2's complement subtraction

Subtraction of two signed binary numbers when negative numbers are in 2's complement form is very simple and can be stated as follows: Take the 2's complement of the subtrahend (including the sign bit) and add it to the minuend (including the sign bit). A carry out of the sign bit position is discarded. This procedure stems from the fact that a subtraction operation can be changed to an addition operation if the sign of the subtrahend is changed.