Carry propagation delay in ripple adders: In a 4-bit ripple-carry adder built from full-adder stages, how does carry propagation delay affect overall speed?

Introduction / Context:
Ripple-carry adders (RCAs) are simple but slow because each full-adder stage must wait for the carry from the previous stage. Understanding carry propagation is key to appreciating faster adder architectures like carry look-ahead.

Given Data / Assumptions:

• A 4-bit adder made of four cascaded full adders.
• Each full adder has a finite propagation delay from inputs to Sum/Cout.
• Worst-case input change requires the carry to ripple through all stages.

Concept / Approach:
The total worst-case delay is approximately the sum of the per-stage carry propagation delays plus the final Sum delay at the most significant stage. Therefore, as you add more bits, the delay grows linearly, directly limiting maximum clock frequency or throughput for arithmetic operations.

Step-by-Step Solution:

Verification / Alternative check:
Compare to carry look-ahead adders where generate/propagate logic reduces the dependency chain, dramatically cutting the worst-case delay.

Why Other Options Are Wrong:
“Not a consideration” is incorrect; even nanosecond delays matter at high frequencies. Claims that delay decreases with more stages are physically wrong. Saying it “never limits speed” ignores real timing constraints.

Common Pitfalls:
Neglecting the Sum output delay at the MSB and ignoring wiring/capacitive loading, which further increases the effective delay.

Final Answer:
is cumulative for each stage and limits the speed at which arithmetic operations are performed