Operating System Viva questions/interview questions 2025

1. What is an Operating System?

An Operating System (OS) is system software that acts as a bridge between users and computer hardware. It manages hardware resources and provides services to application software. An OS handles tasks like memory management, process scheduling, and device control. It ensures that programs and users can run smoothly without interfering with each other. Examples include Windows, Linux, macOS, and Android. The OS also provides a user interface, either graphical (GUI) or command-line (CLI). Without an OS, a computer would be unusable. It organizes files, maintains security, and manages system performance. In short, the OS is the heart of any computing device.

2. What are the main functions of an Operating System?

The primary functions of an Operating System are process management, memory management, file system management, and device management. It ensures efficient execution of processes by scheduling CPU time. Memory management keeps track of each byte in a computer’s memory and allocates memory spaces. File system management organizes, stores, retrieves, and protects data on storage devices. Device management handles communication with input/output devices via drivers. It also manages security and access controls. The OS offers a user interface to interact with the system. It also handles error detection and system performance monitoring. Overall, it makes system resources accessible and optimized.

3. What is a process in an Operating System?

A process is an instance of a program in execution. It includes the program code, its current activity, and a set of resources like CPU registers, memory, files, and I/O devices. Processes are fundamental units that the OS manages for multitasking. Each process has a unique Process ID (PID) and moves through different states like ready, running, waiting, and terminated. The OS allocates resources to processes and ensures smooth scheduling. Processes may be independent or dependent on other processes. Multitasking OSes manage multiple processes simultaneously. Process control blocks (PCBs) store all information about a process. Processes help achieve better CPU utilization.

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

A process is a complete program in execution, while a thread is the smallest unit of execution within a process. Threads share the same memory and resources of their parent process but execute independently. Processes are isolated from each other and require more overhead to create. In contrast, threads are lighter and easier to create and terminate. Communication between threads (in the same process) is simpler than inter-process communication (IPC). Threads are commonly used for parallelism to perform multiple tasks concurrently. Multithreading improves application responsiveness. Each process has its own memory space, but threads within a process share memory. Both enhance multitasking but operate at different levels.

5. What is deadlock in Operating Systems?

Deadlock is a situation in Operating Systems where a set of processes get stuck indefinitely because each process is waiting for a resource held by another. It commonly occurs in multi-processing systems where resources are shared. For deadlock to occur, four conditions must be true simultaneously: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlocks cause system performance issues and can freeze applications. Operating Systems use various strategies to handle deadlocks, such as prevention, avoidance, detection, and recovery. For example, Banker's Algorithm helps avoid deadlocks. Resource allocation graphs can detect potential deadlocks. Solving deadlocks is critical for system stability and reliability.

6. What is virtual memory?

Virtual memory is a memory management technique that gives an application the illusion it has a contiguous block of memory, even if physically it is fragmented or exceeds actual RAM. It uses hardware and software to map memory addresses. Virtual memory uses disk storage (like a hard drive) as an extension of RAM by swapping data back and forth using a technique called paging. This allows more programs to run simultaneously and enables larger programs to run even on small physical memory. It improves multitasking and system stability. However, excessive use of virtual memory can lead to slowdowns, known as thrashing. Operating Systems manage virtual memory efficiently for performance.

7. What is paging in an Operating System?

Paging is a memory management scheme where physical memory is divided into fixed-sized blocks called frames, and logical memory is divided into pages. When a process needs memory, its pages are loaded into any available memory frames. Paging avoids external fragmentation and allows non-contiguous memory allocation. The OS maintains a page table that maps logical addresses to physical addresses. When a page is not in memory, a page fault occurs, and the OS must fetch it from disk. Paging is fundamental to implementing virtual memory. It increases system efficiency and flexibility. Modern OSes like Windows and Linux use paging extensively to manage memory smartly.

8. Explain the concept of context switching.

Context switching is the process of saving the state (context) of a currently running process and loading the state of the next scheduled process. It enables multitasking, allowing multiple processes to share a single CPU efficiently. The saved context includes information like CPU registers, program counter, and memory management data. When the OS decides to switch tasks (based on scheduling policies), it stores the old process's context and loads the new one. Context switching adds overhead to the CPU, as it consumes time and system resources. However, it is crucial for responsiveness and effective process management. Optimized context switching improves system performance and user experience.

9. What are the different types of schedulers in an Operating System?

Operating Systems use three types of schedulers: long-term scheduler, short-term scheduler, and medium-term scheduler. The long-term scheduler decides which processes are admitted to the ready queue. It controls the degree of multiprogramming. The short-term scheduler (CPU scheduler) selects one process from the ready queue to execute. It runs frequently and must be very fast. The medium-term scheduler temporarily removes processes from memory (swapping) to reduce the load and improve performance. Each scheduler plays a specific role in balancing system performance, throughput, and responsiveness. Effective scheduling strategies improve resource utilization, minimize waiting time, and enhance user satisfaction.

10. What is the difference between multitasking, multiprocessing, and multithreading?

Multitasking refers to running multiple tasks (processes) on a single CPU by rapidly switching between them. Multiprocessing uses two or more CPUs to execute multiple processes simultaneously, truly in parallel. Multithreading is running multiple threads within a single process to perform concurrent operations. Multitasking improves user experience by enabling smooth application switching. Multiprocessing boosts performance by dividing tasks among processors. Multithreading optimizes resource sharing within a process and speeds up execution. While multitasking and multiprocessing involve processes, multithreading deals with threads. All three techniques enhance the efficiency and responsiveness of modern computer systems by effectively utilizing hardware and software resources.

11. What is a kernel in an Operating System?

The kernel is the core component of an Operating System that directly interacts with hardware. It manages system resources like memory, CPU, and devices. The kernel provides essential services to all other parts of the OS. There are different types of kernels such as monolithic, microkernel, and hybrid. A monolithic kernel contains all OS services, while a microkernel runs only the most basic services in the kernel space. The kernel also manages process scheduling, system calls, and security. Without the kernel, applications would not be able to use hardware resources. It operates in privileged mode for maximum control and efficiency.

12. What is a system call?

A system call is the programmatic way a user program requests a service from the operating system's kernel. System calls provide an interface between a process and the OS. They allow processes to perform tasks like file operations, process control, device management, and communication. Examples include open(), read(), write(), and fork(). Without system calls, applications couldn’t access hardware resources securely. The OS protects system resources by controlling system call access. Different OSes offer different sets of system calls. They act as gateways for user applications to interact with the kernel in a safe and controlled manner.

13. What are the different types of system calls?

System calls are categorized based on the services they provide. The main types are process control (e.g., fork, exit), file management (e.g., open, close), device management (e.g., read, write), information maintenance (e.g., getpid, alarm), and communication (e.g., pipe, shmget). Process control calls handle process creation and termination. File management calls deal with file operations. Device management system calls manage device communication. Information maintenance calls manage system data. Communication calls enable inter-process communication. Each type ensures secure and efficient interaction between user processes and the system hardware via the kernel interface.

14. What is swapping in Operating Systems?

Swapping is a memory management technique where a process is moved temporarily from main memory to secondary storage like a hard disk. This frees up RAM for other processes. When the swapped-out process needs to execute again, it is brought back into main memory. Swapping improves system multitasking by balancing load among running programs. However, excessive swapping can slow down the system, leading to thrashing. It is mainly used in systems with limited physical memory. Modern Operating Systems use swapping efficiently to maintain performance and system responsiveness, especially under heavy workload conditions.

15. What is thrashing in Operating Systems?

Thrashing occurs when a system spends more time swapping pages in and out of memory than executing actual processes. It happens due to insufficient RAM when too many processes are competing for memory. As a result, CPU utilization drops sharply. Thrashing severely degrades system performance and can cause the system to freeze. Operating Systems handle thrashing by reducing the degree of multiprogramming or by using better page replacement algorithms. Avoiding overloading the system and tuning memory management settings help prevent thrashing. Detecting thrashing early is important for system stability.

16. What is segmentation in memory management?

Segmentation is a memory management scheme where a process is divided into variable-sized segments based on its logical divisions, like functions, arrays, or data structures. Each segment has its own base and limit. Unlike paging, segmentation reflects the logical structure of a program. The OS maintains a segment table with segment numbers, base addresses, and limits. Segmentation simplifies access control, sharing, and protection. However, it can lead to external fragmentation. It is mainly used where logical separation of data improves system efficiency. Some systems combine segmentation with paging for better memory management.

17. What is a file system in an Operating System?

A file system is the method and structure that an Operating System uses to store, retrieve, and organize data on storage devices. It defines how data is named, stored, and accessed. Common file systems include NTFS, FAT32, ext4, and APFS. The file system manages directories, file attributes, permissions, and metadata. It also ensures data security, consistency, and efficient space management. Without a file system, data would be stored in a large, unstructured blob. Good file system design enhances performance, recovery, and security. Every Operating System typically supports multiple file system types.

18. What is the difference between internal and external fragmentation?

Internal fragmentation happens when allocated memory may have small unused spaces inside, because memory is allocated in fixed-size blocks. External fragmentation occurs when free memory is scattered across the system, making it hard to allocate large contiguous spaces even though enough total free space exists. Internal fragmentation wastes space inside allocated memory. External fragmentation wastes memory between allocations. Solutions like paging solve external fragmentation, while compaction or better block size selection reduce internal fragmentation. Both types of fragmentation degrade memory efficiency if not managed properly.

19. What is demand paging?

Demand paging is a memory management technique where a page is loaded into memory only when it is needed during program execution, not in advance. This reduces memory usage and loading time. If a page is not in memory, a page fault occurs, and the OS fetches the required page from disk. Demand paging improves system performance by using memory efficiently. It supports the concept of virtual memory. However, frequent page faults can slow down performance. Proper page replacement strategies are essential for optimizing demand paging systems.

20. What are page replacement algorithms?

Page replacement algorithms decide which memory pages to swap out when a new page needs to be loaded, but there’s no free memory available. Popular algorithms include FIFO (First-In, First-Out), LRU (Least Recently Used), and Optimal Page Replacement. FIFO removes the oldest loaded page. LRU removes the page that hasn’t been used for the longest time. Optimal algorithm replaces the page that will not be used for the longest future duration. Effective page replacement is crucial for minimizing page faults and improving system performance. Operating Systems select the algorithm based on the workload type.

21. What is a race condition?

A race condition occurs when two or more processes or threads access shared resources concurrently, and the outcome depends on the order of execution. It can lead to unpredictable results and bugs. Race conditions often happen in multithreaded programs without proper synchronization. To prevent race conditions, synchronization mechanisms like mutexes, semaphores, or locks are used. Critical sections in code must be protected to ensure only one thread accesses shared data at a time. If not handled correctly, race conditions can cause serious issues like data corruption and security vulnerabilities.

22. What is a critical section?

A critical section is a part of a program where shared resources are accessed and modified. Only one process or thread should execute inside the critical section at a time to avoid race conditions. Operating Systems provide synchronization mechanisms like semaphores, mutexes, and monitors to manage critical sections. Proper design of critical sections ensures data consistency and process coordination. Critical sections should be kept short to minimize waiting time for other processes. Effective management of critical sections is essential for system reliability, especially in multi-user or multi-threaded environments.

23. What is a semaphore?

A semaphore is a synchronization tool used to control access to a shared resource in a concurrent system. It is an integer variable that is incremented or decremented using two atomic operations: wait (P) and signal (V). Semaphores help manage multiple processes trying to access the same resource, preventing race conditions. There are two types of semaphores: binary (mutex) and counting semaphores. Binary semaphores take only 0 and 1 values, suitable for mutual exclusion. Counting semaphores allow a resource pool to be managed. Semaphores are widely used in Operating System kernels.

24. What is mutual exclusion?

Mutual exclusion is a property ensuring that only one process or thread accesses a critical section at a time. It prevents race conditions and ensures data integrity. Mutual exclusion can be implemented using locking mechanisms like mutexes, semaphores, or monitors. When a process enters its critical section, other processes are blocked until it exits. Ensuring mutual exclusion is fundamental in concurrent programming. Without it, systems could suffer from data corruption, inconsistencies, and unexpected behavior. Properly designed mutual exclusion increases system robustness and stability.

25. What is deadlock prevention?

Deadlock prevention ensures that at least one of the four necessary conditions for deadlock (mutual exclusion, hold and wait, no preemption, circular wait) never occurs. Strategies include requesting all resources at once (prevent hold and wait) or using priority-based resource allocation. Allowing preemption or enforcing resource ordering can also prevent deadlocks. Prevention techniques are proactive but can lead to reduced resource utilization or system complexity. They are used in systems where avoiding deadlock is more critical than maximizing performance. Deadlock prevention adds reliability to multitasking environments.