Below are most commonly asked interview questions related to operating systems, along with their answers:
1. What is an operating system?
Answer: An operating system (OS) is a software system that acts as an intermediary between computer hardware and user applications. It manages hardware resources, provides services to user programs, and ensures efficient and secure interaction between users and the computer’s hardware.
2. What are the main functions of an operating system?
Answer: The main functions of an operating system include process management, memory management, file system management, device management, user interface, security, networking, and error handling.
3. What is the difference between process and thread?
Answer: A process is an independent program in execution, whereas a thread is a smaller unit within a process. Multiple threads can exist within a single process and share the process’s resources, such as memory and file handles.
4. Explain the difference between preemptive and non-preemptive scheduling.
Answer: Preemptive scheduling allows the operating system to interrupt a running process and allocate the CPU to another process. Non-preemptive scheduling, also known as cooperative scheduling, allows a process to voluntarily release the CPU to allow other processes to run.
5. What is virtual memory?
Answer: Virtual memory is a memory management technique that allows the operating system to use a portion of the hard disk as an extension of physical RAM. It enables the execution of large programs and allows multiple processes to share the same physical memory while maintaining isolation.
6. What is thrashing, and how can it be prevented?
Answer: Thrashing occurs when the operating system spends most of its time swapping data between main memory and disk due to insufficient RAM. It can be prevented by increasing the amount of RAM, improving memory management algorithms, or reducing the number of active processes.
7. Explain the difference between a mutex and a semaphore.
Answer: Both mutex and semaphore are synchronization mechanisms used to protect shared resources in multi-threaded environments. A mutex allows only one thread to access the resource at a time, while a semaphore can allow multiple threads (up to a certain limit) to access the resource simultaneously.
8. What is deadlock, and how can it be avoided?
Answer: Deadlock is a situation where two or more processes are unable to proceed because each is waiting for the other to release a resource. Deadlock can be avoided by using techniques like resource allocation graphs, deadlock prevention, or deadlock detection and recovery.
9. What is the role of the bootloader in the boot process?
Answer: The bootloader is responsible for loading the operating system into memory when the computer is powered on. It locates the OS on the storage device, loads it into memory, and transfers control to the OS, initiating the boot process.
10. Explain the difference between a monolithic kernel and a microkernel.
Answer: In a monolithic kernel, all kernel services run in a single address space, whereas in a microkernel, only essential services run in kernel space, and other services, such as device drivers, run in user space. Microkernels are more modular and offer better isolation between components.
11. What is the role of the process scheduler in an operating system?
Answer: The process scheduler is responsible for determining which process gets access to the CPU at a given time. It allocates CPU time to processes based on scheduling algorithms, such as First-Come-First-Served (FCFS), Shortest Job Next (SJN), Round Robin, etc.
12. Explain the concept of demand paging in virtual memory.
Answer: Demand paging is a technique in virtual memory management where pages of a process are loaded into main memory only when they are needed, rather than loading the entire process into memory at once. This reduces memory wastage and allows the operating system to handle more processes concurrently.
13. What is a zombie process? How does the operating system handle it?
Answer: A zombie process is a terminated process whose process entry remains in the process table until its parent process reads its exit status. The operating system automatically handles zombie processes by removing their process table entries when the parent process acknowledges the termination using the wait system call.
14. What are system calls? Why are they essential for an operating system?
Answer: System calls are interfaces provided by the operating system that allow user-level processes to request services from the kernel, such as I/O operations, process creation, memory allocation, etc. They are essential for an operating system as they provide controlled access to kernel services while maintaining security and preventing direct access to hardware resources.
15. Explain the concept of a context switch.
Answer: A context switch is the process of saving the state of a running process (e.g., CPU registers, program counter, etc.) and restoring the state of a different process to allow it to run. Context switches occur when the operating system switches the CPU from one process to another, usually due to time-sharing or preemption.
16. What is a page fault? How does the operating system handle it?
Answer: A page fault occurs when a program attempts to access a page that is not currently in main memory (RAM). The operating system handles a page fault by fetching the required page from the secondary storage (e.g., hard disk) into main memory, updating the page table, and then resuming the execution of the program.
17. Explain the role of the I/O scheduler in an operating system.
Answer: The I/O scheduler manages the order in which I/O requests from different processes are executed to optimize disk access and minimize seek times. It helps to improve the overall I/O performance by reordering and prioritizing I/O requests.
18. What is the difference between a fork system call and an exec system call?
Answer: The fork system call creates a new process (child process) that is an exact copy of the calling process (parent process). The exec system call replaces the current process’s memory space with a new program, allowing the process to execute a different program.
19. How does the operating system handle priority inversion?
Answer: Priority inversion occurs when a low-priority process holds a resource needed by a high-priority process, causing the high-priority process to wait. The operating system can handle priority inversion by implementing priority inheritance, where the priority of the low-priority process temporarily inherits the priority of the high-priority process until it releases the resource.
20. What is the difference between a user-level thread and a kernel-level thread?
Answer: User-level threads are managed entirely by the user-level thread library and do not require kernel involvement for thread management. Kernel-level threads, on the other hand, are managed by the operating system kernel, which can provide better concurrency and parallelism.
21. What is the purpose of a deadlock in an operating system?
Answer: Deadlock occurs when two or more processes are unable to proceed because each is waiting for a resource held by another process. The purpose of studying deadlocks is to prevent, detect, and recover from them to ensure the smooth execution of processes.
22. How does the Banker’s algorithm prevent deadlock in a multi-process system?
Answer: The Banker’s algorithm is used to allocate resources to processes in a way that avoids deadlock. It checks if allocating resources to a process would lead to a safe state, i.e., if all processes can complete their execution without getting stuck in a deadlock.
23. Explain the concept of multi-core processors and how an operating system utilizes them.
Answer: Multi-core processors have multiple processing units on a single chip. The operating system utilizes these cores to execute multiple threads or processes concurrently, taking advantage of parallelism to improve performance.
24. What is a page replacement algorithm, and how does it work?
Answer: Page replacement algorithms are used in virtual memory systems to determine which page to evict from memory when a page fault occurs. Common page replacement algorithms include FIFO (First-In-First-Out), LRU (Least Recently Used), and Optimal.
25. Describe the working of the “Copy on Write” technique used during process forking.
Answer: In the Copy on Write (COW) technique, when a process is forked, the child process shares the same memory with the parent process initially. If either process tries to modify a shared memory page, a copy of that page is created, and the modification is made on the copied page to maintain memory isolation between the processes.
26. What are system daemons, and what is their role in an operating system?
Answer: System daemons are background processes that run continuously on a computer. They perform various tasks such as managing system services, handling print jobs, monitoring hardware, and more. Daemons play a crucial role in ensuring the smooth functioning of the operating system and its services.
27. How does the “Fork Bomb” work, and how can the operating system prevent it?
Answer: A Fork Bomb is a malicious program that rapidly and exponentially creates child processes, consuming system resources and causing the system to become unresponsive. The operating system can prevent Fork Bombs by limiting the maximum number of processes a user can create.
28. What is the difference between static linking and dynamic linking of libraries?
Answer: Static linking involves linking the entire library code into the executable file, making the resulting binary independent of the library at runtime. Dynamic linking links the library code during program execution, allowing multiple programs to share a single copy of the library in memory.
29. Explain the concept of context switching overhead and how it affects system performance.
Answer: Context switching overhead refers to the time and resources consumed when the operating system switches the CPU from one process to another. Frequent context switches can lead to reduced system performance due to the additional processing required for saving and restoring process states.
30. What are the main differences between symmetric multiprocessing (SMP) and asymmetric multiprocessing (AMP)?
Answer: In Symmetric Multiprocessing, all processors are identical and share access to memory, and tasks can be scheduled to any available processor. In Asymmetric Multiprocessing, each processor has a specific role, and tasks are explicitly assigned to particular processors, leading to a more controlled environment.
31. What is the critical section problem in concurrency control, and how can it be solved?
Answer: The critical section problem refers to the situation where multiple processes or threads need to access a shared resource, and there is a risk of data inconsistency or race conditions. It can be solved using synchronization techniques like locks, semaphores, or mutexes to ensure that only one process can access the critical section at a time.
32. Explain the difference between internal fragmentation and external fragmentation in memory management.
Answer: Internal fragmentation occurs when a process is allocated more memory than it actually needs, leading to wasted space within a memory block. External fragmentation, on the other hand, occurs when free memory is fragmented into small, non-contiguous blocks, making it challenging to allocate large contiguous blocks of memory to processes.
33. How does the “Round Robin” scheduling algorithm work, and what is its advantage?
Answer: In the Round Robin scheduling algorithm, each process is given a fixed time slice to execute on the CPU. If the process doesn’t complete its task during the time slice, it is moved to the back of the queue, and the next process is given a chance. Its advantage is that it provides fair CPU time to all processes and is suitable for time-sharing systems.
34. What is the purpose of the swap space in virtual memory management?
Answer: The swap space is a reserved area on the hard disk used by the operating system for paging or swapping out pages of memory that are not currently in use. It allows the operating system to free up main memory by temporarily storing less frequently used pages on disk.
35. Explain the concept of multi-level queue scheduling in process management.
Answer: Multi-level queue scheduling involves dividing processes into different priority levels, with each level having its own queue and scheduling algorithm. Processes with higher priority are assigned to queues with higher priority, and each queue is served according to its scheduling policy.
36. What are the advantages and disadvantages of a monolithic kernel compared to a microkernel?
Answer: A monolithic kernel has all kernel services running in a single address space, providing better performance due to direct access to hardware. However, it is less modular and may be less secure. A microkernel, while more modular and secure, incurs some performance overhead due to communication between user-level and kernel-level components.
37. How does virtual memory provide memory protection and isolation between processes?
Answer: Virtual memory provides memory protection and isolation by using virtual addresses, which are translated to physical addresses by the memory management unit (MMU). Each process has its own virtual address space, preventing direct access to another process’s memory and ensuring that a process cannot modify or access memory outside its allocated space.
38. What is the purpose of the init process in Unix-based systems?
Answer: The init process (typically with process ID 1) is the first process started by the kernel during boot-up. It serves as the parent process to all other processes and is responsible for starting system services and daemons.
39. How does the “Least Recently Used (LRU)” page replacement algorithm work?
Answer: The LRU algorithm replaces the least recently used page from memory when a page fault occurs. It tracks the order of page accesses and replaces the page that has not been accessed for the longest time.
40. Explain the role of the “Scheduler” in an operating system.
Answer: The Scheduler is responsible for deciding which process or thread gets access to the CPU at any given time. It selects processes from the ready queue and allocates CPU time based on the scheduling algorithm to achieve better resource utilization and responsiveness.
41. What is the purpose of the Master Boot Record (MBR) in the boot process?
Answer: The Master Boot Record (MBR) is a small program located in the first sector of the bootable storage device (e.g., hard disk). It contains the boot loader, which is responsible for loading the operating system’s kernel into memory during the boot process.
42. Explain the concept of process synchronization and why it is essential.
Answer: Process synchronization ensures that concurrent processes work correctly and avoid conflicts when accessing shared resources. It is crucial to prevent data inconsistency and race conditions in multi-process or multi-threaded environments.
43. What is the purpose of the Superblock in a file system?
Answer: The Superblock contains essential metadata about the file system, such as the total number of blocks, free blocks, and information about the file system’s layout. It helps the operating system manage and access files efficiently.
44. How does the “Shortest Job Next (SJN)” scheduling algorithm work?
Answer: The SJN scheduling algorithm selects the process with the shortest burst time (execution time) first. It prioritizes processes that can be completed quickly, leading to better turnaround time for short processes.
45. Explain the difference between a soft link (symbolic link) and a hard link in Unix-based systems.
Answer: A soft link, also known as a symbolic link, is a pointer to the target file or directory. It creates a new file with a different inode, pointing to the original file or directory. A hard link, on the other hand, creates additional directory entries (filenames) that point directly to the same inode as the original file or directory.
46. What is the purpose of the Page Table in virtual memory management?
Answer: The Page Table maps virtual addresses used by processes to physical addresses in main memory. It allows the operating system to manage memory efficiently and provides memory isolation between processes.
47. How does the Belady’s Anomaly occur in page replacement algorithms?
Answer: Belady’s Anomaly is a phenomenon where increasing the number of frames in a page replacement algorithm causes an increase in page faults. This contradicts the general expectation that more memory should lead to fewer page faults.
48. Explain the concept of “dirty bit” in the context of page tables.
Answer: The “dirty bit” is a flag associated with each page table entry. It is set when the corresponding page in memory is modified (written to). The operating system uses the dirty bit to track which pages need to be written back to disk during page replacement or process termination.
49. What is the purpose of the “initrd” (Initial Ramdisk) in the Linux boot process?
Answer: The “initrd” is an initial ramdisk used during the Linux boot process. It contains a temporary root file system that is loaded into memory during the early stages of booting. The real root file system is later mounted, and the control is transferred to the actual operating system.
50. How does the “First-In-First-Out (FIFO)” page replacement algorithm work?
Answer: The FIFO page replacement algorithm replaces the oldest (first-in) page in memory when a page fault occurs. It uses a queue to track the order in which pages were loaded into memory.