# GATE Papers >> CSE >> 2007 >> Question No 35

Question No. 35

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:

${P}_{i}={A}_{i}\oplus {B}_{i}$ and ${G}_{i}={A}_{i}{B}_{i}$

The expressions for the sum bit Si and the carry bit Ci+1 of the look-ahead carry adder are given by:

${S}_{i}={P}_{i}\oplus {C}_{i}$ and ${C}_{i+1}={G}_{i}+{P}_{i}{C}_{i}$ ,where C0 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: