Operating Systems Interview Questions and Answers

A process is a running program with its own virtual address space, code, data, heap, open file descriptors and at least one thread. The operating system isolates processes from one another, so a crash in one usually does not corrupt another.

A thread is the unit of scheduling inside a process. Threads in the same process share the address space, heap, globals and file descriptors, but each has its own stack, registers and program counter.

Process A
  code + data + heap + file table
  thread 1 (stack, regs)   thread 2 (stack, regs)

Because threads share memory, communication is cheap but synchronisation is required; a data race can corrupt shared state. Processes are isolated, so they are safer but communicating between them needs IPC such as pipes, shared memory or sockets, which is slower and more complex. Creating a process is heavier than creating a thread, so thread pools are common for handling many concurrent requests.

CPUs have at least two privilege levels. In user mode, code cannot execute privileged instructions or touch hardware or protected memory. In kernel mode, the OS kernel can access devices, page tables and privileged registers.

Applications request services through system calls, the controlled entry point into the kernel: read, write, open, fork, mmap and so on. A system call switches to kernel mode, validates arguments, runs the operation and returns to user mode.

ssize_t n = read(fd, buf, size); // traps into the kernel

The boundary matters for performance: each call has overhead, so buffered I/O and batching reduce the number of transitions. It also matters for safety: a bug in user code cannot directly corrupt the kernel, and the kernel checks every pointer and permission. This separation is the foundation of process isolation and of security mechanisms such as address space layout randomisation.

An operating system manages hardware and provides abstractions so programs do not talk to devices directly. Its main responsibilities are:

• Process management: create, schedule, suspend and terminate processes and threads, and provide synchronisation.
• Memory management: allocate memory, maintain virtual address spaces and decide what stays in RAM.
• File systems: organise persistent data into files and directories with permissions.
• Device and I/O management: provide drivers, buffering and a uniform interface to disks, network cards and peripherals.
• Security and protection: isolate processes, enforce access control and authenticate users.
• User interface and services: shells, system calls and utilities.

apps -> system calls -> kernel -> hardware

It balances competing goals: fairness and throughput in scheduling, latency versus utilisation in I/O, and safety versus performance. Examples include Linux and Windows for general use, and real-time kernels where predictable timing is the priority.