Unit 11.1A · Term 1

Operating Systems & Types

The Operating System (OS) is the most important system software on any computer. It manages hardware, runs applications, and provides an interface for the user. Different types of OS serve different needs.

Learning Objectives

11.3.1.3 Describe the purpose and main functions of an OS
11.3.1.4 Compare single-user and multi-user OS
11.3.1.5 Compare one-task and multitasking OS

Lesson Presentation

11.1A-operating-systems.pdf · Slides for classroom use

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Conceptual Anchor

The Traffic Controller Analogy

An OS is like a traffic controller at a busy intersection. It decides which cars (programs) go when, prevents collisions (memory conflicts), directs traffic to the right lanes (CPU/memory allocation), and keeps everything flowing smoothly. Without the controller, there would be chaos.

Rules & Theory

Main Functions of an OS

Function	Description	Example
Memory Management	Allocates RAM to programs, frees it when done	Giving Chrome 2GB, Photoshop 4GB
Process Management	Creates, schedules, terminates processes	Task Manager showing running apps
File Management	Organises files in directories, controls access	File Explorer, Finder
Device Management	Communicates with hardware via drivers	Detecting a USB when plugged in
User Interface	Provides GUI or CLI for interaction	Windows desktop, Linux terminal
Security	User authentication, file permissions	Login password, file permissions

Single-User vs Multi-User OS

Feature	Single-User OS	Multi-User OS
Users	One user at a time	Multiple users simultaneously
Examples	MS-DOS, Windows (home)	Linux Server, Windows Server, UNIX
Use case	Personal computers, phones	Servers, mainframes, universities
Resources	All resources for one user	Resources shared between users
Security	Basic (one account)	Advanced (permissions per user)

Single-Task vs Multitasking OS

Feature	Single-Task OS	Multitasking OS
Programs	One program at a time	Multiple programs simultaneously
Examples	MS-DOS, early Palm OS	Windows 11, macOS, Linux, Android
CPU sharing	Not needed	Time-slicing between processes
Efficiency	Simpler, less overhead	More efficient use of CPU
User experience	Must close one app to open another	Switch between apps freely

Popular Operating Systems

OS	Type	Key Features
Windows	Desktop/Server	Most popular desktop OS, GUI-based, wide software support
macOS	Desktop	Apple-only, Unix-based, creative professionals
Linux	Desktop/Server	Open-source, highly customisable, server standard
Android	Mobile	Linux-based, most popular mobile OS
iOS	Mobile	Apple-only, closed ecosystem, secure

Multitasking ≠ Parallel Execution

On a single-core CPU, multitasking is an illusion — the OS rapidly switches between tasks (time-slicing) so fast that it seems like they run simultaneously. True parallel execution requires multiple CPU cores.

Worked Examples

1 Identify the OS Type

Scenario: A university server runs Linux. 200 students log in remotely
via SSH at the same time, each running their own programs.

OS Type: Multi-user + Multitasking
  Multi-user  → 200 students use it simultaneously
  Multitasking → Each student can run multiple programs

2 OS Function Analysis

User action: You plug in a USB flash drive

OS Functions involved:
  1. Device Management → detects the USB, loads the driver
  2. File Management   → mounts the drive, shows files in Explorer
  3. Security          → scans for autorun malware (if antivirus active)
  4. User Interface    → displays notification "USB detected"

Common Pitfalls

Multi-user = Multitasking

They are different concepts! Multi-user = multiple people using the system. Multitasking = multiple programs running. MS-DOS was single-user AND single-task. Modern Linux servers are both multi-user AND multitasking.

Tasks

Remember

List the 6 main functions of an operating system.

Understand

Explain why a server uses a multi-user OS but a smartphone uses a single-user OS.

Apply

You open Chrome, Spotify, and Word at the same time. Describe which OS functions are active and how multitasking is achieved.

Self-Check Quiz

Q1: What are the main functions of an OS?

Memory management, process management, file management, device management, user interface, security.

Q2: What is the difference between single-user and multi-user OS?

Single-user supports one user at a time (e.g. Windows Home). Multi-user supports multiple users simultaneously (e.g. Linux Server).

Q3: How does a single-core CPU achieve multitasking?

Through time-slicing — the OS rapidly switches between processes, giving each a small time slot, creating the illusion of simultaneous execution.