carry lookahead

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A method that is used in multibit parallel adders whereby the carry into an individual element of the adder can be predicted with a smaller delay than that required for the carry to be produced by rippling through previous adder stages as a result of adding the less significant addend and augend bits. Logic to achieve this examines, in parallel, each pair of addend and augend bits and infers whether carries generated in previous stages will be propagated to the carry input of each adder stage.

Carry lookahead affords a considerable improvement in performance over, say, ripple-carry adders since the carry is generated in parallel at all stages of addition rather than sequentially, as in the ripple-carry adder. Adders using the lookahead technique are thus often described as high-speed adders.

Subjects: Computing.

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