Add option to read initial PC address from the reset vector

Closes #67

CHANGES.rst

@@ -15,6 +15,9 @@
 - Added ``irq()`` and ``nmi()`` methods to the ``MPU`` class, so that
   interrupts can be simulated. Patch by Irmen de Jong.
 
+- The ``MPU`` class constructor now accepts ``None`` for the initial PC, which
+  will cause it to read the address from the reset vector on ``reset()``.
+
 1.1.0 (2018-07-01)
 ------------------
 

py65/devices/mpu6502.py

@@ -39,7 +39,7 @@ class MPU:
         if memory is None:
             memory = 0x10000 * [0x00]
         self.memory = memory
-        self.start_pc = pc
+        self.start_pc = pc # if None, reset vector is used
 
         # init
         self.reset()
@@ -67,6 +67,8 @@ class MPU:
 
     def reset(self):
         self.pc = self.start_pc
+        if self.pc is None:
+            self.pc = self.WordAt(self.RESET)
         self.sp = self.byteMask
         self.a = 0
         self.x = 0

py65/tests/devices/test_mpu6502.py

@@ -18,6 +18,24 @@ class Common6502Tests:
         self.assertEqual(0, mpu.y)
         self.assertEqual(mpu.BREAK | mpu.UNUSED, mpu.p)
 
+    def test_reset_sets_pc_to_0_by_default(self):
+        mpu = self._make_mpu()
+        mpu.reset()
+        self.assertEqual(mpu.pc, 0)
+
+    def test_reset_sets_pc_to_start_pc_if_not_None(self):
+        mpu = self._make_mpu(pc=0x1234)
+        mpu.reset()
+        self.assertEqual(mpu.pc, 0x1234)
+
+    def test_reset_reads_reset_vector_if_start_pc_is_None(self):
+        mpu = self._make_mpu(pc=None)
+        target = 0xABCD
+        mpu.memory[mpu.RESET+0] = target & 0xff
+        mpu.memory[mpu.RESET+1] = target >> 8
+        mpu.reset()
+        self.assertEqual(mpu.pc, 0xABCD)
+
     # ADC Absolute
 
     def test_adc_bcd_off_absolute_carry_clear_in_accumulator_zeroes(self):