Adder performance: A carry look-ahead (CLA) adder uses extra logic to compute carries in parallel and is generally faster than a ripple-carry adder. Evaluate the claim that a look-ahead-carry adder is slower because it requires additional logic.
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ACorrect
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BIncorrect
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CAmbiguous as stated
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DCannot be determined without additional data
Answer
Correct Answer: Incorrect
Explanation
Introduction / Context:Adder architectures trade off complexity and speed. Ripple-carry adders propagate carry serially through each bit, while carry look-ahead (CLA) adders compute carry signals in parallel using generate/propagate logic to reduce delay.
Given Data / Assumptions:
- Ripple adders have O(n) carry-chain delay for n bits.
- CLA adds extra logic (generate/propagate) to shorten the critical path.
- We compare speed, not just gate count.
Concept / Approach:CLA adds logic to compute carries simultaneously across groups, transforming linear carry delay into a shorter, usually logarithmic or block-based delay. Although hardware cost rises, the net effect is faster addition for moderate and wide bit-widths compared to ripple schemes.
Step-by-Step Solution:
Define G = A * B (generate), P = A ⊕ B (propagate).Use CLA equations to compute C1, C2, ... in parallel from Cin and the G/P terms.Observe reduced dependency chain vs. bit-by-bit ripple.Conclude CLA is faster despite additional logic.Verification / Alternative check:ASIC/FPGA timing models consistently show CLA outperforming ripple for nontrivial widths; further enhancements (carry-select, carry-skip, parallel-prefix) push speed even higher.
Why Other Options Are Wrong:
- Correct: Misinterprets added logic as a speed penalty; it is a speed optimization.
- Ambiguous / Cannot be determined: The architectural comparison is well-established.
Common Pitfalls:Equating gate count with speed; in arithmetic circuits, structured parallelism often improves timing at the cost of area.
Final Answer:Incorrect