SBC now properly sets the V flag. Closes #6.

CHANGES.txt

@@ -25,6 +25,10 @@ Next Release
     at that address.  Entering "a" alone will start it at the current
     program counter.
 
+  - Applied patch from Ed Spittles so that SBC now properly sets the
+    Overflow (V) flag.  This fixes a failure in Rob Finch's test suite.
+    Closes #6.
+
 0.6 (2009-08-11)
 
   - Added monitor shortcut "a" for "assemble".

src/py65/devices/mpu6502.py

@@ -382,14 +382,15 @@ class MPU:
       else:
         borrow = 1
 
-      data = self.a - data - borrow
+      result = self.a - data - borrow
       self.flags &= ~(self.CARRY + self.ZERO + self.OVERFLOW + self.NEGATIVE)
+      if ( (self.a ^ data) & (self.a ^ result) ) & 0x80:
+        self.flags |= self.OVERFLOW
+      data = result
       if data == 0:
         self.flags |= self.ZERO + self.CARRY
       elif data > 0:
         self.flags |= self.CARRY
-      else:
-        self.flags |= self.OVERFLOW
       self.flags |= data & self.NEGATIVE
       self.a = data & 0xFF