Adder
Question 1 
Consider an eightbit ripplecarry adder for computing the sum of A and B, where A and B are integers represented in 2’s complement form. If the decimal value of A is one, the decimal value of B that leads to the longest latency for the sum to stabilize is _________.
1  
2  
3  
4 
If we do 2's complement of 1 = 0000 0001, we get 1 = "1111 1111"
So, if B = 1, every carry bit is 1.
Question 2 
A half adder is implemented with XOR and AND gates. A full adder is implemented with two half adders and one OR gate. The propagation delay of an XOR gate is twice that of an AND/OR gate. The propagation delay of an AND/OR gate is 1.2 microseconds. A 4bit ripplecarry binary adder is implemented by using four full adders. The total propagation time of this 4bit binary adder in microseconds is ____________.
19.1  
19.2  
18.1  
18.2 
Here, each Full Adder is taking 4.8 microseconds. Given adder is a 4 Bit Ripple Carry Adder. So it takes 4*4.8 = 19.2 microseconds.
Question 3 
Consider the ALU shown below.
If the operands are in 2's complement representation, which of the following operations can be performed by suitably setting the control lines K and C_{0} only (+ and  denote addition and subtraction respectively)?
A + B, and A – B, but not A + 1  
A + B, and A + 1, but not A – B  
A + B, but not A – B or A + 1  
A + B, and A – B, and A + 1

1) A+B when K=0 and C_{0} = 0. It is binary adder which performs addition of two binary numbers.
2) A  B = A+ B' + 1 when K=1 and C_{0} = 1 ;
Here XOR gates produce B' if K=1. Since 1⊕b= b'.
"1" in (A+B+1) is coming from C_{0}.
Note: 2's complement of B is (B'+1). 3) A+1 when B=0, K=0, C_{0}= 1.
Increments A.
Question 4 
The number of full and halfadders required to add 16bit numbers is
8 halfadders, 8 fulladders  
1 halfadder, 15 fulladders  
16 halfadders, 0 fulladders  
4 halfadders, 12 fulladders 
But for rest of bits we need full address since carry from previous addition has to be included into the addition operation.
So, in total 1 half adder and 15 full adders are required.