ALU 101 Carry and Overflow 1 — Questions and Answers
Question 1: What is a carry bit in binary addition?
- A bit that stores the result
- A bit propagated to the next higher position when the sum exceeds 1 (Correct answer)
- A bit used for error correction
- A bit that resets the accumulator
Correct answer: A bit propagated to the next higher position when the sum exceeds 1
A carry bit is generated when the sum of two bits (plus any incoming carry) is 2 or 3, and it propagates to the next higher bit position.
Question 2: A half adder computes which two outputs?
- Sum and Overflow
- Sum and Carry (Correct answer)
- Carry and Borrow
- Sum and Borrow
Correct answer: Sum and Carry
A half adder computes the Sum (using XOR) and the Carry-out (using AND) for two single-bit inputs.
Question 3: What additional input does a full adder have compared to a half adder?
- Overflow bit
- Carry-in (Correct answer)
- Borrow bit
- Enable signal
Correct answer: Carry-in
A full adder accepts a Carry-in input in addition to the two data bits, allowing chaining of multiple adders.
Question 4: Adding 0111 + 0001 in 4-bit unsigned binary produces what result and carry?
- Result=1000, Carry=0 (Correct answer)
- Result=1000, Carry=1
- Result=0111, Carry=1
- Result=1001, Carry=0
Correct answer: Result=1000, Carry=0
0111 + 0001 = 1000 in 4-bit binary; no carry is generated out of the 4-bit boundary.
Question 5: In a ripple carry adder, why does carry propagation slow down the circuit?
- Each bit's carry must wait for the previous bit's carry to settle (Correct answer)
- Carry signals require extra power
- The carry flip-flop needs clock synchronization
- Carry bits travel over long wire distances
Correct answer: Each bit's carry must wait for the previous bit's carry to settle
In a ripple carry adder, each stage must wait for the carry from the previous stage, so delay accumulates through all stages.
Question 6: Which adder architecture eliminates ripple carry delay by pre-computing carries?
- Carry Save Adder
- Carry Lookahead Adder (Correct answer)
- Wallace Tree Adder
- Booth Multiplier
Correct answer: Carry Lookahead Adder
A Carry Lookahead Adder (CLA) pre-computes carry signals for all bit positions simultaneously, greatly reducing propagation delay.
ALU 101 Carry and Overflow 1
What is a carry bit in binary addition?