Top Operating System Interview Questions — With Short Answers

Revise these before technical rounds. Each answer fits in two lines — expand with an example if asked.

Basics

1. What is an OS? Software that manages hardware and provides services to programs: processes, memory, files, devices, security.
2. Kernel? The core of the OS running in privileged kernel mode.
3. System call? The way a program requests an OS service (e.g. read, fork).
4. Monolithic vs microkernel? Monolithic: all services in the kernel (fast, Linux). Microkernel: minimal kernel, services in user space (stable, modular).

Processes and threads

5. Program vs process? A program is a passive file; a process is a program in execution.
6. Process vs thread? Processes have separate memory; threads share their process's memory and have their own stacks.
7. What is a PCB? The Process Control Block — the OS's record of a process (PID, state, registers, memory info).
8. Context switch? Saving one process's CPU state and loading another's; pure overhead.
9. Zombie vs orphan? Zombie: finished but not yet reaped by its parent. Orphan: parent died; adopted by init.

Scheduling

10. Preemptive vs non-preemptive? Preemptive can interrupt a running process; non-preemptive lets it finish its burst.
11. Which scheduling minimises average waiting time? SJF (non-preemptive) / SRTF (preemptive).
12. Convoy effect? In FCFS, short jobs wait behind a long one.
13. Starvation and aging? Starvation: a process waits indefinitely. Aging: gradually raising its priority.

Synchronization and deadlock

14. Race condition? The output depends on the timing of concurrent access to shared data.
15. Mutex vs semaphore? Mutex: a lock with an owner. Semaphore: a counter with wait/signal; can manage n resources.
16. Deadlock conditions? Mutual exclusion, hold and wait, no preemption, circular wait — all four.
17. Deadlock vs starvation? Deadlock: processes wait for each other forever. Starvation: one process waits while others progress.
18. Banker's algorithm? Deadlock avoidance — grant a request only if the system stays in a safe state.

Memory

19. Paging vs segmentation? Paging: fixed-size pages (no external fragmentation). Segmentation: variable-size logical segments.
20. Internal vs external fragmentation? Internal: waste inside an allocated block. External: free memory split into unusable holes.
21. What is a TLB? A fast cache of page-table entries that speeds up address translation.
22. Virtual memory? Using disk to extend RAM; only needed pages stay in memory (demand paging).
23. Page fault? Accessing a page that isn't in RAM; the OS loads it from disk.
24. Thrashing? Excessive paging that leaves the CPU idle; fix by reducing multiprogramming or adding frames.
25. Belady's anomaly? With FIFO replacement, more frames can cause more page faults.

Bonus: quick calculations interviewers like

• n calls to fork() → 2ⁿ processes in total.
• Page number / offset: page = address ÷ page size, offset = address mod page size.
• Waiting time = turnaround − burst; turnaround = completion − arrival.

Common mistakes

• Mixing up deadlock prevention (break a condition) and avoidance (stay safe).
• Saying threads have separate heaps.
• Forgetting that all four deadlock conditions are needed together.

Quick recap

• Revise: kernel, process vs thread, PCB, scheduling, race conditions, mutex vs semaphore, deadlock, paging, virtual memory, thrashing.
• Answer in two lines, then give an example.