Operating System Interview Questions and Answers

Question 1

What is the difference between a process and a thread?

Accepted Answer

Threads of a process share its memory; processes have separate memory. That's why threads are lighter but need synchronisation.

Question 2

Which scheduling algorithm can cause starvation of long jobs?

Accepted Answer

Shortest Job First. Short jobs keep jumping ahead, so long ones may wait forever. Aging fixes it.

Question 3

Round Robin scheduling uses:

Accepted Answer

A fixed time slice (quantum) for each process in turn. It's fair and good for time-sharing systems.

Question 4

A deadlock requires which four conditions?

Accepted Answer

Mutual exclusion, hold and wait, no preemption, circular wait. Breaking any one condition prevents deadlock.

Question 5

Virtual memory allows:

Accepted Answer

Programs to use more memory than physically available, using disk. Pages not in use are swapped out to disk.

Question 6

A page fault occurs when:

Accepted Answer

A program accesses a page that isn't in RAM. The OS loads the page from disk and resumes the program.

Question 7

What is thrashing?

Accepted Answer

The system spends more time swapping pages than doing work. Too many processes for too little RAM.

Question 8

A semaphore is used for:

Accepted Answer

Controlling access to shared resources between processes or threads. Wait (P) and signal (V) operations coordinate access.

Question 9

A mutex differs from a counting semaphore in that it:

Accepted Answer

Allows only one holder at a time and is owned by the locker. A counting semaphore can allow N simultaneous holders.

Question 10

What is a context switch?

Accepted Answer

Saving one process's state and loading another's. It has overhead, so too many switches slow the system.

Question 11

Which memory is fastest?

Accepted Answer

CPU registers. Registers → cache → RAM → SSD → HDD, from fastest to slowest.

Question 12

The kernel is:

Accepted Answer

The core of the OS that manages CPU, memory and devices. It runs in privileged mode.

Question 13

A zombie process is:

Accepted Answer

A finished process whose parent hasn't collected its exit status. It holds an entry in the process table until the parent calls wait().

Question 14

Which page replacement algorithm replaces the page not used for the longest time?

Accepted Answer

LRU (Least Recently Used). Optimal replaces the page used farthest in the future, which can't be known in practice.

Question 15

A race condition occurs when:

Accepted Answer

The result depends on the timing of threads accessing shared data. Protect shared data with locks.

Question 16

Paging divides memory into:

Accepted Answer

Fixed-size blocks. Fixed-size pages avoid external fragmentation.

Question 17

What is a system call?

Accepted Answer

A request from a program to the OS kernel for a service. e.g. read(), write(), fork().

Question 18

fork() in Unix:

Accepted Answer

Creates a new child process. The child is a copy of the parent; fork returns 0 in the child.

Question 19

Internal fragmentation happens when:

Accepted Answer

Allocated blocks are larger than needed, wasting space inside them. External fragmentation is free memory scattered into unusable gaps.

Question 20

Which of these is NOT an operating system?

Accepted Answer

Python. Python is a programming language; the others are operating systems.

Operating System Interview Questions and Answers

Q1. What is the difference between a process and a thread?

Q2. Which scheduling algorithm can cause starvation of long jobs?

Q3. Round Robin scheduling uses:

Q4. A deadlock requires which four conditions?

Q5. Virtual memory allows:

Q6. A page fault occurs when:

Q7. What is thrashing?

Q8. A semaphore is used for:

Q9. A mutex differs from a counting semaphore in that it:

Q10. What is a context switch?

Q11. Which memory is fastest?

Q12. The kernel is:

Q13. A zombie process is:

Q14. Which page replacement algorithm replaces the page not used for the longest time?

Q15. A race condition occurs when:

Q16. Paging divides memory into:

Q17. What is a system call?

Q18. fork() in Unix:

Q19. Internal fragmentation happens when:

Q20. Which of these is NOT an operating system?