Overflow detection

[PNWMan]

Basically:

• 2's complement overflow occurs if adding 2 similarly signed numbers results in a different sign.
  
•  To detect overflow, see if the XOR of the Couts of the 2 MSBs is 1.
  
• Overflow will never occur when adding 2 differently signed numbers.
  
• This type of overflow is different from just the Cout.