Question

Q3. (15) Add two numbers A 1110 and B 1011 with input carry CO 0 using Carry Lookahead Adder. Determine value of C4, C3, C2, C1 (use the expression derived in Q2) and S4, S3, S2,S1. Determine the overflow bit which is EXOR of carry going into MSB bit and carry coming from MSB bit ie C4eC3.

Answer 1

Carry look ahead adder (CLA adder) c_i H = G_i + P_i c_i where G_i = A_i B_i = carry generate P_i = A_i circledplus B_i = carry propagate I = 0 rightarrow c_1 = G_0 + P_0 C_0 I = 1 rightarrow C_2 = G_1 + P_1 C_1 I = 2 rightarrow C_3 = G_2 + P_2 C_2 I = 3 implies C_4 = G_3 + P_3 C_3 given A_3 A_2 A_1 = 1110 & B_3 B_2 B_1 B_0 = 1011 C_0 = 0 (i) C_1: -G_0 = A_0 middot B_0 = 0.1 = 0 P_0 = A_0 circledplus B_0 = 0 circledplus 1 = 1 C_1 = G_0 + P_0 C_0 = 0 + 1.0 = 0 implies C_1 = 0 (ii) C_2: - G_1 = A_1 B_1 = 1.1 = 1 P_1 = A_1 circledplus B_1 = 1 circledplus 1 = 0 C_2 = G_1 + P_1 C_1 = 1 + 0.0 = 1 implies C_2