Raise OverflowError if assembling exceeds the top of memory

CHANGES.txt

@@ -15,6 +15,9 @@
 
   - Added support for 65C02 opcode 0x7C: JMP (abs,x).
 
+  - Assembling now shows an Overflow Error if the statement won't
+    fit at the top of memory.
+
 0.13 (2012-11-15)
 
   - Fixed a bug where negative numbers could be entered

py65/assembler.py

@@ -130,6 +130,11 @@ class Assembler:
 
                 operands = [int(hex, 16) for hex in operands]
                 bytes.extend(operands)
+
+                # raise if the assembled bytes would exceed top of memory
+                if (pc + len(bytes)) > (2 ** self._mpu.ADDR_WIDTH):
+                    raise OverflowError
+
                 return bytes
 
         # assembly failed

py65/tests/test_assembler.py

@@ -19,6 +19,17 @@ class AssemblerTests(unittest.TestCase):
         self.assertRaises(OverflowError,
                           self.assemble, 'lda #$fff')
 
+    def test_assemble_1_byte_at_top_of_mem_should_not_overflow(self):
+        self.assemble('nop', pc=0xFFFF)  # should not raise
+
+    def test_assemble_3_bytes_at_top_of_mem_should_not_overflow(self):
+        self.assemble('jmp $1234', pc=0xFFFD)  # should not raise
+
+    def test_assemble_should_overflow_if_over_top_of_mem(self):
+        # jmp $1234 requires 3 bytes but there's only 2 at $FFFE-FFFF
+        self.assertRaises(OverflowError,
+                          self.assemble, "jmp $1234", pc=0xFFFE)
+
     def test_assembles_00(self):
         self.assertEqual([0x00],
                          self.assemble('BRK'))