Operating System
An operating system is the most important software on a computer. It acts as a bridge between the user and the hardware — managing all resources so that programs can run smoothly. Without an OS, a computer is just a collection of electronic parts.
A. What is an Operating System?
Definition & Role
The software that manages everything
| Point | Detail |
|---|---|
| Definition | An Operating System (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. It is the first software loaded when a computer starts. |
| Also called | System software · Resource manager · Interface between user and hardware |
| Position in system | Sits between the hardware (CPU, RAM, disk) and the application software (MS Word, Chrome). Applications talk to the OS; the OS talks to the hardware. |
| Why needed? | Without an OS, every application would have to include its own code to control the keyboard, screen, disk — an impossible duplication. The OS handles all this once, centrally. |
| Two main parts | Kernel — the core of the OS that directly controls hardware. Shell — the interface through which users interact with the kernel (can be CLI or GUI). |
| Loaded by | BIOS/UEFI firmware loads the OS from storage into RAM during the booting process. |
Definition
System software that manages hardware and software resources. First program loaded when computer starts.
Also Called
Resource Manager · System Software · Interface between user and hardware
Position
Sits between hardware and application software. Apps talk to OS; OS talks to hardware.
Why Needed?
Without an OS, every application would need its own code to control keyboard, screen, disk. OS handles this centrally — once.
Two main parts
KernelCore — directly controls hardware
ShellInterface for user interaction (CLI or GUI)
Loaded by
BIOS/UEFI firmware loads the OS from storage into RAM during the booting process.
⚡ Exam Tips — OS = system software (not application software). Kernel = core of OS. Shell = user interface of OS. BIOS loads OS during booting. OS is also called resource manager.
B. Functions of an Operating System
Five Core Functions
What the OS does behind the scenes
| Function | What it Does | Example |
|---|---|---|
| Process Management | Creates, schedules, and terminates processes (running programs). Decides which process gets CPU time and for how long. Handles multitasking — running multiple programs at once. | You can play music and browse the web simultaneously |
| Memory Management | Allocates RAM to each running program. Keeps track of which memory is used and which is free. Uses virtual memory (using disk as extra RAM) when physical RAM is insufficient. | Chrome gets 500MB of RAM; Word gets 200MB |
| File System Management | Organises files into directories/folders. Controls who can read, write, or execute each file. Manages storage devices (HDD, SSD, USB). | Creating, copying, deleting, renaming files and folders |
| Device Management | Controls input/output devices using device drivers. Acts as intermediary between hardware and software. Handles device requests in an orderly queue. | Sending a print job to the printer; reading from keyboard |
| Security & Access Control | Authenticates users (passwords, biometrics). Prevents unauthorised access to files and resources. Protects processes from interfering with each other. | Login passwords; file permissions; user accounts |
| User Interface | Provides either a GUI (Graphical User Interface — mouse, windows, icons) or CLI (Command Line Interface — text commands) for users to interact with the system. | Windows desktop (GUI); Linux terminal (CLI) |
Process Management
Creates, schedules, and terminates processes. Enables multitasking — running multiple programs at once.
Memory Management
Allocates RAM to programs. Uses virtual memory (disk as extra RAM) when physical RAM is full.
File System Management
Organises files into folders. Controls who can read/write/execute each file. Manages storage devices.
Device Management
Controls I/O devices via device drivers. Handles keyboard, printer, screen, USB, etc.
Security
Authenticates users via passwords. Controls file permissions. Prevents unauthorised access.
User Interface
Provides GUI (windows, icons, mouse) or CLI (text commands) for users to interact with the system.
⚡ Exam Tip — "OS manages" = Process, Memory, File, Device, Security. Multitasking = multiple programs running at once = process management. Device drivers = device management. Virtual memory = using disk as RAM = memory management.
C. Types of Operating Systems
Seven Types of OS
Classified by how they process jobs and serve users
| Type | Description | Example |
|---|---|---|
| Batch OS | Jobs (programs) are collected in batches and processed one after another without user interaction. No immediate response — user submits job and waits for results. Used in 1st/2nd generation computers. | IBM 7094, early mainframe jobs (payroll, billing) |
| Time-Sharing OS | Multiple users share the same computer simultaneously. CPU time is divided into small slices given to each user in turn — so fast it appears simultaneous. Also called Multitasking OS. | UNIX, early mainframe terminals |
| Real-Time OS (RTOS) | Processes data and responds instantly within a strict time deadline. Used where delays can be dangerous. Two types: Hard RTOS (must meet deadline — e.g. aircraft) and Soft RTOS (deadline preferred — e.g. video streaming). | Aircraft autopilot, missile guidance, medical monitors, VxWorks |
| Distributed OS | Manages a group of networked computers and makes them appear as a single system to the user. Resources (CPU, memory, files) are shared across all machines in the network. | Google's servers, cloud computing systems |
| Network OS | Runs on a server and manages data, users, and security for a network of computers. Allows multiple users to share files and printers over a network. | Windows Server, Novell NetWare, Linux (server) |
| Mobile OS | Designed specifically for smartphones and tablets. Optimised for touch input, battery life, and mobile hardware. | Android, iOS, Windows Phone, Symbian |
| Multiprogramming OS | Multiple programs reside in memory at the same time. When one process waits for I/O, the CPU is given to another — keeps CPU busy at all times. Different from multitasking (which is user-focused). | Most modern OS are multiprogramming |
Batch OS
Jobs processed in batches without user interaction. No immediate response. Used in early computers.
Time-Sharing (Multitasking) OS
Multiple users share CPU in time slices. Appears simultaneous. UNIX is the classic example.
Real-Time OS (RTOS)
Responds instantly within strict time deadlines. Hard RTOS (aircraft) vs Soft RTOS (video). Failure = danger.
Distributed OS
Manages a network of computers as one system. Resources shared across all machines.
Network OS
Runs on a server. Manages data, users, and security for a network. Allows file and printer sharing. Examples: Windows Server, Linux.
Mobile OS
Designed for smartphones/tablets. Touch input, battery optimised. Android, iOS.
Multiprogramming OS
Multiple programs in memory at once. CPU switches to another when one waits for I/O — keeps CPU busy at all times.
⚡ Exam Tips — Batch OS = no user interaction during processing. Time-sharing = multiple users, one CPU, takes turns. Real-time OS = instant response, used in aircraft/missiles. Hard RTOS = must meet deadline (life-critical). Soft RTOS = deadline preferred but not critical. Distributed OS = network of computers appear as one. Mobile OS = Android, iOS.
D. Key OS Concepts & Terms
Important Terms for Exam
Key concepts tested in OS questions
| Term | Definition |
|---|---|
| Process | A program in execution. Every running application is a process. A single program can create multiple processes. |
| Multitasking | Running multiple programs at the same time by rapidly switching between them. Gives the illusion of parallel execution. |
| Multiprogramming | Multiple programs in memory simultaneously. When one waits for I/O, CPU switches to another — maximises CPU utilisation. |
| Multiprocessing | A system with more than one CPU. True parallel processing — multiple processes run simultaneously on different processors. |
| Spooling | Simultaneously Peripheral Operations Online. Jobs are queued on disk before sending to a device (e.g. printer). Allows CPU to work while device is busy. |
| Booting | Starting the computer. BIOS/UEFI checks hardware (POST), then loads the OS from disk into RAM. Cold boot = from powered off. Warm boot = restart while powered. |
| Deadlock | Two or more processes are stuck waiting for each other to release resources — neither can proceed. A critical OS problem. |
| Scheduling | The OS decides the order in which processes get CPU time. Common algorithms: FCFS (First Come First Serve), Round Robin, Priority scheduling. |
| Virtual Memory | A technique that uses part of the hard disk as if it were RAM. Allows programs larger than physical RAM to run. Slower than actual RAM. |
| Thrashing | When the OS spends more time swapping pages in/out of virtual memory than actually executing programs — system becomes very slow. |
Process
A program in execution.
Multitasking vs Multiprogramming
Multitasking = multiple apps for user. Multiprogramming = multiple jobs in memory, keeps CPU busy.
Multiprocessing
Multiple CPUs — true parallel processing.
Spooling
Jobs queued on disk before sending to slow device (e.g. printer). CPU stays free.
Booting
Starting the computer. BIOS → POST → Load OS into RAM. Cold boot = power on. Warm boot = restart.
Deadlock
Two processes waiting for each other — neither can proceed. Critical OS problem.
Scheduling
OS decides the order in which processes get CPU time. Algorithms: FCFS, Round Robin, Priority scheduling.
Virtual Memory
Using hard disk space as extra RAM. Slower but allows large programs to run.
Thrashing
OS spends more time swapping memory pages than executing programs — system becomes very slow.
⚡ Exam Tip — Multitasking ≠ Multiprocessing. Multitasking = one CPU, switching fast. Multiprocessing = multiple CPUs running truly parallel. SPOOL = Simultaneously Peripheral Operations OnLine. Deadlock = circular wait with no exit.
E. Popular Operating Systems
GUI vs CLI & Real-World Examples
Where each OS is used and how users interact with it
| OS | Developer | Type | Interface | Key Facts |
|---|---|---|---|---|
| Windows | Microsoft | Desktop/Laptop | GUI | Most widely used desktop OS. Windows 95 introduced modern GUI. Current: Windows 11. |
| macOS | Apple | Desktop/Laptop | GUI | Unix-based. Only for Apple Mac hardware. Known for stability and design. |
| Linux | Linus Torvalds (1991) | Server/Desktop | CLI + GUI | Free and open-source. Dominant on servers and supercomputers. Many distributions: Ubuntu, Fedora, Kali. |
| Android | Mobile | Touch GUI | Based on Linux kernel. Most popular mobile OS globally. Open-source. | |
| iOS | Apple | Mobile | Touch GUI | Runs only on iPhone/iPad. Known for security and performance. Closed-source. |
| UNIX | Bell Labs (1969) | Server/Mainframe | CLI | Ancestor of Linux and macOS. Multi-user, multitasking. Foundation of modern OS design. |
| MS-DOS | Microsoft | Old PC | CLI only | Disk Operating System. Used before Windows. Text commands only. Still understood for historical context. |
Windows (Microsoft)
Most popular desktop OS. GUI interface. Windows 11 is current version.
macOS (Apple)
Unix-based. Only for Apple Mac hardware. GUI interface. Known for stability and design.
Linux (Linus Torvalds, 1991)
Free, open-source. CLI + GUI. Dominant on servers and supercomputers. Ubuntu, Fedora, Kali are versions.
Android (Google)
Most popular mobile OS. Based on Linux. Open-source. Touch GUI.
iOS (Apple)
Runs only on iPhone/iPad. Touch GUI. Known for security and performance. Closed-source.
UNIX (Bell Labs, 1969)
Ancestor of Linux and macOS. Multi-user, multitasking. Foundation of modern OS design.
MS-DOS (Microsoft)
Disk Operating System. Used before Windows. Text commands (CLI) only. Early PC standard.
| Feature | GUI (Graphical User Interface) | CLI (Command Line Interface) |
|---|---|---|
| Interaction | Mouse clicks, icons, menus, windows | Text commands typed at prompt |
| Ease of use | Beginner-friendly — visual and intuitive | Requires learning commands — for experts |
| Speed | Slower (more resources needed) | Faster for experienced users |
| Memory usage | More RAM required | Very lightweight |
| Examples | Windows, macOS, Android, iOS | MS-DOS, Linux terminal, UNIX shell |
GUI — Graphical User Interface
Mouse + icons + windows. Beginner-friendly. Examples: Windows, macOS, Android.
CLI — Command Line Interface
Text commands only. Faster for experts. Lightweight. Examples: MS-DOS, Linux terminal.
⚡ Exam Tips — Linux created by Linus Torvalds in 1991. UNIX created at Bell Labs in 1969. Android = Linux-based mobile OS. GUI = mouse + icons. CLI = text commands. MS-DOS = Command Line only OS for early PCs.
Quick Fire Revision
- OS stands forOperating System
- OS is also calledResource Manager / System Software
- Core of the OSKernel
- User interface of OSShell
- OS loaded byBIOS/UEFI during booting
- Batch OS characteristicNo user interaction during processing
- Time-sharing OS allowsMultiple users to share one CPU
- Real-time OS used inAircraft, missiles, medical monitors
- Hard RTOS vs Soft RTOSHard = must meet deadline · Soft = preferred
- Distributed OS managesNetwork of computers as one system
- Mobile OS examplesAndroid, iOS
- Multitasking = multiple programs, _ CPUOne CPU (rapid switching)
- Multiprocessing = multiple _CPUs (true parallel)
- SPOOL stands forSimultaneously Peripheral Operations OnLine
- Virtual memory uses _ as RAMHard disk space
- Deadlock meansProcesses waiting for each other — stuck forever
- Device drivers are used byDevice Management function of OS
- Linux created byLinus Torvalds (1991)
- UNIX created atBell Labs (1969)
- Android is based onLinux kernel
- GUI usesMouse, icons, windows — beginner-friendly
- CLI usesText commands — expert-level
- MS-DOS interface typeCLI only (Command Line)
- Cold boot vs Warm bootCold = from off · Warm = restart while on