Lesson 2 of 6

Series 7 - Lesson 2

Operating Systems: What Does an OS Actually Do?

The OS is not just "the thing with the desktop". It has six specific, examinable jobs - and understanding what each one does (and why) is the core of this topic.

45 minutes GCSE Operating Systems

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Right now, your OS is doing at least six things at once

As you read this, your OS is deciding how much RAM each open program gets, switching the CPU between dozens of processes hundreds of times per second, listening for keyboard input, managing your file system, communicating with your screen via its driver, and checking whether any processes are trying to access memory they should not.

None of that is visible. That invisibility is the point. A good OS is one you never notice.

Think: Your laptop runs Chrome, Spotify and a Word document simultaneously. You have one CPU. How is that possible?

Core content

The six core functions of an operating system

Every operating system - Windows, Linux, Android, iOS - performs the same six fundamental jobs. Click each one to understand what it does and why it matters.

1 Memory Management

The OS decides which parts of RAM each running program can use. It ensures programs do not accidentally (or maliciously) read or overwrite each other's memory. When RAM fills up, it uses virtual memory - treating a section of the hard drive as an extension of RAM, though this is much slower.

Real example: When Chrome opens a new tab, the OS allocates a block of RAM for it. If your RAM is nearly full, the OS moves less-used data from RAM onto the SSD (paging) to free space.

2 Process Management (Multitasking)

A process is a program that is currently being executed. The OS manages which processes get CPU time, pausing and resuming them so rapidly it appears simultaneous. This is called scheduling. Single-core CPUs genuinely do one thing at a time - they just switch so fast the user cannot tell.

Real example: Spotify continues playing even while you type in Word. The OS is giving Spotify brief CPU bursts to keep audio going while also allocating time to Word and the kernel itself.

3 File Management

The OS organises files in a hierarchical directory structure (folders within folders). It controls who can read, write or execute each file (permissions), and handles all operations: creating, copying, moving, deleting and searching. Without file management, there would be no way to organise or retrieve stored data.

Real example: When you save a document in Word, Word asks the OS to write bytes to storage. The OS decides where on the disk to write them, updates the file table, and returns control to Word.

4 User Interface (CLI and GUI)

The OS provides a way for users to interact with the computer. A GUI (Graphical User Interface) uses windows, icons, menus and a pointer. A CLI (Command Line Interface) uses text commands typed by the user. GUIs are easier to learn; CLIs are faster and more powerful for experienced users and automation.

Real example: Windows 11 provides a GUI. Linux servers are commonly administered via CLI (bash). Both can coexist - a Windows machine also has PowerShell (CLI).

5 Peripheral and Driver Management

The OS communicates with hardware peripherals (keyboard, mouse, printer, USB devices) through device drivers. When you plug in a USB mouse, the OS detects it, loads the appropriate driver, and begins routing mouse events to the active application. Without drivers, the OS cannot talk to hardware.

Real example: You connect a Wacom drawing tablet. Windows looks for a Wacom driver, installs it, and the tablet immediately works. The application (Photoshop) does not need to know anything about how the tablet works - the OS and driver handle it.

6 Security Management

The OS enforces user authentication (login), access permissions (who can read/write which files), and process isolation (programs cannot spy on each other's memory). It also manages firewalls and user account controls (e.g. UAC in Windows - "Do you want to allow this app to make changes?").

Real example: On a school network, the OS ensures students can save to their own folder but cannot access the teacher's files. This is enforced at the OS level, not by the applications themselves.

Interactive tool

Multitasking: see how the CPU switches between processes

A single CPU can only execute one instruction at a time. Multitasking is an illusion created by switching between processes so rapidly that it appears simultaneous. Press Start to see it happen.

Multitasking Simulator

One CPU - many processes - watch the scheduling happen

Chrome

Waiting

Spotify

Waiting

Word

Waiting

OS Kernel

Waiting

Press Start to begin scheduling

CLI vs GUI

Two ways to talk to an operating system

Creating a new folder called "work" is a simple task. Here is how you do it in a CLI versus a GUI - and the trade-offs of each approach.

CLI - Command Line Interface

Task: create a folder called "work"

C:\Users\Alex> mkdir work

Done. One command.

Fast once learned

Steep learning curve

Scriptable / automatable

Easy to make errors

Uses less system resources

Not intuitive for new users

GUI - Graphical User Interface

Task: create a folder called "work"

Open File Explorer

Navigate to the right location

Right-click in empty space

Select "New" then "Folder"

Type "work" and press Enter

Easy to learn

Slower for repeated tasks

Intuitive and visual

More system resources needed

Exam focus - CLI vs GUI

A 4-mark exam question might ask you to compare CLI and GUI. Always give advantages and disadvantages of each, with a reason. "A CLI is faster than a GUI for experienced users because commands can be typed directly without navigating menus" scores marks. "CLI is better" does not.

Activity

Match the scenario to the OS function

Click a scenario on the left, then click the OS function it belongs to on the right.

OS Function Matcher

Select a scenario, then the matching OS function

Chrome asks to read a file from the SSD

Spotify plays audio while Word is open

Student cannot access the teacher's folder

A new USB mouse is plugged in and immediately works

Chrome gets extra RAM when you open a new tab

You click icons and use menus to navigate

Security management

File management

Memory management

Process management

Peripheral/driver management

User interface (GUI)

Click a scenario, then the matching function

Think deeper

Virtual memory sounds like a great solution - when RAM is full, just use the SSD as extra RAM. So why not just have less RAM and rely on virtual memory instead?

SSDs and hard drives are dramatically slower than RAM - typically 10 to 100 times slower. When the OS is constantly moving data between RAM and storage (called "thrashing"), performance collapses visibly. You have probably seen this: a computer with too little RAM becomes almost unusable, with the hard drive constantly churning. Virtual memory is a safety net for occasional overflow, not a substitute for real RAM.