What is the key advantage of using a carry-lookahead adder instead of a…

Option B:

Why Carry Look Ahead Adder (CLA) is Faster than Ripple Carry Adder (RCA)

1. Ripple Carry Adder (RCA)

In Ripple Carry Adder, each full adder must wait for the carry from the previous stage.

Example:

A = 1011
B = 0110

Addition starts from the LSB (right side).

Step-by-step carry flow:

Bit 0 → generates Carry C1
Bit 1 waits for C1
Bit 2 waits for C2
Bit 3 waits for C3

Thus, carry “ripples” from one stage to another.

Problem:
Each stage must wait for the previous carry, causing large propagation delay.

Therefore:
More bits → More delay

1. Carry Look Ahead Adder (CLA)

CLA removes this waiting problem.

Instead of waiting, CLA calculates carries in advance using:

• Generate (G)

• Propagate (P)

Concepts:

Generate:
A carry is generated when both inputs are 1.

G = A · B

Example:
A = 1, B = 1
Then carry will definitely be generated.

Propagate:
A carry is propagated when exactly one input is 1.

P = A ⊕ B

Example:
A = 1, B = 0
If input carry comes, it will pass to next stage.

Carry Equations in CLA

C1 = G0 + P0C0

C2 = G1 + P1G0 + P1P0C0

C3 = G2 + P2G1 + P2P1G0 + P2P1P0C0

Thus, all carries are calculated simultaneously.

Why CLA is Faster

Ripple Carry Adder:
Carry moves stage-by-stage sequentially.

Carry Look Ahead Adder:
All carries are generated in parallel.

Hence:
CLA greatly reduces propagation delay and increases speed.

Comparison

Ripple Carry Adder:

• Simple design

• Less hardware

• Slow speed

Carry Look Ahead Adder:

• Complex design

• More hardware

• Very fast operation

Final Conclusion

The key advantage of Carry Look Ahead Adder is faster propagation delay because it calculates carry bits in advance using Generate and Propagate logic.