In the case of subtraction, the procedure is more difficult to describe. It involves 1's complement and 2's complement representation. But I shall tell you this, you may subtract two numbers and if there is an under flow, the C flag shall be a 1, otherwise it shall be a 0. An under flow is where you subtract a larger number from a smaller number. Mostly there are four steps for subtraction: (1) converts subtrahend (the number to be subtracted) into its 1's complement (2) add 1 to 1's complement to achieved 2's complement of the subtrahend (3) add 2's complement to the minuend (4) complements the carry flag.