A ripple-carry adder passes each stage's carry to the next stage. Its simplicity comes with a delay that can grow across the word. Carry lookahead derives carry signals from generate and propagate terms so groups can determine carries with fewer serial dependencies.
Worked example
Using G = AB and P = A XOR B for each bit, Cout = G OR (P AND Cin). If A = B = 1, G = 1 and a carry is generated regardless of Cin. If A = 1 and B = 0, P = 1 and Cout follows Cin.
What to watch for
Carry lookahead still has physical gate and wiring delays. It trades additional logic and interconnect for faster carry evaluation rather than making addition instantaneous.
Try it yourself
With A = B = 0, what are G, P and Cout?
Answer: G = 0 and P = 0, so Cout = 0 for either carry-in. This bit position blocks an incoming carry.
How to study this topic
State the operand width and signedness first. Work out a decimal answer, then compare it with the binary result. Keep result bits, carry, borrow and signed overflow distinct when interpreting the circuit's output.
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Digital Electronics