GATE EC 2025 Set 1 — Question 24
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Digital Circuits → Combinational Logic → Adders & Subtractors
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Question
A full adder and an XOR gate are used to design a digital circuit with inputs , , and , and output , as shown below. The input is connected to the carry-in input of the full adder. If the input is set to logic ‘1’, then the circuit functions as ________ with and as inputs.
Correct answer
(B) a subtractor
Solution
The circuit consists of an XOR gate and a full adder. The inputs to the full adder are , , and . The output is a 2-bit vector representing the Sum and Carry-out of the full adder.
When :
In 2's complement arithmetic, the subtraction is performed as .
Since the circuit implements the 2's complement subtraction logic, it functions as a subtractor. Thus, the correct option is (B).
When :
1.The XOR gate output becomes .
2.The carry-in to the full adder is .
The full adder performs the operation .In 2's complement arithmetic, the subtraction is performed as .
Since the circuit implements the 2's complement subtraction logic, it functions as a subtractor. Thus, the correct option is (B).
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