# Digital Circuits Questions and Answers – Half & Full Subtractor

This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Half & Full Subtractor”.

1. Half subtractor is used to perform subtraction of ___________
a) 2 bits
b) 3 bits
c) 4 bits
d) 5 bits

Explanation: Half subtractor is a combinational circuit which is used to perform subtraction of two bits, namely minuend and subtrahend and produces two outputs, borrow and difference.

2. For subtracting 1 from 0, we use to take a _______ from neighbouring bits.
a) Carry
b) Borrow
c) Input
d) Output

Explanation: For subtracting 1 from 0, we use to take a borrow from neighbouring bits because carry is taken into consideration during addition process.

3. How many outputs are required for the implementation of a subtractor?
a) 1
b) 2
c) 3
d) 4

Explanation: There are two outputs required for the implementation of a subtractor. One for the difference and another for borrow.

4. Let the input of a subtractor is A and B then what the output will be if A = B?
a) 0
b) 1
c) A
d) B

Explanation: The output for A = B will be 0. If A = B, it means that A = B = 0 or A = B = 1. In both of the situation subtractor gives 0 as the output.

5. Let A and B is the input of a subtractor then the output will be ___________
a) A XOR B
b) A AND B
c) A OR B
d) A EXNOR B

Explanation: The subtractor has two outputs BOROW and DIFFERENCE. Since, the difference output of a subtractor is given by AB’ + BA’ and this is the output of a XOR gate. So, the final difference output is AB’ + BA’.

6. Let A and B is the input of a subtractor then the borrow will be ___________
a) A AND B’
b) A’ AND B
c) A OR B
d) A AND B

Explanation: The borrow of a subtractor is received through AND gate whose one input is inverted. On that basis the borrow will be (A’ AND B).

7. What does minuend and subtrahend denotes in a subtractor?
a) Their corresponding bits of input
b) Its outputs
c) Its inputs
d) Borrow bits

Explanation: Minuend and subtrahend are the two bits of input of a subtractor. If A and B are the two inputs of a subtractor then A is called minuend and B as subtrahend.

8. Full subtractor is used to perform subtraction of ___________
a) 2 bits
b) 3 bits
c) 4 bits
d) 8 bits

Explanation: Full subtractor is used to perform subtraction of 3 bits, namely minuend bit, subtrahend bit and borrow from the previous stage. However, it also produces 2 outputs BORROW and DIFFERENCE.

9. The full subtractor can be implemented using ___________
a) Two XOR and an OR gates
b) Two half subtractors and an OR gate
c) Two multiplexers and an AND gate
d) Two comparators and an AND gate

Explanation: A full subtractor has 3 input bits and two outputs bits BORROW and DIFFERENCE. The full subtractor can be implemented using two half subtractors and an OR gate.

10. The output of a subtractor is given by (if A, B and X are the inputs).
a) A AND B XOR X
b) A XOR B XOR X
c) A OR B NOR X
d) A NOR B XOR X

Explanation: The difference output of a subtractor is given by (if A, B and X are the inputs) A XOR B XOR X.

11. The output of a full subtractor is same as ____________