Operating systems

The Operating System (OS) is system software that manages hardware resources and provides an environment for application software to run. OCR J277 Paper 1 tests the five main functions of an OS — you need to be able to describe what each function does and why it is needed.

What is an operating system?

An OS acts as an interface between the user, application software and the computer hardware. Without an OS, users would need to program in machine code directly and manage all hardware manually.

Examples: Windows 11, macOS Sonoma, Linux (Ubuntu), Android, iOS.

Five key functions of an OS

1. User interface management

• Provides a way for the user to interact with the computer.
• GUI (Graphical User Interface): uses windows, icons, menus and pointers (WIMP). Easy to use; suitable for general users (Windows, macOS, Android).
• CLI (Command Line Interface): text-based commands typed by the user. Faster for experts; uses less memory; used in servers and programming environments.

2. Memory management

• The OS decides which processes are allocated RAM and how much.
• Paging: divides RAM into fixed-size pages; processes are loaded into available pages (not necessarily contiguous).
• Virtual memory: when RAM is full, less-used pages are swapped to disk (page file) to free RAM.
• The OS tracks which memory addresses belong to which process, preventing processes from overwriting each other's data.

3. Process management (multitasking)

• Modern computers run many processes (programs) simultaneously — this is multitasking.
• The OS uses a scheduler to allocate CPU time between processes.
• Each process gets short time slices of CPU time in turn (time-slicing) — the switching happens so fast it appears simultaneous.
• Process states: running (using CPU), ready (waiting for CPU), blocked (waiting for I/O).

4. Peripheral and device management

• The OS manages communication between the computer and peripheral devices (printers, keyboards, mice, screens).
• Uses device drivers — software that translates OS commands into signals the specific device understands.
• The OS queues print jobs (spooling), manages USB connections, handles input events.

5. File management

• The OS provides a file system that organises data into files and directories (folders).
• Handles: creating, renaming, moving, copying, deleting files.
• Controls read/write permissions so users can only access authorised files.
• Manages different file systems: NTFS (Windows), ext4 (Linux), APFS (macOS).

6. User management

• The OS maintains user accounts with different permission levels.
• Administrator/root: full access to all system settings and files.
• Standard user: restricted access — cannot install software or modify system files.
• Each user logs in with credentials; the OS ensures users can only access their own data.

Common OCR exam mistakes

1. Saying the OS "runs programs" — it manages the environment for programs; the CPU runs the instructions.
2. Confusing a device driver with the OS itself — drivers are separate software components that plug into the OS.
3. Forgetting user management as an OS function — it is commonly omitted from answers.
4. Saying "the OS gives all programs equal RAM" — memory management allocates RAM dynamically based on demand.