Top 50 Operating System Interview Questions And Answers

Operating Systems (OS) form the backbone of software engineering. Aspiring engineers preparing for tech job interviews should be well-versed in OS concepts. Here, we present 50 common OS interview questions divided into three categories: Basic, Intermediate, and Advanced. Each question is accompanied by detailed answers, examples, and tables wherever relevant.

Basic Operating System Questions

1. What is an operating system?

An operating system (OS) is the basic software that manages a computer’s hardware and software resources. It serves as an intermediary between the user and the underlying computer hardware, allowing applications to work properly without concerns about hardware compatibility. For instance, every time you launch a web browser, the OS assigns resources such as memory and processing power to that application.

One of the first annual OS, the GMOs, were established in the early 1950s. Examples of a modern operating system include Windows, macOS, Linux, and Android. From multitasking to file management to security, these systems are what allow computers to be powerful and easy to use.

Do you know? Gary Arlen Kildall, an American computer scientist, is known as the father of the operating system. 

2. What is a process in an OS?

A process is a fundamentally a program in execution. It comprises the program code, the state of activity (e.g., the contents of CPU registers), and related resources. Processes are handled by the operating system, and are tracked via a Process Control Block (PCB). The PCB stores information like process id, state, priority, and context.

To give an example, when you launch a text editor, it gets loaded into the operating system as a process. A chip referred to as the DMA Controller (DMAC) manages these operations. It makes sure that work happens seamlessly.

3. What are threads and their types?

Threads are smaller units within a process that can execute independently. They do not share the same code space but execute the exact same code at the same time, allowing for massively parallel processing. There are two primary types of threads:

• Managed by user-level libraries and are independent of the kernel.

• Managed directly by the OS kernel, offering better integration with hardware.

In a program like a web browser, one thread could be processing user input while another thread is loading web content.

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

Explain with Examples

To explain further, you can provide these examples in your interview. They are clear, relatable, and demonstrate your understanding of processes and threads in practical terms. 

Process

Imagine you open two instances of a word processor, such as Microsoft Word. Each instance runs as a separate process. These processes have their own memory space, meaning one instance crashing will not affect the other. For example, if you’re working on Document A in one instance and Document B in the other, an issue with Document A (like a software crash) will not impact Document B because they are handled by independent processes.

Thread

Think about a web browser, such as Google Chrome, which is designed to use threads within a single process. Each open tab in the browser runs as a thread. These threads share the browser's memory and resources, like the browsing history or cookies. If one tab encounters a problem (like a script error), the browser can isolate it to that thread, and other tabs typically remain unaffected because modern browsers use multi-threading. However, since all threads belong to the same browser process, they share memory, which makes thread communication faster and more efficient.

5. What are the primary functions of an OS?

An OS performs the following main functions:

• Process Management: Manages process creation, execution, scheduling, and termination.
• Memory Management: Allocates, deallocates, and optimizes memory usage.
• File System Management: Handles file storage, retrieval, and organization.
• Device Management: Controls and communicates with hardware devices through drivers.
• Security and Access Control: Protects data and resources against unauthorized access.
• User Interface: Provides an interface (CLI or GUI) for user interaction.
• Networking: Manages data exchange between devices over networks.
• Error Detection and Handling: Monitors system performance and resolves errors.

Also Read: 15 Functions Of Operating System & Services Explained (+Examples)

6. What is meant by deadlock?

Deadlock occurs when two or more processes are waiting for resources that are held by one another. So, none of them can proceed. Process A is queued behind a printer being used by Process B. In the meantime, Process B is blocking on a scanner that’s held by Process A. Neither can advance.

Deadlocks can be prevented with measures including the Banker’s Algorithm, which simulates resource allocation to determine if deadlocks are possible. In fact, studies show that about 70% of deadlocks can be prevented this way.

7. What are the conditions for deadlock?

Four conditions must be met for a deadlock to occur:

1. Mutual Exclusion: Only one process can use a resource at a time.
2. A process is holding at least one resource while waiting for others.
3. Resources cannot be forcibly taken from a process.
4. A set of processes are waiting on each other in a circular chain.

If Process A must wait for Process B, a logjam forms. In turn, if Process B is waiting on Process A, a deadlock is inescapable.

8. What is virtual memory in an OS?

Virtual memory is one of the most powerful and useful OS techniques. It accomplishes this feat by using a part of the hard drive as “virtual” RAM. It allows giant apps to operate by moving dormant data to disk space. The computer in reality has 8 GB of RAM… that’s not changing. When you open several different programs, virtual memory goes into effect, allowing you to expand this capacity and stopping any of your valuable applications from closing unexpectedly.

9. Explain the concept of paging.

Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory. It breaks processes into equally sized chunks called pages and keeps them in physical memory in equally sized frames. This avoids issues such as external fragmentation.

For example, let’s say a program needs 10 pages, the OS immediately locates 10 free frames in memory. This enables more efficient use of memory and faster data retrieval.

10. What is fragmentation in memory?

Fragmentation is the needless waste of memory caused by allocation and freeing. There are two main types of fragmentation:

1. Internal Fragmentation: Occurs when memory is allocated in fixed-sized blocks, leaving unused space within a block.
2. External Fragmentation: Happens when free memory is scattered, making it hard to allocate large contiguous blocks.

Say you have 100 MB of memory that you need, but it's only available in 10 MB chunks. Even if there is enough total memory, a 50 MB process will never be able to allocate a single byte.

11. What is the purpose of a kernel in an OS?

The kernel is sort of the heart of an OS, orchestrating the interaction of hardware and software. It manages processes, such as scheduling and execution, memory allocation, and device management. For example, when you send a document to print, the kernel talks to the printer driver to carry out the action. It’s critical to ensuring a performant, stable system.

12. How do you define spooling in an OS?

Spooling, or Simultaneous Peripheral Operations On-Line, is a process where data is temporarily stored in a buffer. This processed data is subsequently exported to downstream devices including printers for fulfillment. This provides the device the freedom to execute jobs at its own rate. When you print say 10 documents, they are all queued in the buffer. It’s a busy process but it keeps the gears turning without shutting down the entire OS.

13. What is a file system in an OS?

A file system allows you to store and find data on your hard disk drives, flash memory cards, and other storage media. This entails not only directories and files, but also metadata. Examples of file systems are NTFS, FAT32 and ext4. The moment you save a new photo, the file system jumps to work. Then, it saves the image in a special place so that it can be easily retrieved later by using the image’s name and metadata.

14. Explain inter-process communication (IPC).

IPC provides the means for integral processes to exchange information and coordinate tasks. It uses mechanisms such as shared memory, message passing, and pipes. For example, in a multi-device chat application, IPC lets messages instantly traverse between devices. This keeps communications consistent, natural, and integrated.

15. What are the different types of operating systems?

Operating systems can be classified into several types:

1. Batch OS: Processes batches of tasks without user interaction.
2. Time-Sharing OS: Allows multiple users to share system resources efficiently.
3. Distributed OS: Manages a group of computers as one cohesive system.
4. Real-Time OS: Responds to events within a strict time limit, used in applications like air traffic control.
5. Operating Systems designed for smartphones and tablets, like Android and iOS.

Since each type fulfills distinct needs, OS selection is essential based on intended use cases.

Take your OS knowledge a notch up with our well-curated Operating System course taught by seasoned software engineers!

Intermediate Operating System Questions

1. What is context switching in an OS?

Context switching is the process by which the CPU switches from one process to another. This occurs whenever a process is pre-empted, for example as part of the operating system’s support for multitasking, or when a higher-priority process requires the CPU.

The operating system stores all the information about the current state of the process, including CPU registers, program counter and all other execution information in the process control block (PCB). Then it loads the saved state of the next process to run.

The purpose of context switching on a time-sharing system is to quickly and fairly give each process a fair slice of CPU time. This produces the illusion of concurrency. Although context switching allows for multitasking, it comes with an overhead, for example the time it takes to save and load different states. Reducing this overhead should be a critical component of any performance improvement efforts.

2. Explain preemptive and non-preemptive scheduling.

Preemptive scheduling gives the OS the power to interrupt an active process. It quickly gives the CPU to the next process which usually has the most priority. As an example, Round-Robin and Priority Scheduling are both considered preemptive scheduling. This method works well in time-sharing systems where interactive responsiveness is important, like in UNIX and Linux servers.

In non-preemptive scheduling, a process keeps the CPU until it is done running or voluntarily releases it. This approach delegates complete control of their CPU time to processes. These include First-Come, First-Served (FCFS) and Shortest Job First (SJF). It’s easier to program but results in greater delays for less important tasks.

3. What is the role of a dispatcher in CPU scheduling?

The dispatcher is in charge of passing the CPU to the process that the scheduler has chosen to run next. It performs three main tasks: context switching, switching to user mode, and jumping to the proper location in the program to restart execution.

First, the scheduler picks a process to run based on a scheduling algorithm. Next, the dispatcher steps in to make sure the process begins running without taking its sweet time to kick off. Additionally, CPU performance is directly affected by dispatcher efficiency, as more frequent context switches can incur delays that directly degrade performance.

4. What are the different process states in an OS?

Processes in an operating system transition through various states:

• New: The process is being created.
• The process is waiting in the ready queue for CPU allocation.
• Running: The CPU is executing the process.
• The process is waiting for an I/O operation to complete.
• Terminated: The process has completed execution.

For example when you open a text editor, it starts out in the New state. Once it has initialized, it moves to the “Ready” state. Once you save a file, the activity would go into “Waiting” mode as it reads the storage. Once the operation is complete, it will go back to "Ready."

5. Define multithreading and its benefits.

Multithreading is the ability of a CPU or a single process to execute multiple threads concurrently, sharing the same memory and resources.

Benefits:

1. Improved Performance: Tasks run in parallel, enhancing efficiency.
2. Resource Sharing: Threads share memory, reducing overhead.
3. Responsiveness: Applications remain responsive (e.g., UI doesn’t freeze during background tasks).
4. Scalability: Leverages multi-core processors for faster execution.
5. Simplified Program Structure: Breaks complex tasks into manageable threads.

6. What is multitasking, and how does it work?

Multitasking is the ability of an operating system to execute multiple tasks or processes seemingly simultaneously by switching the CPU among them rapidly. This is done via context switching. In a time-sharing OS, the CPU gives each process a very short time slice. This allocation gives the appearance of executing in parallel.

Multitasking would make sure that resources are not wasted. For instance, if one process is waiting for user input, the CPU can work on a different process in the meantime, improving system performance.

7. What is dynamic loading, and why is it used?

Dynamic loading is a very powerful technique. This is accomplished through dynamic linking, which enables a program to load a module or library into memory only when it needs it during execution. This creates a much smaller memory footprint for a program and makes its first load time significantly faster.

For example, a photo editing application only loads high resolution filters when a user selects one. This method prevents the need to load all features upon startup. This method is particularly advantageous in resource constrained systems, such as those with limited memory.

8. Explain swapping in memory management.

Swapping is a memory management technique that temporarily moves processes from the main memory (RAM) to secondary storage (Hard disk). This process makes room, letting other processes operate without a hiccup. When the swapped-out process is needed once more, it is returned to main memory.

In a multi-programming environment, swapping becomes an important factor. It gives the illusion that the system can support more concurrent processes than physical memory would otherwise permit at any given moment in time. Still, too much swapping can create a performance problem called thrashing.

9. What is the difference between logical and physical addresses?

While a program is executing, the CPU creates a logical address. The physical address is the real place in memory that the data is kept. The Memory Management Unit (MMU) is responsible for translating logical addresses into physical addresses.

Each time a program reads or writes an array element, it still uses the logical address defined in the program. The Memory Management Unit (MMU) then computes the physical address of the data and fetches it. This separation provides stability for programs to run without the hassle of tracking down real memory addresses.

10. What are semaphores, and why are they used?

Semaphores are a type of synchronization primitive that are used to control access to shared resources in a concurrent system. They avoid situations such as race conditions, where a number of threads or processes attempt to access and change the same resource at the same time.

A counting semaphore is a great way to actively control access to a database connection pool. Second, it makes sure that only a finite number of threads can access the pool at the same time. This ensures data integrity and avoids unexpected system behavior.

11. Describe common synchronization problems.

Synchronization problems arise when processes or threads interact without proper controls, leading to issues like:

• Multiple processes modify shared data simultaneously, resulting in unpredictable outcomes.
• Two or more processes wait indefinitely for resources held by each other.
• Starvation: A process waits indefinitely due to resource allocation policies.

For instance, without proper synchronization, two threads incrementing a shared counter could result in one thread overwriting the other’s increment, producing an incorrect result.

12. What are orphan processes, and how do they occur?

An orphan process is when a parent process ends before its child process. In these situations, the orphaned child is adopted by the init process in UNIX/Linux systems.

For example, if a parent process crashes unexpectedly, its children are orphan processes. Operating systems try to keep these orphans from wasting resources.

13. What is the difference between a monolithic kernel and a microkernel?

With a monolithic kernel, all of the necessary services and drivers are included as part of a single, massive codebase. In comparison, a microkernel has a very limited number of core services, with most tasks being handled in user space.

Linux uses a monolithic kernel, for instance, which has better performance thanks to the reduced overhead. At the same time, microkernels, such as those used in Minix, offered improved modularity and stability by isolating services.

14. Explain the purpose of a process control block (PCB).

A Process Control Block (PCB) is a data structure that holds important details about a process. It contains information such as the process’s status, program counter, CPU registers, memory allocation, and I/O state.

Like when a process is context-switched, its PCB makes sure that everything is saved and restored correctly. This prevents processes from attempting to execute with corrupted state.

15. What are the goals of CPU scheduling?

The objectives of CPU scheduling are maximizing CPU and process resource utilization and throughput, reducing waiting time and turnaround time and providing fairness. It further plays a big role in realizing system responsiveness and throughput.

Round-Robin scheduling algorithms guarantee that all processes get their due portion of CPU time. This method increases system performance and improves the user experience.

Advanced Operating System Questions

1. What is demand paging, and how does it work?

Demand paging is a memory management optimization. It only loads data pages from secondary storage into main memory when they’re required by the executing program. This technique is very memory efficient. It does so by only loading the parts of a program that are currently needed into memory, instead of the entire program.

As an illustrative example, imagine a situation in which an application only needs to access a restricted set of modules during its execution. Rather than keep every module loaded in advance, the OS downloads pages as needed when the program loads a file. A page table is used to track which pages are currently in memory. It helps visualize where they sit on the disk. When a process accesses a page that isn’t currently in memory, it results in a page fault. Only then does the operating system jump in to pull the data from storage.

Demand paging has a positive effect on systems with small memory, where improved resource usage is even more valuable. An excess of page faults can drastically degrade the performance of a system, leading to the idea of thrashing discussed further below.

2. Explain the concept of overlays in memory management.

Overlays are an early form of memory management. They let us execute applications that are larger than the size of our physical main memory. The concept is to break a program into smaller, bite-sized pieces known as overlays. As each new overlay is entered, the previous one is evicted from memory.

As an example, in a big enough program, one function could be thought of as calling another. Overlays make the best use of scarce memory by loading only the functions in use at the moment. They continue to store the inactive ones on disk. This technique was a crutch for older systems due to the lack of memory available to them. Thanks to advances in virtual memory, this concern is largely moot.

3. What is thrashing, and what causes it to occur?

Thrashing is occurring when the system is spending more time managing page faults than it is running real processes. When the real memory is overcommitted, it usually means constant thrashing of the pages. This never-ending ping-ponging can hamper our system performance on a system-wide basis.

Imagine a system where multiple processes are competing for memory, but the system runs out of resources to give each process what it needs. As a consequence, page faults skyrocket, and performance tumbles. Thrashing usually only happens on systems with too little RAM or badly configured memory management policies. Fixing thrashing usually requires reducing the number of processes in memory, or adding more physical memory.

4. How does caching improve system performance?

Caching dramatically increases system performance through the temporary storage of often-used data. It leverages a second, smaller, faster layer of memory that’s located much closer to the CPU. This reduces the time needed to access data from much slower non-volatile memory, such as hard disks or even main memory.

A practical example is CPU cache, which stores instructions and data the processor will need next, with a high degree of probability. By minimizing access times, caching greatly increases the speed of processing, especially in applications that often access the same data multiple times.

Real-world implementations frequently utilize multi-tiered caches (L1, L2, L3, etc.) to provide a compromise between high speed and high capacity.

5. What is the role of interrupts in an OS?

Interrupts are signals sent to the CPU to notify it that an important event has happened and needs to be processed right away. They are an unsung, often hidden, critical layer that allows the operating system to effectively leverage hardware and software in a highly efficient manner.

For instance, when you push a button on your keyboard, an interrupt tells the CPU to handle that input. In the same way, the hardware interrupts from devices such as printers or network cards prevent the starving of work from these devices. Software interrupts are generated by software applications to request resources or services from the operating system. Interrupt handling enables the CPU to quickly address urgent matters without having to allow long-running tasks to finish.

6. Explain the GUI and its importance in modern systems.

A Graphical User Interface (GUI) enables users to interact with a system through graphical icons. This is through graphical elements that include windows, icons, and menus as opposed to just text-based commands. It allows for more natural human-computer interaction, opening up powerful new systems to users without deep technical expertise.

Just look at today’s operating systems, whether it’s Windows or macOS. Their GUIs offer logical and easy-to-use navigation, allowing users to easily complete tasks such as opening a file or adjusting a setting. GUIs are particularly useful for maximizing productivity and minimizing the learning curve, which makes them a foundation of intuitive design.

Also Read: GUI vs CUI: Demystifying The Differences Between GUI and CUI

7. How does a RAID system work in an OS?

RAID, or Redundant Array of Independent Disks, is a remarkable data storage technology. It enables you to group several physical drives into a single logical unit for added efficiency and reliability. Its main use case is increased performance, data redundancy, or both, based on the RAID level that is applied.

RAID 0 increases both read and write speeds through a technique called striping, which splits data across multiple drives. In comparison, RAID 1 achieves redundancy through data mirroring. Even higher levels, such as RAID 5 and RAID 6, fuse both of those concepts together, providing redundancy while boosting performance as well. RAID systems are commonly used in servers and data centers to provide reliability and reduce system wide downtime.

8. What is the role of system calls in an operating system?

System calls are the interface between user programs and the operating system. They allow programs to request services like file operations, memory allocation, and process management.

For example, a system call lets a program read data from a file. The OS validates the request, performs the operation, and returns results. This abstraction ensures security and prevents user programs from directly manipulating hardware, thereby maintaining system stability and integrity.

9. What is the difference between cooperative and preemptive multitasking?

In cooperative multitasking, processes voluntarily yield control of the CPU, relying on each process to act fairly. Conversely, preemptive multitasking allows the OS to forcibly switch the CPU between processes based on priority or time slices. Cooperative multitasking is simpler but less robust, as a misbehaving process can monopolize the CPU. Preemptive multitasking ensures better system responsiveness and is the standard for modern operating systems.

10. Explain the concept of memory-mapped I/O.

Memory-mapped I/O assigns a range of memory addresses to I/O devices, allowing the CPU to interact with them using standard memory instructions. For instance, writing to a specific memory location could trigger data transfer to a disk. This approach simplifies programming by treating devices like regular memory, eliminating the need for separate I/O instructions, and is common in modern architectures.

11. What is a monolithic kernel?

A monolithic kernel integrates all essential OS services, such as file systems, device drivers, and process management, into a single large program. While this design offers high performance due to reduced context-switching overhead, it risks stability since a bug in any service can crash the entire OS. Examples include Linux and older versions of Unix.

12. What is a microkernel, and how does it differ from a monolithic kernel?

A microkernel keeps only essential functions, like process and memory management, in kernel mode, delegating other services to user-space modules. This design improves modularity and stability since user-space crashes don’t affect the kernel. However, it incurs performance overhead due to frequent communication between the kernel and user-space modules. Examples include QNX and Minix.

13. Explain the concept of disk scheduling.

Disk scheduling determines the order in which I/O requests are serviced to optimize disk performance. Algorithms like FCFS, SSTF, and SCAN reduce seek time and latency. For instance, SSTF serves the request closest to the current head position, minimizing movement. Effective disk scheduling enhances throughput and response time, crucial for database and server applications.

14. What is a hypervisor in virtualization?

A hypervisor, or virtual machine monitor, enables multiple virtual machines (VMs) to run on a single physical machine by abstracting hardware resources. There are two types: Type 1 (bare-metal) runs directly on hardware, while Type 2 runs atop a host OS. Hypervisors like VMware ESXi and VirtualBox enable efficient resource sharing, isolation, and scalability.

15. How does the OS handle interrupt-driven I/O?

In interrupt-driven I/O, devices notify the CPU via interrupts when ready for data transfer, eliminating the need for continuous polling. For example, a keyboard sends an interrupt when a key is pressed. The OS pauses current execution, services the interrupt via an interrupt handler, and resumes normal operation. This approach enhances efficiency and responsiveness.

16. What are the benefits of journaling in file systems?

Journaling ensures file system integrity by recording changes in a log before applying them. In case of a crash, the log can replay or roll back incomplete operations, preventing data corruption. File systems like ext4 and NTFS use journaling to maintain reliability, particularly in environments with frequent updates or power outages.

17. What is the difference between soft and hard real-time systems?

Soft real-time systems prioritize tasks but tolerate occasional deadline misses, suitable for multimedia and gaming. Hard real-time systems require strict adherence to deadlines, as failures can cause catastrophic results, as in medical or aerospace applications. The OS's ability to guarantee timing constraints differentiates the two.

18. Explain the concept of priority inversion.

Priority inversion occurs when a higher-priority task is waiting for a resource held by a lower-priority task, but a medium-priority task preempts the lower-priority one.

For example, in a rover mission, tasks are typically scheduled based on priority because resources like power, memory, CPU time, or specific hardware are limited. Solutions include priority inheritance, where the lower-priority task temporarily inherits the higher priority.

To explain your answer better, you can state the following example:

Priority Inversion in a Rover Mission

Priority inversion occurs when a high-priority task is waiting for a resource currently held by a lower-priority task, and a medium-priority task interferes by preempting the lower-priority task. This scenario can delay the high-priority task indefinitely.

Example:

1. A critical task (Task A) with high priority needs to use a robotic arm to analyze a rock.
2. A low-priority task (Task B) is holding the robotic arm to perform a routine maintenance operation.
3. Before Task B can finish and release the robotic arm, a medium-priority task (Task C) preempts Task B because it has a higher priority than Task B but lower than Task A.
4. Task C continues running, preventing Task B from completing its operation and releasing the robotic arm for Task A.
5. Task A remains blocked, even though it is the most critical task.

This situation can jeopardize the mission by causing critical operations to miss their deadlines.

Solution: Priority Inheritance

One effective solution to priority inversion is priority inheritance. In this mechanism:

• The lower-priority task (Task B) temporarily "inherits" the priority of the blocked higher-priority task (Task A).
• Task B is allowed to finish its operation without being preempted by medium-priority tasks (like Task C).
• Once Task B releases the resource (the robotic arm), it reverts to its original lower priority.

In Practice:

• Task B (low-priority) inherits the priority of Task A (high-priority).
• Task C (medium-priority) cannot preempt Task B because Task B now has high priority.
• Task B finishes its use of the robotic arm and releases it.
• Task A immediately gains access to the robotic arm and completes its critical operation.
• Task B reverts to its original low priority.

19. What is the purpose of the Master Boot Record (MBR)?

The MBR is a small section on a storage device containing bootloader code and partition table information. It directs the system to load the OS during startup. If the MBR is corrupted, the system may fail to boot. Modern systems often use GUID Partition Table (GPT) for more features and reliability.

20. Explain the concept of real-time scheduling.

Real-time scheduling ensures tasks meet strict deadlines by prioritizing time-sensitive processes. Algorithms like Rate Monotonic Scheduling (RMS) or Earliest Deadline First (EDF) are commonly used.

For instance, in an embedded system controlling industrial machinery, real-time scheduling ensures actions occur within predefined timeframes, maintaining system accuracy and safety.

Conclusion

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