Question 1

Consider an eight-bit ripple-carry 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 _________.

 A -1 B -2 C -3 D -4
Digital-Logic-Design       Adder       GATE 2016 [Set-2]       Video-Explanation
Question 1 Explanation:
In the question, longest LATENCY means longest DELAY for the sum to get settle.
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 4-bit ripple-carry binary adder is implemented by using four full adders. The total propagation time of this 4-bit binary adder in microseconds is ____________.

 A 19.1 B 19.2 C 18.1 D 18.2
Question 2 Explanation: 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 C0 only (+ and - denote addition and subtraction respectively)?

 A A + B, and A – B, but not A + 1 B A + B, and A + 1, but not A – B C A + B, but not A – B or A + 1 D A + B, and A – B, and A + 1
Question 3 Explanation:
The circuits performs
1) A+B when K=0 and C0 = 0. It is binary adder which performs addition of two binary numbers.
2) A - B = A+ B' + 1 when K=1 and C0 = 1 ;
Here XOR gates produce B' if K=1. Since 1⊕b= b'.
"1" in (A+B+1) is coming from C0.
Note: 2's complement of B is (B'+1). 3) A+1 when B=0, K=0, C0= 1.
Increments A.
 Question 4

The number of full and half-adders required to add 16-bit numbers is