A1.3.1 · Operating systems and control systems
The role of the operating system
By the end you can
Describe what an operating system is for: managing the hardware and sharing it fairly, and hiding its complexity behind a simple interface (abstraction).
A computer has one CPU (with a few cores), one block of RAM, one screen and one network connection, yet at any moment dozens of programs and hundreds of background tasks want to use them. Something has to decide who gets what, stop programs trampling on each other, and let each program use the hardware without knowing how it works. That is the job of the .
User
clicks, taps, types
Applications
browser, game, word processor
Operating system
scheduling, memory, files, devices, security, user interface
Hardware
CPU, RAM, storage, screen, keyboard, network card
Manager of resources#
The OS owns the hardware and lends it out. For each kind of resource it has a job:
- CPU: decides which process runs next and for how long, switching between them so quickly that they all seem to run at once (scheduling, A1.3.3).
- Memory: gives each process its own area, keeps the areas apart, and moves data out to disk when RAM runs short (virtual memory).
- Storage: organises the disk into files and folders, and controls who may read or change them.
- Devices: drives the keyboard, screen, printer and anything plugged in by USB or Bluetooth, and responds when they need attention (interrupts, A1.3.4).
- Network: runs the network stack that sends and receives data, and shares the connection between every program using it.
- Security: decides who may log in and which users and programs may use each file, device and setting.
- Power: dims the screen, slows or sleeps idle cores and devices, and suspends the machine, to save battery and heat.
It shares all of this fairly (no program can hog the CPU), safely (a crash in one program does not take down the others) and efficiently (no resource sits idle while work is waiting). Think of a conductor: the musicians are capable, but without someone deciding who plays when, the result is noise.
Hiding the hardware: abstraction#
The second half of the job is : hiding complicated detail behind something simpler. A program that wants to save a file does not find free blocks on the disk, work out how that particular SSD wants to be addressed, or wait for it to finish. It makes a , “write these bytes to report.txt”, and the OS does the rest. The same goes for drawing on the screen, playing a sound or sending a message across the internet.
This pays off three ways:
- Programs are far simpler to write, because the hard, hardware-specific code is written once, in the OS.
- The same program runs on machines with different hardware, because the OS (through device drivers) handles the differences.
- Users work with ideas that make sense to people (files, folders, windows, apps) instead of addresses and signals.
- Applicationuser mode
- Operating systemkernel mode
- Device driverpart of the OS
- Hardwarethe device itself
Step 1 of 7 · Application
You press Save. The word processor makes one system call: “write these bytes to Essay.docx”.
The app makes one request at the top; 5 steps happen below it that it never sees. That is abstraction.
Check yourself
The role of the OS
Quick questions, made fresh every time and marked here. Do a few until they feel easy.
Exam practice
Paper 1-style questions
4 questions · 13 marks
Write your answer as you would in the exam, then show the mark points and tick each one your answer made. These are our own questions in the style of the exam, not past-paper questions.
1.Describe the role of an operating system.
[3]Describe: A detailed account: say what happens, step by step if it is a process.
2.Explain how an operating system uses abstraction to make a computer easier to use and to program.
[3]Explain: Give reasons: say what happens AND why, or how one thing causes another.
3.Identify three hardware resources that an operating system manages, and for each state one thing the OS does with it.
[3]Identify: Pick out the answer from the options or the context.
4.Discuss what would happen if every application had to manage the hardware directly, without an operating system.
[4]Discuss: Look at the question from more than one side, weigh the points against each other, and come to a conclusion.
Past papers
Old exam questions on this topic
From HL Paper 1, May 2014 to November 2025. These papers were set on the previous syllabus, so the wording may differ from today's exam, but the topic is the same. Find the questions and markschemes in your school's past-paper pack.
| Paper | Question | Marks | About |
|---|---|---|---|
| November 2024 | 13(e) | 3 | Dedicated versus generic OS |
| May 2023 TZ1 | 12(c) | 4 | One OS function; why a dedicated OS |
| May 2021 | 4 | 2 | Why OS patches are needed |
| May 2021 | 15(b) | 2 | How the OS hides hardware complexity |
| May 2019 | 16(b) | 3 | Dedicated versus generic OS |
| November 2019 | 11(b) | 4 | The OS managing hardware resources |
| November 2016 | 10(c) | 4 | The OS’s role in a control scenario |
| May 2015 | 7 | 3 | How the OS hides network complexity |
| November 2015 | 8(d) | 3 | Two OS functions; where the OS is stored when off |
| November 2014 | 1(b) | 1 | Definition of an operating system |
In a nutshell
- An operating system (OS) manages the hardware (CPU, memory, storage, devices, network) and shares it between all the programs running.
- It sits between users and applications on one side and the hardware on the other: programs ask the OS for services through system calls.
- It hides the hardware’s complexity behind a simpler interface: abstraction. Programs see files and windows, not disk blocks and signals.
- Without it, every program would control devices itself, could interfere with others, and would only run on one exact machine.
- Examples: Windows, macOS, Linux on computers; Android and iOS on phones; specialised systems in consoles, cars and servers.