Carry Lookahead Adder (CLA)
Computes all carries in parallel using generate/propagate logic.
Problem
How do you eliminate the sequential carry chain in addition?
How it works
Each bit precomputes generate G = A·B and propagate P = A⊕B. A carry lookahead unit (CLU) uses G and P to compute all carries simultaneously as Boolean functions of inputs and C₀. Groups of 4 bits share one CLU; larger adders stack CLU levels hierarchically.
Key insightEvery carry is just a Boolean function of the inputs: C₁ = G₀ + P₀·C₀ can be computed in constant time, no ripple needed.
Tradeoffs
- Area
- ~2.25n FA-eq
- Delay
- 4 + 2(⌈log₄n⌉−1) gate delays
- Best when: Speed is critical and ~2× area overhead is acceptable.