Operating system functions
The operating system (OS) is the most important piece of system software. It sits between the hardware and applications, sharing resources fairly and securely. AQA expects you to describe its main functions in detail.
1. Process management
A process is a running program. The OS:
- Loads a program from storage into RAM and creates a process.
- Schedules CPU time among many processes (multitasking).
- Switches between processes — saves the state of one and restores another (context switch).
- Prioritises processes (e.g. interactive UI > background indexing).
- Terminates a process when it finishes or crashes.
On a single-core CPU, only one process truly runs at a time, but the OS switches so fast it appears simultaneous. Multi-core CPUs run several processes in parallel.
2. Memory management
RAM is finite and shared between all running processes. The OS:
- Allocates RAM to each process when it starts.
- Tracks which areas are used and which are free.
- Protects processes from each other — process A can't read process B's memory.
- Uses virtual memory when RAM fills up: less-used pages are swapped to a disk (the page file). Slow but allows the system to keep running.
3. File management
Storage devices hold huge numbers of files. The OS:
- Provides a file system with folders, paths and metadata.
- Reads and writes files on behalf of applications.
- Manages permissions (who can read/write/delete).
- Handles fragmentation and optimisation (depending on the file system).
4. Peripheral / device management
The OS controls every input/output device — keyboard, mouse, printer, screen, network adapter — through drivers. Drivers are small programs supplied with the device or by the OS that translate generic OS commands into device-specific signals.
The OS:
- Loads appropriate drivers when a device is connected (plug and play).
- Buffers data flowing between devices and processes.
- Polls or handles interrupts when devices need attention.
5. Security and user management
The OS protects the system from unauthorised access:
- User accounts with passwords / biometrics.
- Permissions on files (read/write/execute) and on actions (install software, change settings).
- Logs of system events (login, file access).
- Authentication of programs (signed code) to detect tampering.
6. User interface
The OS provides the surface the user interacts with:
- GUI (Graphical User Interface) — windows, icons, menus, pointer (WIMP).
- CLI (Command Line Interface) — text-based commands. Powerful for scripting and remote work.
- Voice / touch interfaces on phones, smart speakers.
✦Worked example— Worked example — opening a file
When you double-click a document:
- OS user interface receives the click.
- Process management starts the appropriate app (e.g. Word).
- Memory management allocates RAM for the new process.
- File management opens the document file from disk.
- Peripheral management displays the contents on the screen.
- Security checks your account has read access to the file.
Six OS functions in one click.
Multitasking — preemptive vs cooperative
- Preemptive multitasking: OS forces context switches at fixed intervals — the kind used by Windows, macOS, Linux. Fair and robust.
- Cooperative multitasking: each program gives up the CPU voluntarily — historically used in early Mac OS. A buggy program could freeze the whole system.
GCSE focus: just know modern OSes are preemptive.
⚠Common mistakes— Pitfalls
- Confusing virtual memory with cache. Cache is fast RAM near the CPU; virtual memory uses slow disk to extend RAM.
- Calling drivers part of hardware. Drivers are software.
- Listing only "GUI" as the user interface. CLIs are equally valid.
- Treating user management as separate from security. They overlap heavily.
- Saying the OS runs programs. It schedules CPU time so programs run; the CPU runs the actual instructions.
Why we need an OS
Without an OS, every application would need to:
- Talk directly to every hardware device.
- Coordinate access with other apps.
- Manage its own memory.
- Implement its own user interface.
Practically impossible. The OS provides shared, standardised, safe access to all of this.
➜Try this— Quick check
Match each task to the OS function responsible:
- Allocating RAM to a new app → memory management.
- Recognising a USB stick when plugged in → peripheral management.
- Stopping a frozen program → process management.
- Storing your password securely → security / user management.
- Showing the desktop wallpaper → user interface.
- Saving a document to disk → file management.
AI-generated · claude-opus-4-7 · v3-deep-computer-science