GATE 2016 EC (EC2) – Question 28
A 4:1 multiplexer must generate the output carry of a full adder. A and B are the added bits and select inputs, with A most significant; $C_{in}$ is input carry. Which signals should connect to $I_0,I_1,I_2,I_3$ to produce $C_{out}$?
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Correct answer: (A) $0,C_{in},C_{in},1$
Explanation
Evaluate the carry for the four select combinations. If AB=00, even input carry 1 cannot produce two ones, so output carry is 0. If AB=01 or 10, the output carry equals $C_{in}$. If AB=11, the output carry is always 1.
Thus the ordered multiplexer inputs are $0,C_{in},C_{in},1$, giving **A**.