Carry-lookahead adder

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A carry-lookahead adder (CLA) or fast adder is a type of electronics adder used in digital logic. A carry-lookahead adder improves speed by reducing the amount of time required to determine carry bits. It can be contrasted with the simpler, but usually slower, ripple-carry adder (RCA), for which the carry bit is calculated alongside the sum bit, and each stage must wait until the previous carry bit has been calculated to begin calculating its own sum bit and carry bit. The carry-lookahead adder calculates one or more carry bits before the sum, which reduces the wait time to calculate the result of the larger-value bits of the adder.

Already in the mid-1800s, Charles Babbage recognized the performance penalty imposed by the ripple-carry used in his Difference Engine, and subsequently designed mechanisms for anticipating carriage for his never-built Analytical Engine.^[1][2] Konrad Zuse is thought to have implemented the first carry-lookahead adder in his 1930s binary mechanical computer, the Zuse Z1.^[3] Gerald B. Rosenberger of IBM filed for a patent on a modern binary carry-lookahead adder in 1957.^[4]

Two widely used implementations of the concept are the Kogge–Stone adder (KSA) and Brent–Kung adder (BKA).