Digital Arithmetic

Carry-Lookahead Adder (CLA)

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2,6

2,6=1

F ! G U R S 2.11 Optimal distribution of group sizes in carry-skip adder.

Further reduction of the worst-case delay can be obtained by putting several

groups into blocks and providing carry-skip around blocks. This process is gen-

eralized to multilevel carry-skip adders (see Exercise 2.9).

Carry-Lookahead Adder (CLA)

The basic idea of this adder is to compute several carries simultaneously. In the

extreme, all carries could be computed at the same time. As stated in expression

(2.6), if we call x (i) and y

(i)

the integers represented by the bit-vector from bit 0

to bit i; that is,

i

X