Question

One way to make a four-bit adder to perform subtraction is by ___________

(a) Inverting the output

(b) Inverting the carry-in

(c) Inverting the B inputs

(d) Grounding the B inputs

I got this question during an online interview.

Question is taken from 4-Bit Parallel Adder/Subtractor in division Arithmetic Circuits of Digital Circuits

Answer 1

Right answer is (c) Inverting the B inputs

For explanation: A adder is a digital circuit which adds bits along with a carry bit from a previous stage, thus producing 2 outputs SUM and CARRY. Since, a four bit adder has four A, four B and a carry at the input end. So, for subtraction to be performed, all the Bs terminal should be inverted.