Fix BRK on 65C02 so it clears the decimal flag

CHANGES.txt

@@ -4,6 +4,9 @@
     the program counter to overflow to $10000.  It now wraps around
     to $0000 as it should.  Thanks to Gábor Lénárt for reporting it.
 
+  - Fixed BRK on 65C02 so it clears the decimal flag.  On NMOS 6502, the
+    decimal flag is unaffected on BRK.  On 65C02, it is always cleared.
+
 0.17 (2013-10-26)
 
   - Added support for Python 3.2 and 3.3 based on work by David Beazley.

py65/devices/mpu65c02.py

@@ -68,6 +68,21 @@ class MPU(mpu6502.MPU):
 
     # instructions
 
+    @instruction(name="BRK", mode="imp", cycles=7)
+    def inst_0x00(self):
+        # pc has already been increased one
+        pc = (self.pc + 1) & self.addrMask
+        self.stPushWord(pc)
+
+        self.p |= self.BREAK
+        self.stPush(self.p | self.BREAK | self.UNUSED)
+
+        self.p |= self.INTERRUPT
+        self.pc = self.WordAt(self.IrqTo)
+
+        # 65C02 clears decimal flag, NMOS 6502 does not
+        self.p &= ~self.DECIMAL
+
     @instruction(name="RMB0", mode="zpg", cycles=5)
     def inst_0x07(self):
         self.opRMB(self.ZeroPageAddr, 0xFE)

py65/tests/devices/test_mpu6502.py

@@ -1919,7 +1919,7 @@ class Common6502Tests:
 
     def test_brk_pushes_pc_plus_2_and_status_then_sets_pc_to_irq_vector(self):
         mpu = self._make_mpu()
-        mpu.p = mpu.BREAK | mpu.UNUSED
+        mpu.p = mpu.UNUSED
         self._write(mpu.memory, 0xFFFE, (0xCD, 0xAB))
         # $C000 BRK
         mpu.memory[0xC000] = 0x00
@@ -5661,6 +5661,28 @@ class MPUTests(unittest.TestCase, Common6502Tests):
         mpu = self._make_mpu()
         self.assertTrue("6502" in repr(mpu))
 
+    # BRK
+
+    def test_brk_preserves_decimal_flag_when_it_is_set(self):
+        mpu = self._make_mpu()
+        mpu.p = mpu.DECIMAL
+        # $C000 BRK
+        mpu.memory[0xC000] = 0x00
+        mpu.pc = 0xC000
+        mpu.step()
+        self.assertEqual(mpu.BREAK, mpu.p & mpu.BREAK)
+        self.assertEqual(mpu.DECIMAL, mpu.p & mpu.DECIMAL)
+
+    def test_brk_preserves_decimal_flag_when_it_is_clear(self):
+        mpu = self._make_mpu()
+        mpu.p = 0
+        # $C000 BRK
+        mpu.memory[0xC000] = 0x00
+        mpu.pc = 0xC000
+        mpu.step()
+        self.assertEqual(mpu.BREAK, mpu.p & mpu.BREAK)
+        self.assertEqual(0, mpu.p & mpu.DECIMAL)
+
     def _get_target_class(self):
         return py65.devices.mpu6502.MPU
 

py65/tests/devices/test_mpu65c02.py

@@ -430,6 +430,18 @@ class MPUTests(unittest.TestCase, Common6502Tests):
         self.assertEqual(0x01, mpu.a)
         self.assertEqual(0x00, mpu.memory[0x0010 + mpu.x])
 
+    # BRK
+
+    def test_brk_clears_decimal_flag(self):
+        mpu = self._make_mpu()
+        mpu.p = mpu.DECIMAL
+        # $C000 BRK
+        mpu.memory[0xC000] = 0x00
+        mpu.pc = 0xC000
+        mpu.step()
+        self.assertEqual(mpu.BREAK, mpu.p & mpu.BREAK)
+        self.assertEqual(0, mpu.p & mpu.DECIMAL)
+
     # EOR Zero Page, Indirect
 
     def test_eor_zp_ind_flips_bits_over_setting_z_flag(self):