Operating System – Complete Notes for IBPS, SSC, RRB & Govt Exams

Introduction: Why Operating System is a Must-Master Chapter

The Operating System is the single most important software on any computer. Every computer interaction — from turning on your laptop to processing an NEFT transaction at a bank — runs through the OS. It is the invisible engine that makes everything possible.

In government job exams, Operating System is one of the highest-weightage topics in Computer Awareness:

• "Which is the first OS on IBM PC?" → MS-DOS
• "Which OS was developed by Linus Torvalds?" → Linux
• "What is the shortcut for Warm Booting?" → Ctrl + Alt + Del
• "The first GUI was designed by ___?" → Xerox Corporation (1970s)
• "Android is based on ___?" → Linux
• "Windows 11 requires TPM ___?" → TPM 2.0

Questions come from every sub-topic — OS types, DOS commands, Windows features, mobile OS facts, and booting concepts. This chapter is also directly relevant to banking — core banking systems run on mainframe OS (IBM z/OS), ATMs run on Windows or Linux, and bank employees use Windows daily.

This post covers every testable detail — from MS-DOS commands to Windows 11 AI features, from Linux kernel facts to Android version history.

What is an Operating System?

An Operating System (OS) is a program (or collection of programs) that acts as an intermediary between the user and the computer hardware. It is the most essential system software — a computer cannot boot, run any program, or perform any useful function without it.

Formal Definition:

An Operating System is a set of programs that manages all hardware resources, provides a platform for application software to run, and enables communication between the user and the hardware — hiding the complexity of hardware from the user.

Key Roles of an OS:

1. Resource Manager — manages CPU, memory, storage, and I/O devices
2. Platform Provider — provides an environment where application software can run
3. User Interface — provides a way for users to interact with the hardware (GUI or CLI)
4. Traffic Controller — decides which program gets CPU time and when

Without an OS:

• No programs can run
• No files can be saved or read
• No hardware device works
• No user can interact with the computer

Functions of an Operating System

Types of Operating Systems

Batch Processing OS

Single-User OS

Multi-User OS

Multi-Tasking OS

Time-Sharing OS

Real-Time OS (RTOS)

Multitasking Types - Preemptive vs Cooperative

RTOS Types - Hard vs Soft

User Interface - How Users Interact with the OS

GUI (Graphical User Interface)

CLI / CUI (Command Line Interface / Character User Interface)

Booting - Starting the Computer

Booting is the process of starting up a computer — loading the OS from the storage device into RAM so the computer becomes ready for use. The word comes from the phrase "pulling yourself up by your bootstraps."

Cold Booting

Warm Booting

MS-DOS (Microsoft Disk Operating System)

MS-DOS was developed by Microsoft in 1980 and became the first OS on IBM PCs in 1981. It is a single-user, single-tasking, command-line operating system.

Key Facts about MS-DOS:

• Full form: Microsoft Disk Operating System
• Developer: Microsoft Corporation
• Year: 1980 (developed); 1981 (first used on IBM PC)
• Interface: CLI (Command Line Interface) — text only, no graphics
• User type: Single-user only
• Multitasking: No — can run only one program at a time
• Primary hard drive: C: (always)

MS-DOS File Structure:

• Boot Record — contains instructions for loading the OS
• BIOS.sys — hardware management
• MSDOS.sys — core OS functions
• Command.com — command interpreter (processes user commands)
• Config.sys — system configuration file; adjusts system settings at startup
• Autoexec.bat — batch file that executes automatically at every startup

MS-DOS Commands

Commands in MS-DOS are of two types:

Internal Commands (loaded into memory at boot — always available):

External Commands (stored as separate files on disk — need to be loaded):

Important File Extensions

Windows Operating System

Windows is the world's most widely used desktop OS, developed by Microsoft Corporation. It introduced the GUI (Graphical User Interface) to mass-market computing.

Windows Version History

Windows 11 - In Depth

Windows 11 was released on October 5, 2021, and represents the biggest visual and functional overhaul of Windows since Windows 10.

Minimum System Requirements:

Key New Features of Windows 11:

Internet Explorer was officially discontinued on June 15, 2022 — replaced by Microsoft Edge.

macOS - Apple's Desktop Operating System

macOS is the proprietary desktop OS developed by Apple Inc. for its Mac computers. It is Unix-based, known for stability, security, and tight hardware-software integration.

Linux - The Open-Source Powerhouse

Linux is a free, open-source operating system kernel first released by Linus Torvalds on September 17, 1991 at the University of Helsinki, Finland. Linux powers everything from Android smartphones to web servers to supercomputers.

Key Facts:

Key Components of Linux

Key Linux Concepts:

• Thread — The smallest unit of execution within a process
• Process — A running instance of a program
• Spooling — Sending data to a temporary buffer (e.g., print spooling — print jobs queue before the printer)
• Daemon — A background service process (equivalent to Windows Services)

Popular Linux Distributions (Distros):

Mobile Operating Systems

Mobile OS are designed specifically for smartphones and tablets — they manage touchscreens, battery, wireless connectivity, and mobile-specific hardware.

Android

Android Version History (Key Versions for Exams):

iOS

Other Mobile OS

Memory Tricks

🔑 OS Functions — "PMFDS EU":

Process Management | Memory Management | File Management | Device Management | Security | Error Handling | User Interface Mnemonic: "Please Manage Files Daily — Security Ensures Us"

🔑 Types of OS — "BSM MTR":

Batch | Single User | Multi User | Multi Tasking | Time Sharing | Real Time Mnemonic: "Big Schools Make Many Talented Readers"

🔑 Booting Types:

Cold Boot = Computer is COLD (OFF) — press power button Warm Boot = Computer is WARM (ON) — press Ctrl+Alt+Del to restart

🔑 First GUI Designer:

Xerox designed the first GUI in the 1970s — then Apple popularised it, then Microsoft followed Remember: "X came first" (Xerox)

🔑 Linux Creator:

Linus Torvalds created Linux in 1991 — "Linus made Linux"

🔑 Android Base:

Android is based on Linux kernel — "Android Loves Linux"

🔑 Windows 11 Key Requirement:

TPM 2.0 = Trusted Platform Module 2.0 — security chip mandatory Remember: "Windows 11 needs T.P.M. — Trust Pays More"

🔑 MS-DOS Key Facts:

Developed 1980 | IBM PC 1981 | Single user | CLI | C: is primary drive Mnemonic: "DOS Is CLI, C: Is Primary"

One-Liner Recap (Quick Revision)

1. An Operating System is the most essential system software that acts as an intermediary between the user and hardware, managing all resources and providing a platform for applications to run.
2. The six key functions of an OS are Process Management, Memory Management, File Management, Device Management, User Interface, and Security.
3. Batch Processing OS groups similar jobs and executes them in batches without user interaction — ideal for repetitive high-volume tasks like payroll and bank statements.
4. Multi-Tasking OS allows multiple processes to appear to run simultaneously by rapidly switching the CPU between them — examples include Linux, Unix, and Windows 95+.
5. A Real-Time OS (RTOS) guarantees response to inputs within a predetermined time limit and is used in flight control, ATMs, missiles, and medical devices.
6. GUI (Graphical User Interface) was first designed by Xerox Corporation in the 1970s and uses icons, windows, and pointing devices; CLI (Command Line Interface) uses text commands like in MS-DOS and Linux Terminal.
7. Cold Booting starts the computer from a completely powered-off state, while Warm Booting restarts a running computer using Ctrl + Alt + Del without a full power cycle.
8. MS-DOS (Microsoft Disk Operating System) was developed in 1980, first used on IBM PCs in 1981, and is a single-user, single-tasking, command-line operating system with C: as the primary drive.
9. Internal MS-DOS commands (DIR, COPY, DEL, MD, CD, CLS) are loaded into memory at boot, while External commands (FORMAT, CHKDSK, DISKCOPY) are stored as separate files on disk.
10. Windows 11 was released in October 2021 and requires TPM 2.0, 64-bit CPU, 4 GB RAM, and 64 GB storage — key features include Centred Start Menu, Snap Layouts, Android app support, and Copilot AI.
11. Linux was created by Linus Torvalds and first released in September 1991 — it is free, open-source, Unix-based, and powers 90%+ of internet servers and all Android phones.
12. The Linux Kernel is the core of the OS that directly manages hardware, while the Shell is the command interpreter that takes user commands and passes them to the kernel.
13. Android, developed by Google and based on the Linux kernel, is the world's most widely used mobile OS with ~72% market share; Android 15 is the latest version (2024).
14. iOS, developed by Apple, is the closed-source mobile OS for iPhones and iPads; iOS 18 (2024) introduced Apple Intelligence AI features.
15. HarmonyOS was developed by Huawei as an alternative to Android after US sanctions prevented Google from licensing Android to Huawei for its new devices.

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