OS Interview Questions & Answers

This section is designed to help students prepare for interviews that test knowledge of Operating System (OS) concepts. It includes both theoretical questions and coding-oriented problem types commonly asked in technical interviews.

Part 1: Theoretical Interview Questions

These questions test your understanding of how operating systems work internally. They’re frequently asked in HR screens, tech rounds, and CS fundamentals assessments.

Q1. What is the difference between a Process and a Thread?

Answer:

• A process is an independent executing program with its own memory space.
• A thread is the smallest unit of execution within a process and shares memory with other threads of the same process.
• Threads are lightweight and faster to create, whereas processes have more overhead due to isolated memory.

Q2. What happens during a Context Switch?

Answer:

A context switch occurs when the CPU switches from one process or thread to another. During the switch, the OS saves the state (registers, program counter, etc.) of the old process and loads the state of the new one. This allows multitasking and CPU sharing.

Q3. Explain Deadlock and the four necessary conditions.

Answer:

A deadlock occurs when a group of processes are stuck, each waiting for resources held by others. The four necessary conditions:

1. Mutual exclusion
2. Hold and wait
3. No preemption
4. Circular wait

Preventing any one of these conditions avoids deadlock.

Q4. What is Virtual Memory? How does it work?

Answer:

Virtual memory allows a process to use more memory than physically available by using disk space as an extension of RAM. It relies on paging or segmentation, and uses a page table to translate virtual addresses into physical addresses.

Q5. Compare Preemptive and Non-Preemptive Scheduling.

Answer:

• Preemptive scheduling allows the OS to interrupt a running process to assign the CPU to another.

• Non-preemptive scheduling allows a process to run until it voluntarily yields or terminates.

• Preemptive models are more responsive; non-preemptive models are simpler and fairer in cooperative systems.

Part 2: Coding-Oriented OS Questions

These questions often test your understanding of OS principles through algorithm design and implementation, commonly in competitive programming or tech interviews.

Q1. Simulate Round Robin Scheduling

• Task: Given a list of processes with burst times and a time quantum, simulate the Round Robin algorithm and compute waiting and turnaround times.

• Concepts Tested: Scheduling algorithms, queue data structures, time simulation.

Q2. Implement a Semaphore using Mutex and Condition Variables

Task: Build a simple counting semaphore class that supports wait() and signal().

Concepts Tested: Concurrency, synchronization primitives, thread safety.

Q3. Producer-Consumer Problem (Bounded Buffer)

• Task: Create a multithreaded solution where producers generate data and consumers use it, ensuring no race conditions occur.

• Concepts Tested: Critical sections, semaphores or mutexes, inter-thread communication.

Q4. Simulate Page Replacement Algorithms

Task: Write code for FIFO, LRU, or Optimal page replacement and report the number of page faults.

Concepts Tested: Memory management, caching, data structures (queue, stack, hash map).

Q5. Dining Philosophers Problem

Task: Implement a solution that avoids deadlock or starvation when five philosophers pick up and put down forks.

Concepts Tested: Deadlock avoidance, mutexes, resource allocation.

Summary Tips

Revise core OS concepts: processes, threads, scheduling, synchronization, memory.

Practice pseudocode and multithreading logic, especially for problems involving race conditions or deadlock.

Focus on real-world use cases like OS scheduling and page replacement—these are common interview themes.