Carry Lookahead Adder
Question 1 
6, 3  
10, 4
 
6, 4  
10, 5

Question 1 Explanation:
Formula: n(n+1)/2 AND gates and n OR gates are needed for an nbit carry look ahead circuit for addition of two binary numbers.
Question 2 
A 4bit carry lookahead adder, which adds two 4bit numbers, is designed using AND, OR, NOT, NAND, NOR gates only. Assuming that all the inputs are available in both complemented and uncomplemented forms and the delay of each gate is one time unit, what is the overall propagation delay of the adder? Assume that the carry network has been implemented using twolevel ANDOR logic.
4 time units
 
6 time units
 
10 time units
 
12 time units

Question 2 Explanation:
The 4bit addition will be calculated in 3 stages:
1) (2 time units) In 2 time units we can compute G_{i} and P_{i} in parallel, 2 time units for P_{i} since its an XOR operation and 1 time unit for G_{i} sinceits an AND operation.
2) (2 time units) Once G_{i} and P_{i} are available, we can calculate the carries, C_{i}, in 2 time units.
Level1 we can compute all the conjunctions (AND). Example P_{3}G_{2}, P_{3}P_{2}G_{1}, P_{3}P_{2}P_{1}G_{0} and P_{3}P_{2}P_{1}P_{0}C_{0} which are required for C_{4}.
Level2 we get the carries by computing the disjunction (OR).
3) (2 time units) Finally, we compute the sum in 2 time units, as its a XOR operation.
Hence, the total is 2+2+2=6 time units.
1) (2 time units) In 2 time units we can compute G_{i} and P_{i} in parallel, 2 time units for P_{i} since its an XOR operation and 1 time unit for G_{i} sinceits an AND operation.
2) (2 time units) Once G_{i} and P_{i} are available, we can calculate the carries, C_{i}, in 2 time units.
Level1 we can compute all the conjunctions (AND). Example P_{3}G_{2}, P_{3}P_{2}G_{1}, P_{3}P_{2}P_{1}G_{0} and P_{3}P_{2}P_{1}P_{0}C_{0} which are required for C_{4}.
Level2 we get the carries by computing the disjunction (OR).
3) (2 time units) Finally, we compute the sum in 2 time units, as its a XOR operation.
Hence, the total is 2+2+2=6 time units.
There are 2 questions to complete.