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

For a pipelined CPU with a single ALU, consider the following situations I. The j + 1-st instruction uses the result of the j-th instruction as an operand II. The execution of a conditional jump instruction III. The j-th and j + 1-st instructions require the ALU at the same time

Which of the above can cause a hazard?

  1. I and II only

  2. II and III only

  3. III only

  4. All the three
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Case 1 is here of data dependency, this can't be safe with single ALU so read after write. Case 2 Conditional jumps are always hazardous they create conditional dependency in pipeline Case 3 this is write after read problem or concurrency dependency so hazardous All the three are hazardous.

Multiple choice

Consider the following assembly language program for a hypothetical processor A, B, and C are 8 bit registers. The meanings of various instructions are shown as comments. MOV B, #0 ; B $\leftarrow$ 0 MOV C, #8 ; C $\leftarrow$ 8 Z: CMP C, #0 ; Compare C with 0 JZX ; Jump to X if zero flag is set SUB C, #1 ; C $\leftarrow$ C - 1 RRC A, #1 ; right rotate A through carry by one bit. Thus: ; if the initial values of A and the carry flag are a7… a0 and ; instruction will be c0a7…a1 and a0 respectively JCY ; jump to Y if carry flag is set JMP Z ; jump to Z Y: ADD B, #1 ; B $\leftarrow$ B + 1 X: If the initial value of register A is A0, the value of register B after the program execution will be

  1. the number of 0 bits in A0

  2. the number of 1 bits in A0

  3. A0

  4. 8
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Here value of B is incremented by 1 only if carry flag is 1, carry is filled using right rotation, so B will store the no. of 1s in A0.

Multiple choice

Consider the following assembly language program for a hypothetical processor A, B, and C are 8 bit registers. The meanings of various instructions are shown as comments. MOV B, #0 ; B $\leftarrow$0 MOV C, #8 ; C$\leftarrow$8 Z: CMP C, #0 ; Compare C with 0 JZX ; Jump to X if zero flag is set SUB C, #1 ; C $\leftarrow$ C -1 RRC A, #1 ; right rotate A through carry by one bit. Thus: ; if the initial values of A and the carry flag are a7… a0 and ; instruction will be c0a7…a1 and a0 respectively JCY ; jump to Y if carry flag is set JMP Z ; jump to Z Y: ADD B, #1 ; B $\leftarrow$ B + 1 X:

Which of the following instructions when inserted at location X will ensure that the value of register A after program execution is the same as its initial value?

  1. RRC A, #1

  2. NOP ; no operation

  3. LRC A, #1 ; left rotate A through carry flag by one bit

  4. ADD A, #1
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In the end of program execution to check whether both initial and final value of register A is A0, we need to right rotate register A through carry by one bit.

Multiple choice

A 1-input, 2-output synchronous sequential circuit behaves as follows: Let zk, nk denote the number of 0's and 1's respectively in initial k bits of the input (zk+nk=k). The circuit outputs 00 until one of the following conditions holds. zk - nk=2. In this case, the output at the k-th and all subsequent clock ticks is 10. nk - zk = 2. In this case, the output at the k-th and all subsequent clock ticks is 01.

What is the minimum number of states required in the state transition graph of the above circuit?

  1. 5

  2. 6

  3. 7

  4. 8
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The sequential circuit has 3 variables to decide the state in which input & 2 inputs are present. Output for particular inputs decide states.

i/p op 1 op 2 State
0 0 0 Initial
0 0 1 $n_k - z_k = 2$
0 1 0 $z_k - n_k = 2$
1 0 0 Not applicable
1 0 0 Initial
1 0 1 $n_k - z_k = 2$
1 1 0 $z_k - n_k = 2$
0 0 1 $n_k - z_k = 2$
1 1 1 is correct

using 3 bits we require $ 2^3 - 1 = 7 $ states here

Multiple choice

We consider the addition of two $2's$ complement numbers $ b_{n-1}b_{n-2}\dots b_{0}$ and $a_{n-1}a_{n-2}\dots a_{0}$. A binary adder for adding unsigned binary numbers is used to add the two numbers. The sum is denoted by $ c_{n-1}c_{n-2}\dots c_{0}$ and the carry-out by $ c_{out}$. Which one of the following options correctly identifies the overflow condition?
  1. $ c_{out}\left ( \overline{a_{n-1}\oplus b_{n-1}} \right )$
  2. $ a_{n-1}b_{n-1}\overline{c_{n-1}}+\overline{a_{n-1}b_{n-1}}c_{n-1}$
  3. $ c_{out}\oplus c_{n-1}$
  4. $ a_{n-1}\oplus b_{n-1}\oplus c_{n-1}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Binary adder generates C out only if

Multiple choice

Consider a Boolean function $ f(w,x,y,z)$. Suppose that exactly one of its inputs is allowed to change at a time. If the function happens to be true for two input vectors $ i_{1}=\left \langle w_{1}, x_{1}, y_{1},z_{1}\right \rangle $ and $ i_{2}=\left \langle w_{2}, x_{2}, y_{2},z_{2}\right \rangle $ , we would like the function to remain true as the input changes from $ i_{1}$ to $ i_{2}$ ($ i_{1}$ and $ i_{2}$ differ in exactly one bit position) without becoming false momentarily. Let $ f(w,x,y,z)=\sum (5,7,11,12,13,15)$ . Which of the following cube covers of $f$ will ensure that the required property is satisfied?

  1. $ \overline{w}xz,wx\overline{y},x\overline{y}z,xyz,wyz$
  2. $ wxy, \overline{w}xz,wyz$
  3. $ wx\overline{y} \overline{z}, xz, w\overline{x}yz$
  4. $ wx\overline{y}, wyz, wxz, \overline{w}xz, x\overline{y}z, xyz$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Multiple choice

Consider the circuit in the diagram. The $\oplus$ operator represents Ex - OR. The D flip - flops are initialized to zeroes (cleared).

The following data: 100110000 is supplied to the “data” terminal in nine clock cycles. After that the values of q2 q1 q0 are:

  1. 000

  2. 001

  3. 010

  4. 101
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

|||||| |---|---|---|---|---| | | q2| q1| q0| I/P| | initially | 0| 0| 0| -| || 0| 0| 1| 1| || 0| 1| 0| 0| || 1| 0| 0| 0| || 0| 0| 0| 1| || 0| 0| 1| 1| || 0| 1| 0| 0| || 1| 0| 0| 0| || 0| 0| 1| 0| || 0| 1| 0| 0|

Multiple choice

Consider a new instruction named branch-on-bit-set (mnemonic bbs). The instruction “bbs reg, pos, label” jumps to label if bit in position pos of register operand reg is one. A register is 32 bits wide and the bits are numbered 0 to 31, bit in position 0 being the least significant. Consider the following emulation of this instruction on a processor that does not have bbs implemented. temp$\leftarrow$reg & mask Branch to label if temp is non-zero. The variable temp is a temporary register. For correct emulation, the variable mask must be generated by
  1. mask $\leftarrow$ 0 ×l << pos
  2. musk $\leftarrow$ 0 × ffffffff >> pos
  3. mask $\leftarrow$pos
  4. masdk $\leftarrow$0×f
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation