0x100
     
     def indirect_mode(self):
-        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)
+        return self.memory.read_word_bug(self.absolute_mode())
     
     def indirect_x_mode(self):
-        return self.memory.read_word((self.read_pc_byte() + signed(self.x_index)) % 0x100)
+        return self.memory.read_word_bug((self.read_pc_byte() + signed(self.x_index)) % 0x100)
     
     def indirect_y_mode(self):
-        return self.memory.read_word(self.read_pc_byte()) + signed(self.y_index)
+        return self.memory.read_word_bug(self.read_pc_byte()) + signed(self.y_index)
     
     def relative_mode(self):
         pc = self.get_pc()

tests.py

@@ -949,5 +949,54 @@ class TestSystemFunctionOperations(unittest.TestCase):
         self.cpu.NOP()
 
 
+class Test6502Bugs(unittest.TestCase):
+    
+    def setUp(self):
+        self.memory = Memory(0x10000)
+        self.cpu = CPU(self.memory)
+    
+    def test_zero_page_x(self):
+        self.cpu.x_index = 0x01
+        self.memory.load(0x1000, [0x00, 0x7F, 0xFF])
+        self.cpu.program_counter = 0x1000
+        self.assertEqual(self.cpu.zero_page_x_mode(), 0x01)
+        self.assertEqual(self.cpu.zero_page_x_mode(), 0x80)
+        self.assertEqual(self.cpu.zero_page_x_mode(), 0x00)
+    
+    def test_indirect(self):
+        self.memory.load(0x20, [0x00, 0x20])
+        self.memory.load(0x00, [0x12])
+        self.memory.load(0xFF, [0x34])
+        self.memory.load(0x100, [0x56])
+        self.memory.load(0x1000, [0x20, 0x20, 0xFF, 0xFF, 0x00, 0x45, 0x23])
+        self.memory.load(0x2000, [0x05])
+        self.memory.load(0x1234, [0x05])
+        self.memory.load(0x2345, [0x00, 0xF0])
+        
+        self.cpu.program_counter = 0x1000
+        
+        self.cpu.x_index = 0x00
+        self.cpu.LDA(self.cpu.indirect_x_mode())
+        self.assertEqual(self.cpu.accumulator, 0x05)
+        
+        self.cpu.y_index = 0x00
+        self.cpu.LDA(self.cpu.indirect_y_mode())
+        self.assertEqual(self.cpu.accumulator, 0x05)
+        
+        self.cpu.y_index = 0x00
+        self.cpu.LDA(self.cpu.indirect_y_mode())
+        self.assertEqual(self.cpu.accumulator, 0x05)
+        
+        self.cpu.x_index = 0x00
+        self.cpu.LDA(self.cpu.indirect_x_mode())
+        self.assertEqual(self.cpu.accumulator, 0x05)
+        
+        self.cpu.x_index = 0xFF
+        self.cpu.LDA(self.cpu.indirect_x_mode())
+        self.assertEqual(self.cpu.accumulator, 0x05)
+        
+        self.assertEqual(self.cpu.indirect_mode(), 0xF000)
+
+
 if __name__ == "__main__":
     unittest.main()