A parallel (ripple-carry) adder consist of N full adders chained together. The c

Question

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.

Explanation / Answer

Answer:

(a)

When a CLA cost much more than a parallel adder:

Why because the CLA cost and process delays be high, owing to, CLA difficulty - carry logic-

Block is gets extremely much complex for more-than 4-bits. So as to cause, the CLAs are more often than not

Need to implement as for 4-bit module, are old for, in hierarchical arrangement of its, and to

Realize adders, they contain that multiples of the 4-bits.

Ripple-carry adder is incredibly cheaper in cost incredibly fast in haste. Accurate marks, simply intend by

Compare with CLAs.

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