GATE 2024 CS (CS1) – Question 64

Digital Logic · Combinational Circuits · 2 marks · Numerical answer

Consider three 2-to-1 multiplexers M1, M2, M3. X1 and X2 are inputs of M1, X3 and X4 are inputs of M2, and A, B, C are the select lines of M1, M2, M3 respectively. M3 selects Q1 (C=0) or Q2 (C=1). For inputs X1=1, X2=1, X3=0, X4=0, the number of combinations of A, B, C that give the output Y=1 is _________

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Correct answer: 4

Explanation

Q1 = 1 always (X1=X2=1) and Q2 = 0 always (X3=X4=0). Y = Q1 when C=0, so Y=1 iff C=0, regardless of A and B. That gives 2×2×1 = 4 combinations.