📘 RTOS Interview Questions & Answers (Quick Guide)

🔹 Basic Concepts

Q1. What is an RTOS? How is it different from a general OS?
👉 An RTOS (Real-Time Operating System) is designed to process data within a strict time constraint (deterministic). Unlike general OS (like Windows/Linux), RTOS guarantees timely task execution.

Q2. Difference between hard real-time and soft real-time systems?
👉 Hard RTOS: Missing a deadline = system failure (e.g., pacemaker).
👉 Soft RTOS: Occasional deadline miss is acceptable (e.g., video streaming).

Q3. What is a task in RTOS?
👉 A task (thread) is the smallest unit of execution managed by the RTOS.

Q4. What is the kernel in RTOS?
👉 The kernel is the core that manages tasks, scheduling, synchronization, interrupts, and timing.

Q5. Difference between multitasking, multithreading, multiprocessing?
👉 Multitasking: CPU runs multiple tasks by switching.
👉 Multithreading: A task is divided into multiple threads.
👉 Multiprocessing: Multiple CPUs working together.

🔹 Task Management

Q6. What is task scheduling?
👉 The process of deciding which task runs when.

Q7. What is priority-based scheduling?
👉 Tasks are assigned priorities; the RTOS always runs the highest-priority ready task.

Q8. What is context switching?
👉 Saving the state of a task and loading another. It enables multitasking but adds overhead.

Q9. How does RTOS handle starvation?
👉 By using priority inheritance or aging, where task priority increases if it waits too long.

Q10. Difference between preemptive and cooperative scheduling?
👉 Preemptive: High-priority tasks can interrupt lower-priority tasks.
👉 Cooperative: Tasks voluntarily yield control.

🔹 Synchronization & Communication

Q11. What are semaphores? Binary vs Counting?
👉 Semaphore: Sync tool for tasks.
👉 Binary Semaphore: Value 0/1, used like a lock.
👉 Counting Semaphore: Allows multiple resources to be shared.

Q12. What is a mutex? Difference from semaphore?
👉 Mutex (Mutual Exclusion): Ensures only one task accesses a resource at a time. Unlike a semaphore, it is owned by a task.

Q13. What is priority inversion?
👉 A high-priority task is blocked by a low-priority one. Fixed with priority inheritance.

Q14. What are IPC methods in RTOS?
👉 Message queues, pipes, semaphores, shared memory, signals.

Q15. What are message queues?
👉 Buffers that allow tasks to send/receive messages asynchronously.

🔹 Timing & Performance

Q16. What is a tick timer?
👉 A periodic timer interrupt that drives task scheduling.

Q17. What is latency?
👉 Delay between an event occurring and the RTOS responding.

Q18. What is jitter?
👉 Variation in task response time. Low jitter = better real-time performance.

Q19. How are deadlines handled in RTOS?
👉 By using deadline-based scheduling algorithms like EDF (Earliest Deadline First).

Q20. Factors affecting context switch time?
👉 CPU speed, number of registers, memory access speed, cache performance.

🔹 Advanced Concepts

Q21. What is RTC in RTOS?
👉 Real-Time Clock keeps track of calendar time and generates interrupts for scheduling.

Q22. How does RTOS achieve deterministic behavior?
👉 Through bounded (predictable) task switching and interrupt handling.

Q23. Static vs Dynamic memory allocation in RTOS?
👉 Static: Allocated at compile time (safer, preferred).
👉 Dynamic: Allocated at runtime (can cause fragmentation, less predictable).

Q24. What is ISR? Interaction with tasks?
👉 Interrupt Service Routine: Handles external/internal interrupts. Usually signals a task via semaphores or queues.

Q25. Common scheduling algorithms in RTOS?
👉 Rate Monotonic Scheduling (RMS) and Earliest Deadline First (EDF).

🔹 Practical / Applications

Q26. Example where RTOS is essential?
👉 Pacemakers, automotive ABS, aerospace systems.

Q27. Why not use Linux/Windows for strict real-time?
👉 They are non-deterministic; cannot guarantee deadlines due to background processes.

Q28. Differences between FreeRTOS, VxWorks, RTEMS?
👉 FreeRTOS: Open-source, lightweight.
👉 VxWorks: Commercial, robust, used in aerospace.
👉 RTEMS: Open-source, POSIX-compliant.

Q29. How to design RTOS system for IoT device?
👉 Identify tasks → Assign priorities → Use RTOS kernel → Implement communication → Optimize power.

Q30. Challenges in debugging RTOS apps?
👉 Concurrency issues, timing bugs, race conditions, deadlocks.

👉 These 30 questions cover fundamentals, scheduling, synchronization, timing, and practical applications—perfect for interviews, viva, and exam prep.