Question

When performing subtraction by addition in the 2’s-complement system is?

(a) The minuend and the subtrahend are both changed to the 2’s-complement

(b) The minuend is changed to 2’s-complement and the subtrahend is left in its original form

(c) The minuend is left in its original form and the subtrahend is changed to its 2’s-complement

(d) The minuend and subtrahend are both left in their original form

This question was addressed to me during an interview for a job.

My enquiry is from Controlled Inverter topic in portion Arithmetic Circuits of Digital Circuits

Answer 1

The correct answer is (c) The minuend is left in its original form and the subtrahend is changed to its 2’s-complement

To explain: When performing subtraction by addition in the 2’s-complement system, the minuend is left in its original form and the subtrahend is changed to its 2’s-complement. It is then added to the minuend. If the result has carry, then it’s dropped and that’s the final answer. Else, if the result has no carry, then the result is again converted to it’s 2’s complement form and that’s the final answer with a ‘negative’ sign.