Subtraction (indirect method)
If you write subtraction as addition with a negative number, then the previous method can be used.
For example:
{2 – 6} can be done by performing {2 + (-6)}
-5 1 0 1 1
+(-6) 1 0 1 0
----------
0 1 0 1 Result is +5 which is wrong
since -5 +(-6) = -11
1 0 1 0 Carry bits
The sign is 0 (+) where
the two operands had sign
1 (-), indicating an
overflow.
The carries in and out of sign-
bit are different, indicating an
overflow.