Computer Knowledge

Digital Logic and Microprocessors

749 Questions

Digital logic and microprocessors form the core of computer science, covering logic gates, combinational and sequential circuits, and CPU architecture. These topics are crucial for computer knowledge sections in technical exams. Test your digital electronics basics here.

Logic gates and circuitsCombinational vs sequential circuitsMicroprocessor instructionsTruth tables and boolean functions

Digital Logic and Microprocessors Questions

Multiple choice

Consider a 4-bit ripple counter and a 4-bit synchronous counter made using flip-flops. Propagation delay = 10 nano-seconds. R = worst case delay in the ripple counter, S = worst case delay in the synchronous counter. Which of the following options is correct?

  1. R = 10 ns, S = 40 ns

  2. R = 40 ns, S = 10 ns

  3. R = 10 ns, S = 30 ns

  4. R = 30 ns, S = 10 ns
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Propagation delay in 4-bit ripple counter = 4 Td = 40 ns. Propagation delay in synchronous counter = Td = 10 ns.

Multiple choice

A gate having two inputs (A and B) and one output (Y) is implemented using a 4-to-1 multiplexer as shown in the figure below. A1 (MSB) and A0 are the control bits and I0 - I3 are the inputs to the multiplexer. The gate is

  1. NAND

  2. NOR

  3. XOR

  4. OR
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A1 = A, A0 = B, I0 = I2 = A, I1 = I3 = BY = $\bar A_1$$\bar A_0$I0 + $\bar A_1$A0 I1 + A1$\bar A_0$I2 + A1 A0 I3Y = $\bar A$$\bar B$A + $\bar A$BB + A$\bar B$A + ABBY = $\bar A$B + A$\bar B$ + AB = $\bar A$B + A = A + B

Multiple choice

Which of the following statements is correct?

  1. A flip-flop is used to store 1-bit of information.
  2. Race around condition occurs in a J-K flip-flop when both inputs are 1.
  3. Master slave configuration is used in flip-flops to store 2-bits of information.
  4. A transparent latch consists of a D-type flip-flop. Select the correct answer using the codes given above.

  1. 1,2 and 3

  2. 1, 3 and 4

  3. 1, 2 and 4

  4. 2, 3 and 4
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

 According to the definition of flip-flop, it consists of a latch used to store 1-bit of information. Race around condition occurs in J-K flip-flop, if the output continuously toggles. D-type flip-flop's output is equal to input.

Multiple choice

The circuit in the figure below has 4 boxes each described by inputs P, Q, R and output Y, Z with Y = P$\oplus$Q$\oplus$R Z = RQ + $\bar P$R + Q$\bar P$

The circuit acts as a

  1. 4-bit adder giving P + Q

  2. 4-bit subtractor giving P – Q

  3. 4-bit subtractor giving Q – P

  4. 4-bit adder giving P + Q + R
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Explanation Let P = 1011 and Q = 1001. Given Yn = Pn $\oplus$Qn$\oplus$RnZn = Rn Qn $\oplus$ $\bar p_n$ Rn $\oplus$ Qn$\bar p_n$

Pn Qn Rn Zn Yn
1 1 0 0 0
1 0 0 0 1
0 0 0 0 0
1 1 0 0 0
0

                      Where, Y = P + Q + RZ = RQ + $\bar PR$ +  $Q\bar P$   From the circuit given, we have Rn + 1 = Zn, 1$\le$n$\le$3Z4 = Ys (MSB). Hence, the output is 00010, which shows a 4-bit subtractor P – Q. 

Multiple choice

Consider a vending machine which sells product ‘A’ and ‘B’ and has push buttons X1 and X2. On pressing the button of the vending machine, the price of either product is displayed in a seven – segment display. Case: No buttons pressed - ‘0’ is displayed signifying Rs.’0’. Case: If only X1 is pressed - ‘2’ is displayed signifying Rs. 2. Case: If only X2 is pressed - ‘5’ is displayed signifying Rs. 5. If both X1 and X2 are pressed - ‘E’ is displayed signifying error.

The figure given below shows the name of the segment in the seven – segment display and the glow of the display for ‘0’, ‘2’, ‘5’, and ‘E’. If segments ‘a’ to ‘g’ are considered as functions of X1 and X2, then which of the following is true?
  1. g = $\bar P_1$ + P2

  2. g = P1 + P2
  3. g = $\bar P_1$+ P2

  4. g = P1 + P2
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

0 = $\bar X_1\bar X_2$, 2 = X1$\bar X_2$,  5 = X2$\bar X_1$,  E = X1X2 G = 2 + 5 + E = X1 + X2 a = 1b = 0 + 2 = $\bar X_2$c = 0 + 5 = $\bar X_1$d = 1e = 0 + 2 + E = X1 + X2 f = 0 + 5 + E = X1 X2

Multiple choice

Following is the Boolean expression for one truth table of an unknown function.

A B C F
0 0 0 0
0 0 1 0
0 1 0 0
0 1 1 1
1 0 0 0
1 0 1 0
1 1 0 1
1 1 1 0

If the above function is implemented using D flip-flop, then
  1. $ D_A = \bar AB + \bar BC + AC$
  2. $ D_A = A \bar B \bar C + BC$
  3. $ D_A = ABC + AB$
  4. $ D_A = A \bar B + C \bar A$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$J_A = \bar ABC + AB\bar C = B (\bar C + A\bar C) $ $K_A = A + \bar A\bar C$
$D = A\bar B\bar C + BC $

Multiple choice

For the given Boolean expression for one truth table of an unknown function.

A B C F
0 0 0 1
0 0 1 0
0 1 0 0
0 1 1 0
1 0 0 1
1 0 1 1
1 1 0 0
1 1 1 0

The output recycles after the last state. If the above function is implemented using J-K flip flop then,
  1. $ J_A = B(\bar AC + A\bar C), K_A = A + \bar A\bar C$
  2. $ J_A = \bar B(A\bar C +\bar AC), K_A = A + \bar A\bar B$
  3. $ J_A = B(\bar AC + A\bar C), K_A = B + AC$
  4. $ J_A = \bar C(\bar AB + B\bar A), K_A = A + \bar A\bar C$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
A B C F JA KA D
0 0 0 1 X 1 0
0 0 1 0 0 X 0
0 1 0 0 0 X 0
0 1 1 0 1 X 1
1 0 0 1 X 0 1
1 0 1 1 X 1 0
1 1 0 0 0 X 0
1 1 1 0 1 x 1

$J_A = \bar ABC + AB\bar C = B (\bar AC + A\bar C)$

$K_A = A + \bar A\bar C $

$D = A\bar B\bar C + BC$