Adder architectures — carry performance: In which adder design is the conventional ripple carry delay effectively eliminated by computing carries in parallel?

Introduction / Context:
Adder speed is often limited by how quickly carry signals propagate from the least significant bit to the most significant bit. Ripple-carry adders pass each carry sequentially, leading to linear delay with bit-width. Faster architectures precompute carry signals using generate/propagate logic to reduce or eliminate ripple delay.

Given Data / Assumptions:

• We compare common adder types by carry behavior.
• Goal: identify the design that removes linear ripple bottlenecks.
• Basic knowledge of generate (G) and propagate (P) concepts is assumed.

Concept / Approach:
A carry-look-ahead (CLA) adder forms carry outputs as Boolean functions of operand bits and the initial carry, using propagate (P = A ⊕ B) and generate (G = A * B) signals. Closed-form expressions compute c1, c2, …, cN in parallel or in grouped blocks, dramatically reducing delay compared to ripple chains.

Step-by-Step Solution:
Recognize ripple adders have O(n) carry delay.Identify CLA as using parallel carry equations (e.g., c2 = G1 + P1*c1, etc.).Select “Carry-look-ahead adder” as the architecture eliminating ripple delay.

Verification / Alternative check:
Timing comparisons in digital design texts show CLA and its variants (carry-skip, carry-select, parallel prefix) outperform ripple adders by shortening the critical path.

Why Other Options Are Wrong:
Half and full adders are single-bit cells, not multi-bit architectures. “Parallel adder” generally means a multi-bit ripple structure unless carry acceleration is specified.

Common Pitfalls:
Equating any “parallel adder” with fast carry; without look-ahead/skip/select, it still ripples.

Final Answer:
Carry-look-ahead adder.