Operating Systems

This lesson covers the purpose and functions of an operating system (OS), a key topic in AQA and OCR GCSE Computer Science. You need to understand what an operating system does, why it is needed, and its core management functions.

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What Is an Operating System?

An operating system is a piece of systems software that manages the computer's hardware and software resources and provides a platform for application software to run. Without an operating system, you would need to write low-level code to interact directly with every piece of hardware — an impractical task for everyday users.

Examples of operating systems:

• Microsoft Windows
• macOS (Apple)
• Linux (Ubuntu, Fedora, etc.)
• Android (mobile)
• iOS (Apple mobile)

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The Purpose of an Operating System

The OS acts as an intermediary between the user, application software, and the hardware. Its main purposes are:

1. Provide a user interface so humans can interact with the computer.
2. Manage hardware resources (CPU, memory, storage, I/O devices).
3. Manage files and control access to stored data.
4. Manage security and user accounts.
5. Provide a platform for running application software.

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Layered Architecture

The operating system sits between the user (and their applications) and the hardware, providing services through well-defined layers.

graph TD
    A["User"] --> B["Application Software"]
    B --> C["Operating System"]
    C --> D["Memory Manager"]
    C --> E["Process Scheduler"]
    C --> F["File System"]
    C --> G["Device Drivers"]
    C --> H["Security / User Accounts"]
    D --> I["Hardware (CPU, RAM, Storage, I/O)"]
    E --> I
    F --> I
    G --> I

Key Functions of an Operating System

1. User Interface

The OS provides the interface through which users interact with the computer:

Interface Type	Description	Example
GUI (Graphical User Interface)	Windows, icons, menus, and pointers (WIMP)	Windows 11 desktop, macOS
CLI (Command-Line Interface)	Text-based; user types commands	Linux terminal, Windows Command Prompt
Touch interface	Optimised for touchscreen gestures	Android, iOS

• GUI is easier for beginners — visual and intuitive.
• CLI is preferred by advanced users and system administrators — faster for repetitive tasks and scripting.

2. Memory Management

The OS manages RAM to ensure all running programs have enough memory:

• Allocates memory to each running program.
• Deallocates memory when a program closes.
• Manages virtual memory when RAM is full (using part of secondary storage).
• Prevents programs from accessing each other's memory space (memory protection).

Without memory management, programs could overwrite each other's data, causing crashes.

3. Processor (CPU) Management

When multiple programs are running, the OS decides which program gets to use the CPU and for how long:

• Multitasking: The OS rapidly switches between programs, giving each a small time slice of CPU time. This switching is so fast that it appears as though all programs are running simultaneously.
• Scheduling: The OS uses scheduling algorithms to decide the order and priority of tasks.
• Interrupt handling: The OS responds to interrupts — signals from hardware or software that require immediate CPU attention (e.g., a key press, a printer error).

4. File Management

The OS organises and manages files stored on secondary storage:

• Provides a file system (e.g., NTFS, FAT32, ext4) that organises files into folders/directories.
• Controls file naming, moving, copying, and deleting.
• Manages file permissions — who can read, write, or execute a file.
• Keeps track of where files are physically stored on the disk.

5. Peripheral (I/O) Management

The OS manages input and output devices (peripherals) such as printers, keyboards, mice, and monitors:

• Uses device drivers — small programs that translate between the OS and specific hardware devices.
• Without the correct driver, the OS cannot communicate with a device.
• Manages print queues (a line of documents waiting to be printed).
• Handles plug-and-play — automatically detecting and configuring new devices.

Peripheral	Driver Required?	Managed By OS?
Keyboard	Yes (usually built-in)	Yes
Printer	Yes (often needs installing)	Yes (print queue)
USB drive	Yes (usually built-in)	Yes (file system)
Graphics card	Yes (needs installing)	Yes

6. Security Management

The OS manages security to protect the system and data:

• User accounts and passwords — ensures only authorised users can access the system.
• Access levels — different users may have different permissions (e.g., administrator vs standard user).
• Encryption — the OS may encrypt data to protect it.
• Firewall — monitors incoming and outgoing network traffic.
• Automatic updates — patches security vulnerabilities.

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Multitasking in Detail

Modern operating systems are multitasking — they appear to run multiple programs at the same time. In reality, the CPU processes one instruction at a time (per core), but the OS switches between programs so quickly that the user perceives simultaneous execution.

How it works:

1. The OS assigns each program a small time slice of CPU time.
2. After the time slice expires, the OS saves the program's state (its register values, PC, etc.).
3. The OS loads the next program's state and gives it CPU time.
4. This rapid switching is called context switching.

Exam Tip: Be careful — the OS does not make programs run truly simultaneously on a single core. It gives the illusion of multitasking through rapid context switching. True simultaneous processing requires multiple cores.

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Summary

• An operating system is systems software that manages hardware and software resources.
• Key functions: user interface, memory management, processor management, file management, peripheral management, and security management.
• The OS provides a platform for application software and acts as an intermediary between users and hardware.
• Device drivers allow the OS to communicate with peripherals.
• Multitasking uses rapid context switching to give the illusion of running multiple programs simultaneously.

Key Vocabulary: operating system, GUI, CLI, memory management, virtual memory, multitasking, scheduling, interrupt, file system, device driver, peripheral, context switching, user account, firewall.

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The OS and the Hardware it Manages