Top 20 Operating System (OS) Interview Questions

An OS is a software program that acts as an intermediary between the computer hardware and the user. Its main functions include managing memory, executing processes, allocating resources, and providing a platform for applications to run.

A system call is a programmatic way for a user application to request a privileged service or resource directly from the OS kernel, such as creating a file or allocating memory.

User mode is a restricted execution environment where standard applications run to prevent them from directly altering hardware. Kernel mode is a highly privileged state where the OS has unrestricted access to system hardware and memory to execute critical tasks.

In a monolithic kernel, all core OS services (like file management and device drivers) run in the single kernel space, which makes it fast but means a single service crash can take down the whole system. A microkernel runs only absolute essentials in kernel space and moves other services to user space, maximizing stability.

A process is an executing program that has its own independent memory space. A thread is a lightweight unit of execution that lives inside a process. Threads share the same memory space as their parent process, making thread creation and context switching much faster than process operations.

A PCB is a data structure maintained by the OS for every running process. It stores crucial context about the process, including its current state, program counter, CPU registers, and memory management information.

Context switching is the procedure where the OS saves the state of a currently running process or thread and loads the state of the next one to be executed. It enables multitasking but introduces a slight CPU overhead because the system isn't doing useful work during the switch.

CPU scheduling is how the OS decides which process in the ready queue gets to use the CPU next. Round Robin is a fair, preemptive scheduling algorithm that assigns a fixed time slice to each process in a cyclic order, ensuring no single process starves.

An orphan process is a child process whose parent has finished or terminated, so it is adopted by the init process. A zombie process has finished executing, but its entry remains in the process table because the parent hasn't yet read its exit status.

Virtual memory is a memory management technique that uses physical disk space to simulate additional RAM. It allows the OS to run applications that are larger than the available physical memory by swapping data between the disk and RAM as needed.

Paging eliminates the need for contiguous physical memory by dividing memory into fixed-size blocks called pages and frames. Segmentation divides memory into logical, variable-sized segments based on user views, such as functions, objects, or data arrays.

Demand paging is a lazy-loading concept where pages are only brought into RAM when an application actually needs them. A page fault occurs when a program tries to access memory that is mapped into its virtual address space but isn't currently loaded in physical RAM.

Thrashing occurs when the physical memory is overcommitted, causing the CPU to spend more time frantically swapping pages in and out of the disk than executing actual program instructions.

Internal fragmentation happens when a process is allocated a memory block larger than it needs, leaving wasted space inside that block. External fragmentation occurs when there is enough total free memory in the system to satisfy a request, but it is scattered in non-contiguous small blocks.

Belady's Anomaly is a counterintuitive phenomenon in memory management where increasing the number of physical memory frames actually increases the number of page faults. It most commonly happens when using the First-In-First-Out (FIFO) page replacement algorithm.

A deadlock is a system freeze that happens when a set of processes are permanently blocked because each process is holding a resource while waiting for another resource held by someone else in the cycle.

A deadlock can only occur if these four conditions happen simultaneously: mutual exclusion, hold and wait, no preemption, and circular wait.

Banker's Algorithm is a strategy used by the OS for resource allocation and deadlock avoidance. It tests resource requests by simulating the maximum possible allocation and only grants the request if it leaves the system in a safe state.

A critical section is the part of a program's code where shared memory or variables are accessed and modified. A race condition is a software bug that occurs when multiple threads try to write to that critical section simultaneously, leading to unpredictable results based on which thread finishes first.

Both are synchronization mechanisms used to protect critical sections and prevent race conditions. A mutex is a strict locking mechanism that ensures only one thread can access a resource at a time. A semaphore is an integer variable that acts as a counter, allowing a specified number of threads to access a pool of resources.