Q. 50 In a look-ahead carry generator, the carry generate function Gi and the carry
propagate function Pi for inputs Ai and Bi are given by:
Pi = Ai ⨁ Bi and Gi = AiBi
The expressions for the sum bit Si and the carry bit Ci+1 of the look-ahead carry adder are given by:
Si = Pi ⨁ Ci and Ci+1 = Gi + PiCi , where C is the input carry.
Consider a two-level logic implementation of the look-ahead carry generator. Assume that
all Pi and Gi are available for the carry generator circuit and that the AND and OR gates can
have any number of inputs. The number of AND gates and OR gates needed to implement
the look-ahead carry generator for a 4-bit adder with S3, S2, S1, S0 and C4 as its outputs
are respectively:
(A) 6,3
(B) 10,4
(C) 6,4
(D) 10,5
Answer: (B)
Explanation:
