Introduction to Operating System | NPTEL | Week 4 Answers

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Week 3:
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Week 4: Assignment solutions

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Introduction to Operating System NPTEL Week 4 Answers

Q1. Let the number of bits for Interrupt gate descriptor be 48, ie 32:16 (segment selector:offset). Each  entry in the interrupt descriptor table is 5 Bytes. What will the total size of the IDT  in bytes ?

a) 1256
b) 1024
c) 1020
d) 1280

Answer: d) 1280

Introduction to Operating System NPTEL Week 4 Answers

Q2. State True or False
The user to kernel stack switching is done using Task Segment Descriptor, by changing SS to SS1 and ESP to ESP1

a) True
b) False

Answer: b) False

Q3. Which of the following is FALSE.
An interrupt occurs in the protected mode.

a) Determine the IRQ number from the interrupt controller.
b) Switch from user to kernel stack
c) Save the process states.
d) None of these

Answer: b) Switch from user to kernel stack

Q4. Interrupt vector number used for system call is xv6 _____________.

a) 16
b) 32
c) 48
d) 64

Answer: d) 64

Introduction to Operating System NPTEL Week 4 Answers

Q5. What does the the system call sys_uptime()  return?

a) Time the when system call is invoked.
b) Time since the start of the process
c) The number of clock ticks interrupts that have occurred since the start.
d) Maximum time the interrupt taken to complete execution.

Answer: c) The number of clock ticks interrupts that have occurred since the start.

Q6. The signal sent from the processor to the device after receiving an interrupt is

a) Interrupt-acknowledge
b) Return signal
c) Service signal
d) Permission signal

Answer: a) Interrupt-acknowledge

Q7. The return address from the interrupt-service routine is stored on the

a) Interrupt handler
b) Processor register
c) Kernel stack
d) Memory

Answer: c) Kernel stack

Introduction to Operating System NPTEL Week 4 Answers

Q8. Which of the following are TRUE in a multiprogrammed OS?
a. More than one program can be in the ready state at the same time.
b. When a program exits, other programs are loaded into ready queue immediately.
c. After a software interrupt occurred, a process in the ready queue may switch to running state
d. If a higher priority process is present in the ready queue, then the running process will immediately switching from running to ready state.

a) a, b, c
b) a, b, d
c) a, c, d
d) a, b, c, d

Answer: c) a, c, d

Q9. Which of the following does not interrupt a running process?

a) A device
b) Timer
c) Power Failure
d) None of the above

Answer: d) None of the above

Q10. State True/False for each of the following statements.
1. In a multitasking operating system, interrupt (either software, hardware, or exception) is required for the operating system to execute.
2. A context switching occurs whenever the OS executes.

a) True, True
b) True, False
c) False, True
d) False, False

Answer: b) True, False

Introduction to Operating System NPTEL Week 4 Answers

Q11. Which of the following need not necessarily be saved when context switch occurs?

a) Registers
b) TLB
c) Program counter
d) None of the above

Answer: b) TLB

Q12. Consider a system having n CPUsk processes, and k > n. Calculate the upper bound for the number of processes in the READY, RUNNING and BLOCKED states

a) k, k, k
b) n, n, n
c) k, k, n
d) k, n, k

Answer: d) k, n, k

Q13. State True or False
Double fault is a fault that occurs when a CPU is trying to call an exception handler.

a) True
b) False

Answer: b) False

Q14. Devices that turn on interrupt in xv6?

a) UART
b) IDE
c) Keyboard
d) All of these

Answer: d) All of these

Introduction to Operating System NPTEL Week 4 Answers

Q15. Match the following
a. Trapframe 1. Overflow
b. Trap 2. Difficult to recover
c. Fault 3. Restore the context
d. Abort 4. Segment not present

a) a-3, b-1 c-4, d-2
b) a-3, b-4,c-1, d-3
c) a-4, b-1, c-3, d-2
d) a-4, b-3, c-2, d -1

Answer: a) a-3, b-1 c-4, d-2

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