# Digital Circuits Questions and Answers – 4-Bit Parallel Adder/Subtractor – 1

This set of Digital Electronics/Circuits Multiple Choice Questions & Answers (MCQs) focuses on “4-Bit Parallel Adder/Subtractor – 1”.

1. Controlled inverter is also known as ____________
a) Controlled buffer
b) NOT gate
c) Both controlled buffer and NOT gate
d) Controlled gate

Explanation: Controlled inverter is also known as controlled buffer and NOT gate as well. It is used between output and a bus so that one can control whether the output is fed to the bus or not.

2. Why XOR gate is called an inverter?
a) Because of the same input
b) Because of the same output
c) It behaves like a NOT gate
d) It behaves like a AND gate

Explanation: The XOR (Exclusive Or) gate has a true output when the two inputs are different. When one input is true, the output is the inversion of the other. When one input is false, the output is the non-inversion of the other.

3. Controlled buffers can be useful ____________
a) To control the circuit’s output into the bus
b) In comparison of component’s output with its input
c) In increasing the output from its low input
d) All of the Mentioned

Explanation: Controlled buffers can be useful when you have a wire (often called a bus) whose value should match the output of one of several components. By placing a controlled buffer between each component output and the bus, you can control whether that component’s output is fed onto the bus or not.

4. A logic circuit that provides a HIGH output for both inputs HIGH or both inputs LOW is ____________
a) Ex-NOR gate
b) OR gate
c) Ex-OR gate
d) NAND gate

Explanation: EX-OR gate gives 1 if both inputs are different means 0 or 1 and gives 0 if both are same and EX-NOR is opposite of EX-OR gate, so it provides a HIGH output for both inputs HIGH or both inputs are LOW. Thus, EX-NOR produces output for even number of 1’s or all 0s, while EXOR produces output for odd number of 1’s.

5. What is the first thing you will need if you are going to use a macro-function?
a) A complicated design project
b) An experienced design engineer
c) Good documentation
d) Experience in HDL

Explanation: HDL stands for Hardware Description Language. In order to use a macro function, one needs to have experience in HDL for representing the structure and behaviour of digital circuits.

b) Full adders can handle double-digit numbers
c) Full adders have a carry input capability
d) Half adders can handle only single-digit numbers

Explanation: Half adders have only two inputs A and B. When we add two 4 bit binary number like 0001 and 0011, then half adder can not be used because if the first bit of both the numbers is 1, then the sum would be 0 and carry would be 1. But this carry can not be added with the second bits addition of the number. So, half adders are useless. But in full adders, one more carry input is present, so that, if carry of one stage is present, it can be added with the next stage as it is done in normal addition. So, therefore, full adders have a carry input capability.

7. The binary subtraction of 0 – 0 = ?
a) Difference = 0, borrow = 0
b) Difference = 1, borrow = 0
c) Difference = 1, borrow = 1
d) Difference = 0, borrow = 1

Explanation: The binary subtraction of 0 – 0 = 0. Thus, it’s difference is 0 as well as it’s borrow.

8. How many basic binary subtraction operations are possible?
a) 1
b) 4
c) 3
d) 2

Explanation: 4 basic binary subtraction operations (0-0, 1-0, 0-1, 1-1) are possible.
0 – 0 = 0
0 – 1 = 1 ( Borrow 1)
1 – 0 = 1
1 – 1 = 0

9. When performing subtraction by addition in the 2’s-complement system ____________
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

Explanation: 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.

10. What are the two types of basic adder circuits?
a) Sum and carry
c) Asynchronous and synchronous
d) One and two’s-complement

Explanation: There are two types of adder circuits: half-adder and full-adder. Half-Adder has 2 inputs while Full-Adder has 3 inputs. Whereas, both have two outputs SUM and CARRY.

11. Which of the following is correct for full adders?
c) Full adders are limited to two inputs since there are only two binary digits
d) In a parallel full adder, the first stage may be a half adder

Explanation: By using maximum of two half adders we can make a full adder for the first stage of a Parallel Full adder.

12. The selector inputs to an arithmetic/logic unit (ALU) determine the ____________
a) Selection of the IC
b) Arithmetic or logic function
c) Data word selection
d) Clock frequency to be used