Fix program counter overflow bug in RTS

CHANGES.txt

@@ -1,5 +1,9 @@
 0.18-dev (Next Release)
 
+  - Fixed a bug in RTS where popping $FFFF off the stack would cause
+    program counter to overflow to $10000.  It now wraps around to
+    $0000 as it should.  Thanks to Gábor Lénárt for reporting it.
+
 0.17 (2013-10-26)
 
   - Added support for Python 3.2 and 3.3 based on work by David Beazley.

py65/devices/mpu6502.py

@@ -61,6 +61,7 @@ class MPU:
         self.excycles = 0
         self.addcycles = self.extracycles[instructCode]
         self.instruct[instructCode](self)
+        self.pc &= self.addrMask
         self.processorCycles += self.cycletime[instructCode] + self.excycles
         return self
 

py65/tests/devices/test_mpu6502.py

@@ -4329,12 +4329,25 @@ class Common6502Tests:
         # $0000 RTS
         mpu.memory[0x0000] = 0x60
         self._write(mpu.memory, 0x01FE, (0x03, 0xC0))  # PCL, PCH
+        mpu.pc = 0x0000
         mpu.sp = 0xFD
 
         mpu.step()
         self.assertEqual(0xC004, mpu.pc)
         self.assertEqual(0xFF,   mpu.sp)
 
+    def test_rts_wraps_around_top_of_memory(self):
+        mpu = self._make_mpu()
+        # $0000 RTS
+        mpu.memory[0x1000] = 0x60
+        self._write(mpu.memory, 0x01FE, (0xFF, 0xFF))  # PCL, PCH
+        mpu.pc = 0x1000
+        mpu.sp = 0xFD
+
+        mpu.step()
+        self.assertEqual(0x0000, mpu.pc)
+        self.assertEqual(0xFF,   mpu.sp)
+
     # SBC Absolute
 
     def test_sbc_abs_all_zeros_and_no_borrow_is_zero(self):