The big idea: The role of an OS breaks down into a set of named functions.
Each one manages a different resource, and the exam expects you to name them and say what each actually does.
All at once, all the time: These run constantly in the background while you use the machine — and must keep running correctly even as programs start, crash and close.
That is what "maintaining system integrity" means.
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Memory management
Gives each program its own region of memory, tracks what is free, and reclaims it when a program closes.
It also stops a program reading or writing memory that is not its own — which is what keeps one crash from spreading.
Scheduling
Decides which program gets the CPU next and for how long, switching between them fast enough to look simultaneous.
The methods it can use are the subject of the next micro-topic.
File system
Keeps track of where every file physically lives on storage, and presents it as folders and filenames instead.
It also records sizes, dates and who may open each one.
Device management
Talks to every keyboard, printer, drive and network card through its driver, and queues requests so two programs never use one device at once.
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Security
- Checks who is logged in and what they may do
- Refuses a program access it has not been granted
- Keeps the OS's own memory out of reach of programs
Accounting
- Records how much CPU time, memory and storage is used
- Used for billing on shared or cloud systems
- Also how the OS spots a program consuming too much
Graphical user interface
- Draws the windows, menus and pointer
- Turns clicks and keystrokes into messages for programs
- Gives every program the same look and controls
Virtualization
- Lets one machine appear to be several separate ones
- Each virtual machine has its own OS and cannot touch the others
- How one physical server hosts many customers
Networking
- Sends and receives data over the network
- Handles addressing and the rules for the conversation
- So a program just says "fetch this"
System integrity: All nine run while programs start, crash and close around them.
A program that fails must not take the OS with it — which is why the OS keeps its own memory separate and never lets a program reach hardware directly.
How this is tested — you must NAME the function and say what it manages — a name alone is not an answer. It comes up two ways:
Paper 1 Section A
- Describe the functions of an OS, 3-6 marks
- Name the function responsible for a stated job
- Explain how one function protects the system
Paper 1 Section B — case study
- Say which functions matter most to the case study
- Justify it from that organisation's setup
The classic trap: Listing names and stopping. "Memory management, scheduling, security" is a list, not a description. Each name needs a clause: what it manages, and what would go wrong without it.
A shared office computer is used by several staff, each with their own account, running many programs at once. Describe two functions of the operating system that make this possible.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.