ISRO CS 2023 Q.37: solution and answer

ISRO 2023 · CSQ.37 · 1 mark · MCQ

37. A full adder circuit requires

(A)
two inputs and two outputs
(B)
two inputs and three outputs
(C)
three inputs and two outputs
(D)
three inputs and one output

See the question in the paper

Answer and solution

ANSWER

A full adder has three inputs ( $A$ , $B$ , $C_{in}$ ) and two outputs ( $S$ , $C_{o u t}$ ), so the answer is option (c).

Official answer: CSource: ISRO's revised and final answer key of 16 February 2024

A full adder adds three one-bit quantities: the two operand bits $A$ and $B$ , and the carry-in $C_{in}$ from the previous stage. So it has three inputs.
It produces two results: the sum bit $S = A \oplus B \oplus C_{in}$ and the carry-out $C_{o u t} = A B + C_{in} (A \oplus B)$ . So it has two outputs.
Option (a) describes a half adder, which has no carry-in: inputs $A, B$ and outputs $S = A \oplus B$ and $C = A B$ .
Option (b) is wrong because the adder has three inputs, not two, and only two outputs, not three. Option (d) is wrong because a single output cannot hold both the sum and the carry, since the sum of three bits can be as large as $3$ , which is $11$ in binary and needs two bits.
Hence a full adder needs three inputs and two outputs.

KEY IDEA

A full adder adds three bits (two operands plus carry-in) and gives a two-bit result: sum and carry-out. A half adder has only two inputs because it has no carry-in.

WATCH OUT FOR

Choosing two inputs and two outputs, which is the half adder, by forgetting the carry-in bit from the previous stage.

From the ISRO CS 2023 paper. Every question of this paper · The full paper (PDF, ISRO) · All 6 ISRO Combinational questions, with notes

ISRO CS 2023 Q.37: solution and answer

Answer and solution

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