A parallel (ripple-carry) adder consist of N full adders chained together. The carry out of one stage acts as the carry in of the next stage as shown in Figure 3 below. A carry-lookahead adder (CLA) divides the adder into blocks and provides circuitry to quickly determine the carry out of a block as soon as the carry in is known. Thus it is said to look ahead across blocks rather than waiting to ripple through all the full adders inside a block. An example is shown in Figure 4 below. When would a digital circuit designer choose a parallel (ripple-carry adder) over a carry-lookahead adder?

Figure 3

Figure 4

(a) When arithmetic operations must be more accurate.

(b) When chip area is not critical and propagation and gate delays are critical.

(c) When chip area is critical and propagation and gate delays are not critical.

(d) When a CLA cost much more than a parallel adder.

0 Full Adder 3 Full Full Adder C1 Full Adder Adder 0 B3 end-around carry for I's complement