Then all that is needed to convert a half adder to a half subtractor is the inversion of the minuend input X. The two Boolean expressions for the binary subtractor BORROW is also very similar to that for the adders CARRY. If we compare the Boolean expressions of the half subtractor with a half adder, we can see that the two expressions for the SUM (adder) and DIFFERENCE (subtractor) are exactly the same and so they should be because of the Exclusive-OR gate function.
