Microsoft Word - Digital Logic Design v_4_6a

 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.