Operation System

What do you mean by an operating system?

An Operating System (OS) is system software that manages computer hardware and software resources and provides common services for programs.

• Manages CPU, memory, storage, I/O devices, and processes.
• Schedules tasks and allocates resources fairly and efficiently.
• Provides abstractions (files, processes, threads) and security.
• Acts as an intermediary between users/applications and hardware.

Scheduling Algorithm

Common CPU scheduling algorithms:

• FCFS (First Come First Serve): processes execute in arrival order; easy to implement (FIFO queue) but may have high average waiting time.
• SJF/SJN (Shortest Job First/Next): runs the process with the smallest burst time; minimizes average waiting time; may cause starvation for long jobs.
• Priority Scheduling: runs the highest priority process first; equal priorities use FCFS; risk of starvation for low-priority processes (can use aging).
• SRTF (Shortest Remaining Time First): preemptive version of SJF; always runs the job with the shortest remaining time; good for short jobs; can preempt longer jobs.
• Round Robin (RR): each process gets a fixed time slice (quantum); preempted after quantum expires; improves responsiveness in time-sharing systems.

Difference between Counting and Binary Semaphores

• Binary semaphore: takes values 0/1; used for mutual exclusion (mutex-like behavior).
• Counting semaphore: holds a non-negative integer; controls access to a resource pool with multiple instances.
• Binary semaphores are simpler and enforce exclusive access; counting semaphores manage capacity.

What is Deadlock?

Deadlock occurs when processes are blocked, each holding a resource and waiting for others to release theirs.

Necessary conditions (Coffman):

• Mutual exclusion
• Hold and wait
• No preemption
• Circular wait

Ram vs Cache

• Cache: very fast, small memory close to the CPU; stores frequently used data/instructions; organized in levels (L1/L2/L3).
• RAM: larger main memory; stores programs and data currently in use; slower than cache but faster than disk.

Thread vs Process

• Process: an executing program with its own memory space.
• Thread: lightweight execution unit within a process; shares process memory.

Multi-thread vs multi-process

• Multiprocessing: multiple processes (often across CPUs/cores) with isolated memory; improves parallelism and isolation; IPC required.
• Multithreading: multiple threads within one process sharing memory; lower overhead; easier sharing; needs careful synchronization.

Virtual Memory (VM), VM paging and Page Fault

• Virtual Memory: abstraction that gives processes the illusion of a large, contiguous memory space; enables memory protection and isolation.
• Paging: divides virtual memory into fixed-size pages mapped to physical frames; avoids external fragmentation.
• Page Fault: occurs when a process accesses a page not in physical memory; OS loads the page from disk (swap/page file) into RAM.

Ram vs Rom

• RAM (Random Access Memory): volatile; temporary working memory for active programs/data.
• ROM (Read-Only Memory): non-volatile; permanently stores firmware/instructions.

Solution to the Critical Section Problem

• A correct solution must satisfy:
  • Mutual Exclusion: only one process in the critical section at a time.
  • Progress: if no process is in the critical section, selection of the next entrant cannot be postponed indefinitely.
  • Bounded Waiting: each process has a bounded waiting time before entering the critical section.