Operating System – Complete Study Notes (Exam Edition)

Operating System — Complete Study Notes

Simple, exam-focused notes for SSC, Banking (IBPS/SBI/RBI), Railway (RRB), UPSC, and other government exams. Built for fast learning and last-minute revision.

★ Concepts + Tables ★ Full Forms ★ 70+ Solved MCQs ★ Quick Revision

1 OS Basics & Definition

The single most common starting topic in every computer-awareness section.

Definition An Operating System (OS) is system software that acts as an interface between the user and computer hardware. It manages all hardware and software resources and lets the user run programs. Without an OS, a computer cannot function.

OS is the first program loaded into memory after booting and the last to close at shutdown.
It is also called a resource manager or master control program.
It runs in kernel mode (privileged), while user apps run in user mode.

Common Examples (memorise the maker)

OS	Made By	Used In	Type
Windows	Microsoft	PCs / Laptops	GUI, Desktop
Linux	Linus Torvalds (open source)	Servers, supercomputers	Open-source, Multi-user
macOS / iOS / iPadOS	Apple	Mac, iPhone, iPad	Unix-based
Android	Google	Smartphones	Linux-kernel based, Mobile
UNIX	Bell Labs	Servers, workstations	Multi-user
MS-DOS	Microsoft	Old PCs	CLI, Single-user
Chrome OS	Google	Chromebooks	Linux-based, Cloud

Exam Tip The most-used desktop OS is Windows; the most-used mobile OS in the world is Android; servers/supercomputers mostly run Linux/Unix. These three facts appear repeatedly.

2 Functions of an Operating System

Function	What it does	Example
Process Management	Creates, schedules & ends running programs	Chrome + Word open together
Memory Management	Allocates & frees RAM for programs	Gives memory to each app
File Management	Stores, names, organises files/folders	Create, copy, delete, rename
Device (I/O) Management	Controls input/output via drivers	Keyboard, printer, mouse
Security & Protection	Controls access & protects data	Passwords, permissions
User Interface	Lets user interact	GUI icons / CLI commands
Error Detection	Finds & reports problems	“Not responding” message
Resource Allocation	Shares CPU, memory & devices fairly	CPU time-sharing

Remember A quick way to recall functions: “PMFDS-URE” → Process, Memory, File, Device, Security, User-interface, Resource-allocation, Error-handling.

3 Components: Kernel & Shell

Kernel The core / heart of the OS. It directly controls hardware, CPU, memory and devices. It always stays in memory.

Shell The interface between the user and the kernel. It takes user commands and passes them to the kernel (e.g. command interpreter in Linux/DOS).

Types of Kernel (frequently asked)

Kernel Type	Idea	Example
Monolithic	Whole OS runs in kernel space; fast but large	Linux, UNIX
Microkernel	Only basics in kernel; services in user space; stable & secure but slower	QNX, Minix, L4
Hybrid	Mix of both	Windows NT, macOS (XNU)

Exam Tip Other key terms: Bootloader (loads OS into memory), Device Driver (software that controls a hardware device), System Call (the way a program requests a service from the OS, e.g. fork(), read(), write()).

4 Types of Operating Systems

Type	Meaning	Example
Batch OS	Similar jobs grouped & run in batches; no user interaction	Old payroll / bank processing
Time-Sharing OS	Many users share CPU using small time slices	Mainframes, Multics
Real-Time OS (RTOS)	Gives output within a strict time limit	Flight control, missiles, ATMs, medical devices
Distributed OS	Many connected computers act as one system	LOCUS, cloud systems
Multi-user OS	Many users use one system together	UNIX, Linux
Multiprocessing OS	Uses two or more CPUs together	Servers, modern PCs
Mobile OS	OS for smartphones/tablets	Android, iOS
Network OS	Manages computers on a network	Windows Server, Novell NetWare
Embedded OS	Runs inside dedicated small devices	VxWorks, washing machines, smart TVs

Common Trap RTOS is the one with guaranteed/immediate response — used in flight control, ATMs and medical equipment. Don’t confuse it with Distributed OS (multiple computers as one).

5 Process Management & Threads

Process A program in execution (a running program). A thread is the smallest unit of execution — a lightweight part of a process.

Process States

NEW  →  READY  →  RUNNING  →  TERMINATED
                ↑          │
                └──────────┘   (back to READY when time slice ends)
RUNNING → WAITING/BLOCKED (for I/O) → READY

PCB (Process Control Block): stores all info about a process (ID, state, registers, priority).
Context switching: CPU saves one process’s state and loads another’s.
fork(): system call used to create a new (child) process.
Daemon: a background process (e.g. antivirus scan, print service).

Multi- terms (very frequently confused)

Term	Meaning	Example
Multitasking	One CPU runs many tasks (by switching fast)	Music + browsing
Multiprocessing	Many CPUs work at the same time	Quad-core processor
Multithreading	Many threads run inside one program	Download + scroll in a browser
Multiprogramming	Many programs kept in memory to keep CPU busy	Improves CPU utilisation

6 CPU Scheduling Algorithms

Scheduling decides which process gets the CPU next. Done by the scheduler; the dispatcher actually hands over the CPU.

Algorithm	Rule	Preemptive?	Key Point
FCFS	First Come First Served	No	Simple; suffers “convoy effect”
SJF	Shortest Job First	No	Lowest average waiting time; risk of starvation
SRTF	Shortest Remaining Time First	Yes	Preemptive version of SJF
Round Robin	Each process gets a fixed time quantum	Yes	Best for time-sharing / interactive systems
Priority	Highest priority runs first	Both	Starvation solved by “aging”

Exam Tip Preemptive: Round Robin, SRTF, Preemptive-Priority. Non-preemptive: FCFS, basic SJF. A Gantt chart is used to show scheduling order.

7 Synchronization & Deadlock

Critical Section: code that accesses a shared resource.
Race Condition: output depends on the timing/order of processes accessing shared data.
Mutual Exclusion: only one process uses a resource at a time.
Semaphore / Mutex: tools used to achieve synchronization.
Starvation: a process waits indefinitely for the CPU/resource.

Deadlock A situation where two or more processes wait for each other forever, so none can proceed.

4 Necessary Conditions for Deadlock (Coffman conditions)

Condition	Meaning
Mutual Exclusion	Resource can be used by only one process at a time
Hold and Wait	Process holds one resource & waits for another
No Preemption	Resource cannot be forcibly taken away
Circular Wait	Processes form a circular chain of waiting

Remember All four conditions must hold for a deadlock. Banker’s Algorithm is used for deadlock avoidance.

8 Memory Management

Memory Hierarchy (fast → slow)

Registers  →  Cache  →  RAM  →  SSD  →  Hard Disk
(fastest, smallest, costliest) ........ (slowest, largest, cheapest)

RAM – volatile, temporary, primary memory. ROM – non-volatile, permanent (stores BIOS).
Cache – very fast memory between CPU and RAM for frequently-used data.
Virtual Memory – uses part of the hard disk as extra RAM (swap / page file).

Paging vs Segmentation

Paging

Memory split into fixed-size pages/frames
Causes internal fragmentation
Uses a page table

Segmentation

Memory split into variable-size logical segments (code, data, stack)
Causes external fragmentation
Matches programmer’s view

Key Memory Terms

Term	Meaning
Page Fault	Required page is not in RAM → must be loaded from disk
Thrashing	Too much paging/swapping → CPU utilisation drops sharply
TLB	Translation Lookaside Buffer — speeds up address translation
MMU	Memory Management Unit — converts virtual→physical address
Demand Paging	Load a page only when it is actually needed
Belady’s Anomaly	More frames → more page faults (occurs in FIFO)

Page Replacement Algorithms

FIFO     → removes the oldest loaded page (can show Belady's anomaly)
LRU      → removes the Least Recently Used page
Optimal  → removes the page not needed for the longest future time
            (lowest faults — but only theoretical)

9 File & Device Management

File System Basics

Term	Meaning
File	Named collection of related data
Directory / Folder	Container that stores files
Path	Address of a file (absolute = from root; relative = from current folder)
File Extension	Type of file: .exe, .docx, .txt, .jpg
FAT / NTFS	Windows file systems (NTFS = New Technology File System)
ext4	Common Linux file system

Exam Tip File allocation methods: Contiguous, Linked, Indexed. Directory structures: Single-level, Two-level, Tree, Acyclic-graph.

Device (I/O) Management

Term	Meaning
Device Driver	Software that lets the OS control a hardware device
Spooling	Simultaneous Peripheral Operations On-Line — queues jobs (esp. printing)
Buffering	Temporary storage to match speeds of CPU and device
DMA	Direct Memory Access — data transfer without using the CPU
Interrupt	Signal that tells the CPU to pause and attend a device
RAID	Redundant Array of Independent Disks — reliability/speed

Remember Disk scheduling algorithms: FCFS, SSTF (Shortest Seek Time First), SCAN (elevator), C-SCAN, LOOK.

10 Booting (BIOS / UEFI)

Power ON → BIOS/UEFI → POST (self-test) → Bootloader → OS loaded into RAM → Ready

Term	Meaning
BIOS	Basic Input Output System — firmware that starts the PC (stored in ROM)
UEFI	Modern replacement for BIOS (faster, supports large disks)
POST	Power-On Self-Test — checks hardware at startup
Bootloader	Loads the OS into memory (e.g. GRUB in Linux)
Cold Boot	Starting the computer from power-off
Warm Boot	Restarting an already-on computer (Ctrl+Alt+Del)

11 OS Security & Protection

A high-value area as exams increasingly add computer-security questions.

Authentication — verifying identity (password, OTP, biometric, smart card).
Authorization — deciding what an authenticated user is allowed to do.
Access Control — file permissions Read / Write / Execute (rwx); uses ACL (Access Control List).
Encryption — scrambling data so only authorised users can read it.
Firewall — filters network traffic to block unauthorised access.
User mode vs Kernel mode — protects core OS from faulty/malicious programs.
Sandboxing — running apps in an isolated space.

Common OS-Level Threats (quick definitions)

Threat	Meaning
Virus	Malicious code that attaches to files & spreads when run
Worm	Self-replicating malware that spreads on its own (no host file)
Trojan	Harmful program disguised as useful software
Ransomware	Locks/encrypts files & demands payment
Spyware	Secretly collects user information
Privilege Escalation	Gaining higher access rights than allowed

Common Trap A virus needs a host file and human action to spread; a worm spreads by itself over a network. This difference is frequently asked.

12 Important Full Forms

Abbr.	Full Form	Abbr.	Full Form
OS	Operating System	BIOS	Basic Input Output System
GUI	Graphical User Interface	UEFI	Unified Extensible Firmware Interface
CLI	Command Line Interface	POST	Power-On Self-Test
RAM	Random Access Memory	ROM	Read Only Memory
FCFS	First Come First Served	SJF	Shortest Job First
RR	Round Robin	RTOS	Real-Time Operating System
PCB	Process Control Block	DMA	Direct Memory Access
TLB	Translation Lookaside Buffer	MMU	Memory Management Unit
FAT	File Allocation Table	NTFS	New Technology File System
SSTF	Shortest Seek Time First	RAID	Redundant Array of Independent Disks
SPOOL	Simultaneous Peripheral Operations On-Line	ACL	Access Control List
IPC	Inter-Process Communication	GRUB	GRand Unified Bootloader

13 Inventors & History Facts

Fact	Answer
UNIX developed at	Bell Labs — by Ken Thompson & Dennis Ritchie (≈1969)
Linux created by	Linus Torvalds (1991)
Windows created by	Microsoft — first version (Windows 1.0) in 1985
C language created by	Dennis Ritchie
macOS / iOS made by	Apple
Android made by	Google (now Open Handset Alliance)
Most OSes are written in	C language

14 Comparison Charts

GUI

Uses icons, windows, mouse
Easy for beginners
Examples: Windows, Android

CLI

Uses typed commands
Fast & powerful for experts
Examples: MS-DOS, Linux terminal

RAM

Volatile (data lost on power-off)
Read + Write
Temporary working memory

ROM

Non-volatile (permanent)
Mostly read-only
Stores BIOS / firmware

Process

Running program
Has its own memory
Heavyweight

Thread

Part of a process
Shares process memory
Lightweight

🧠 Solved MCQs — Based on Exam Trends

Curated, high-frequency questions with answers + one-line explanations. Click “Show Answer” to self-test.

Set A · Basics & Types

1. An Operating System is a type of:

A) Application Software
B) System Software
C) Firmware
D) Utility Software

Show Answer

B) System Software. The OS manages hardware/software resources, unlike application software (e.g. MS Word) which performs user tasks.

2. Which of the following is NOT an operating system?

A) Linux
B) Windows
C) Android
D) Oracle

Show Answer

D) Oracle. Oracle is a database management system (DBMS), not an OS.

3. The core component of an OS is called:

A) Shell
B) Kernel
C) Application
D) Processor

Show Answer

B) Kernel. The kernel is the heart of the OS that controls hardware. The shell is the user interface to the kernel.

4. Android OS is based on:

A) UNIX
B) Windows
C) Linux kernel
D) Macintosh

Show Answer

C) Linux kernel. Android is built on a modified Linux kernel and is owned by Google.

5. Which OS is an example of open-source software?

A) Windows
B) macOS
C) Linux
D) iOS

Show Answer

C) Linux. Linux source code is freely available and modifiable.

6. The program that loads the OS into memory during startup is the:

A) Compiler
B) BIOS
C) Bootloader
D) Kernel

Show Answer

C) Bootloader. BIOS/UEFI starts the machine, then the bootloader loads the OS into RAM.

7. UNIX was originally developed by:

A) Microsoft
B) Bell Labs
C) IBM
D) Intel

Show Answer

B) Bell Labs — by Ken Thompson and Dennis Ritchie.

8. Restarting a computer without turning off the power is called:

A) Hard boot
B) Warm boot
C) Cold boot
D) Power boot

Show Answer

B) Warm boot. Cold boot = starting from power-off; warm boot = restart of a running system.

9. The OS used in ATM machines and flight control is usually:

A) Batch OS
B) Real-Time OS
C) Distributed OS
D) Multi-user OS

Show Answer

B) Real-Time OS. RTOS guarantees response within a strict time limit.

10. Which is NOT a function of an OS?

A) Memory management
B) Process management
C) File handling
D) Weather forecasting

Show Answer

D) Weather forecasting. It is an application task, not an OS function.

Set B · Process & Scheduling

11. A process in the “ready” state means it is:

A) Waiting for I/O
B) Currently executing
C) Waiting to be assigned the CPU
D) Completed

Show Answer

C) Waiting to be assigned the CPU. It is loaded and ready, just waiting for CPU time.

12. The scheduling method used in time-sharing systems is:

A) FCFS
B) Round Robin
C) SJF
D) LIFO

Show Answer

B) Round Robin. Each process gets a fixed time quantum — ideal for interactive systems.

13. Switching the CPU from one process to another is called:

A) Swapping
B) Paging
C) Context switching
D) Spooling

Show Answer

C) Context switching. The state of the current process is saved and the next process’s state is loaded.

14. The system call used to create a new process is:

A) init()
B) fork()
C) create()
D) load()

Show Answer

B) fork(). It creates a child process in UNIX/Linux.

15. Banker’s Algorithm is used for:

A) Booting
B) Deadlock avoidance
C) Page replacement
D) Process creation

Show Answer

B) Deadlock avoidance. It checks whether allocating resources keeps the system in a “safe state”.

16. Which is NOT one of the four conditions for deadlock?

A) Mutual exclusion
B) Hold and wait
C) Circular wait
D) Fast switching

Show Answer

D) Fast switching. The four Coffman conditions are mutual exclusion, hold & wait, no preemption, and circular wait.

17. When a process waits indefinitely for resources, it is called:

A) Deadlock
B) Starvation
C) Thrashing
D) Buffering

Show Answer

B) Starvation. It is solved using “aging” (gradually raising waiting processes’ priority).

18. Multiple CPUs working together is called:

A) Multithreading
B) Multiprocessing
C) Multitasking
D) Multiprogramming

Show Answer

B) Multiprocessing. Two or more processors execute simultaneously.

Set C · Memory Management

19. The fastest memory among these is:

A) Hard disk
B) RAM
C) Cache
D) Pen drive

Show Answer

C) Cache. Among the options, cache is fastest (CPU registers are faster still, but not listed).

20. Virtual memory means:

A) Backup memory
B) Using disk space as extra RAM
C) Flash memory
D) Cache memory

Show Answer

B) Using disk space as extra RAM. It lets large programs run with limited physical RAM.

21. Paging divides memory into:

A) Variable segments
B) Fixed-size pages/frames
C) Logical blocks
D) Sectors

Show Answer

B) Fixed-size pages/frames. Paging causes internal fragmentation; segmentation causes external fragmentation.

22. Thrashing occurs due to:

A) Too many files
B) Excessive paging/swapping
C) A booting error
D) Disk crash

Show Answer

B) Excessive paging/swapping. The CPU spends more time swapping pages than executing — utilisation drops.

23. A page fault occurs when:

A) The disk crashes
B) The required page is not in RAM
C) The CPU overheats
D) A program ends

Show Answer

B) The required page is not in RAM. It must then be fetched from disk.

24. Belady’s anomaly is associated with which page replacement algorithm?

A) LRU
B) FIFO
C) Optimal
D) LFU

Show Answer

B) FIFO. Here, increasing the number of frames can surprisingly increase page faults.

25. TLB stands for:

A) Table Lookup Buffer
B) Translation Lookaside Buffer
C) Time Local Buffer
D) Transfer Level Buffer

Show Answer

B) Translation Lookaside Buffer. It caches recent address translations to speed up paging.

Set D · File & Device Management

26. NTFS stands for:

A) Network Table File System
B) New Technology File System
C) New Terminal Format System
D) National Transfer File System

Show Answer

B) New Technology File System — the modern Windows file system supporting permissions & journaling.

27. Spooling is mainly used with:

A) Monitor
B) Printer
C) Scanner
D) Mouse

Show Answer

B) Printer. Spooling queues print jobs so the CPU need not wait for the slow printer.

28. A device driver is:

A) A hardware component
B) Software that controls hardware
C) A data file
D) A type of shell

Show Answer

B) Software that controls hardware and lets the OS communicate with a device.

29. The DOS command to list files in a directory is:

A) LIST
B) DIR
C) SHOW
D) FILES

Show Answer

B) DIR. (In Linux, the equivalent is ls.)

30. DMA stands for:

A) Direct Memory Address
B) Direct Memory Access
C) Dynamic Memory Allocation
D) Data Memory Activity

Show Answer

B) Direct Memory Access. It transfers data between device and memory without the CPU, freeing it for other work.

31. RAID stands for:

A) Random Access Input Drive
B) Redundant Array of Independent Disks
C) Remote Access Internet Disk
D) Real-time Array Integrated Device

Show Answer

B) Redundant Array of Independent Disks — used for data reliability and improved performance.

Set E · Security & Misc

32. A firewall protects mainly against:

A) Hardware failure
B) Unauthorised network access
C) Disk corruption
D) Power cuts

Show Answer

B) Unauthorised network access. It filters incoming/outgoing traffic by rules.

33. Self-replicating malware that spreads without a host file is a:

A) Virus
B) Worm
C) Trojan
D) Cookie

Show Answer

B) Worm. Unlike a virus, a worm spreads by itself across networks.

34. File permissions Read/Write/Execute are managed using:

A) Page table
B) Access Control List (ACL)
C) Gantt chart
D) Bootloader

Show Answer

B) Access Control List (ACL). It defines what each user can do with a file.

35. A virtual machine allows:

A) Running without an OS
B) Running multiple OSes on one hardware
C) Running only BIOS
D) Running RAM only

Show Answer

B) Running multiple OSes on one hardware. A hypervisor creates isolated virtual machines.

🎯 Expected / High-Probability MCQs

Newer and likely-to-appear questions on modern OS and security concepts.

1. UEFI is best described as:

A) A type of RAM
B) A modern replacement for BIOS firmware
C) A scheduling algorithm
D) A file system

Show Answer

B) A modern replacement for BIOS. UEFI boots faster and supports very large hard disks.

2. Which kernel type is used by the Linux operating system?

A) Microkernel
B) Monolithic kernel
C) Exokernel
D) No kernel

Show Answer

B) Monolithic kernel. The entire core OS runs in kernel space. (Windows/macOS use hybrid kernels.)

3. Malware that encrypts a user’s files and demands payment is called:

A) Adware
B) Spyware
C) Ransomware
D) Rootkit

Show Answer

C) Ransomware. A leading modern cyber threat that targets individuals and organisations.

4. Running an application in an isolated environment to limit damage is called:

A) Spooling
B) Sandboxing
C) Paging
D) Mounting

Show Answer

B) Sandboxing. It contains a program so it cannot affect the rest of the system.

5. Which provides the interface between an application program and the OS?

A) System call
B) Compiler
C) Firewall
D) Bootloader

Show Answer

A) System call. Programs request OS services (file, memory, process) through system calls.

6. In a multi-core processor, the OS feature that most directly uses extra cores is:

A) Spooling
B) Multiprocessing
C) Demand paging
D) Defragmentation

Show Answer

B) Multiprocessing. Multiple cores execute different tasks at the same time.

7. Two-factor authentication usually combines a password with:

A) A second password
B) An OTP / biometric
C) A faster CPU
D) A larger disk

Show Answer

B) An OTP / biometric. It mixes “something you know” with “something you have/are” for stronger security.

8. The elevator algorithm in disk scheduling is also known as:

A) FCFS
B) SSTF
C) SCAN
D) LRU

Show Answer

C) SCAN. The disk head moves in one direction servicing requests, then reverses — like a lift.

📄 One-Page Quick Revision

Concept	One-line Recall
OS	Interface between user & hardware (system software)
Kernel	Heart/core of OS that controls hardware
Shell	User’s command interface to the kernel
Bootloader	Loads OS into memory at startup
Cold / Warm boot	Start from power-off / restart while running
RTOS	Guaranteed response time (ATM, flight control)
Process / Thread	Running program / lightweight part of a process
Round Robin	Time-quantum scheduling for time-sharing
Deadlock	Processes wait for each other forever (4 conditions)
Banker’s Algorithm	Deadlock avoidance
Virtual Memory	Disk used as extra RAM
Paging / Segmentation	Fixed pages (internal frag.) / variable segments (external frag.)
Thrashing	Excessive swapping; CPU use drops
Spooling	Job queue, mainly for printers
DMA	Data transfer without CPU
Firewall	Blocks unauthorised network access
Virus vs Worm	Virus needs host + action; worm spreads itself

5-Second Memory Hooks Kernel = Heart · Shell = Speaker · RTOS = Fast & on-time · Deadlock = Stuck forever · Virtual Memory = Disk pretending to be RAM.

✦ Operating System — Complete Study Notes · GyanDesk · Made for quick learning & smart revision ✦

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