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UNIT 1 ▪ CHAPTER 3
03
Operating System
Functions · User Interfaces · Types · Boot
Learning Outcome 1: Explain the functions of an operating system and the types of user interfaces it provides.
An Operating System is the system software that sits between the user and the hardware. It is the first program that loads when you power on a computer, and the last to shut down. Without an OS, every application would have to know every detail of every device — an impossible task.
An Operating System is a program that acts as an interface between the user and the computer hardware, manages all resources (CPU, memory, I/O devices, files), and provides a convenient environment for executing user programs.
OS as the middle layer:
3.1 Functions of an Operating System
Every OS — from Windows on your laptop to Android on your phone — performs the same core set of jobs:
Process Management
Decides which program runs on the CPU and for how long. Creates, schedules and ends processes (the running copies of programs).
Memory Management
Keeps track of which part of RAM is free and which is in use, and allocates memory to new programs as they start.
File Management
Organises data into files and folders on secondary storage, controls who can read / write them.
Device Management
Uses device drivers to send commands to hardware (printer, scanner, Wi-Fi card) and coordinates input/output.
Security & Protection
Uses passwords, file permissions and user accounts to stop unauthorised access to data.
User Interface
Provides a way for the user to give commands — typing them, clicking icons, tapping the screen or speaking.
Whenever you double-click a .docx file and MS Word pops up, the OS has (i) loaded Word into memory (memory mgmt), (ii) scheduled it on the CPU (process mgmt), (iii) fetched the .docx from disk (file mgmt) and (iv) verified you have permission to open it (security) — all in a fraction of a second.
3.2 Types of User Interface
The way the OS talks to you is called its User Interface (UI). Over the decades UIs have evolved from plain text to touch and voice.
| UI Type | How you interact | Example OS |
|---|---|---|
| CLI (Command Line Interface) | Type text commands at a prompt. Powerful but requires memorising commands. | MS-DOS, Linux terminal, Windows PowerShell |
| GUI (Graphical User Interface) | Click icons, drag windows, use menus. Easy for beginners. | Windows 11, macOS, Ubuntu Desktop |
| Touch UI | Tap, swipe and pinch directly on the screen. | Android, iOS, Windows tablets |
| Voice UI | Give spoken commands to a virtual assistant. | Siri (iOS), Google Assistant (Android), Alexa |
3.3 Popular Operating Systems
| OS | Developer | Used on | Licence |
|---|---|---|---|
| Windows 11 | Microsoft | Desktop & laptop PCs | Proprietary / paid |
| macOS | Apple | MacBook, iMac | Proprietary (ships with hardware) |
| Linux (Ubuntu, Fedora…) | Community / Red Hat / Canonical | Servers, desktops, Raspberry Pi | Open-source / free |
| Android | Most smart-phones, tablets | Open-source (AOSP) | |
| iOS / iPadOS | Apple | iPhone, iPad | Proprietary |
| Chrome OS | Chromebooks in schools | Proprietary (Linux-based) |
Tiny Linux demo. Try these on any Linux terminal or Windows PowerShell — both give the same result on their respective systems:
# Print the current working directory pwd # List files ls # Make a new folder mkdir class11 # Move into it cd class11 # Create an empty file touch notes.txtEach line is an OS command — the CLI equivalent of right-click → New Folder. The OS's file-management subsystem actually does the work.
3.4 Types of Operating System
Though every OS performs the same core jobs, the style in which it serves users differs. The main classical types are:
| Type | Idea | Example |
|---|---|---|
| Single-user, single-task | One user runs one program at a time. | MS-DOS (early PCs) |
| Single-user, multi-tasking | One user, but many programs appear to run at once (browser + music + Word). | Windows, macOS on a laptop |
| Multi-user | Many users share the same computer, each with their own account & files. | Linux server, school lab PC |
| Real-time OS (RTOS) | Guarantees a response within a strict time limit; used where delay is dangerous. | Car ABS, pacemaker, satellite, traffic-signal controller |
| Distributed OS | One OS coordinates many connected computers as a single system. | Google / Amazon data-centres |
| Mobile OS | Designed for touch screens and small batteries. | Android, iOS |
Boot sequence — when you press the power button:
- The CPU jumps to a fixed address in ROM and runs the BIOS / UEFI firmware.
- BIOS performs POST (Power-On Self Test) — checks RAM, keyboard, disk.
- BIOS loads the bootloader from the disk, which loads the OS kernel into RAM.
- The OS starts up drivers, services and finally shows the login screen. You're in.
📌 Quick Revision — Chapter 3 at a Glance
- Operating System = program between user and hardware. Loads first on power-on; shuts down last.
- Six functions: Process, Memory, File, Device management + Security + User Interface.
- UI types: CLI (text commands), GUI (icons & windows), Touch UI, Voice UI.
- Popular OSes: Windows, macOS, Linux, Android, iOS, Chrome OS.
- Types of OS: single-user single-task, single-user multi-tasking, multi-user, real-time (RTOS), distributed, mobile.
- Boot sequence: power on → BIOS/UEFI from ROM → POST → bootloader → kernel → login screen.