Which unit combines addition and subtraction capabilities in a single arithmetic module?

Which unit combines addition and subtraction capabilities in a single arithmetic module?

• A. Adder
• B. Subtractor
• C. Adder-Subtractors
• D. Counter

Answer: (C)

A unit that can do both addition and subtraction uses an adder-subtractor configuration. It shares the same adder hardware but uses a mode control to decide the operation. For subtraction, the second operand is inverted and the carry-in is set to 1, effectively performing A + (~B) + 1, which equals A − B in two's complement arithmetic. For addition, the operand is passed through as-is and the carry-in remains 0, giving A + B. This reuse is efficient and common in arithmetic units. A pure adder handles only addition, a pure subtractor only subtraction, and a counter is not an arithmetic unit. Hence the combined capability is provided by the adder-subtractor.

A unit that can do both addition and subtraction uses an adder-subtractor configuration. It shares the same adder hardware but uses a mode control to decide the operation. For subtraction, the second operand is inverted and the carry-in is set to 1, effectively performing A + (~B) + 1, which equals A − B in two's complement arithmetic. For addition, the operand is passed through as-is and the carry-in remains 0, giving A + B. This reuse is efficient and common in arithmetic units. A pure adder handles only addition, a pure subtractor only subtraction, and a counter is not an arithmetic unit. Hence the combined capability is provided by the adder-subtractor.