AQA 7517 · Paper 2 · 4.6.3a

Operating System
Functions

Memory · Process · File · Device management · Section 4.6

WHAT YOU'LL LEARN

Memory management · Process scheduling · Virtual memory · Device drivers · CLI vs GUI

AQA SPEC LINK

4.6.3 — Operating system functions in detail

Memory Management

OS allocates portions of RAM to each running process to prevent processes accessing each other's memory

Paging — RAM divided into fixed-size frames; programs divided into same-size pages; pages loaded as needed

Segmentation — program divided into logical sections (code, stack, heap) of varying size

Virtual memory — uses part of secondary storage as if it were RAM; allows programs larger than physical RAM to run

Virtual Memory

Virtual Memory in Detail

When RAM is full, the OS moves least recently used pages to a page file on the hard drive, freeing RAM for active processes. Called paging/swapping.

Page fault — when a needed page is not in RAM; OS must fetch it from disk (slow)

Thrashing — system spends more time swapping than running code; caused by insufficient RAM

Solution to thrashing: add more RAM or close applications

Process Management

Process Scheduling

A process is a program in execution. The OS manages multiple processes concurrently

Round Robin — each process gets a fixed time slice (quantum); fair but can be slow for I/O-bound tasks

Priority scheduling — higher priority processes get CPU first; lower priority may starve

Process states: Running (on CPU) → Ready (waiting for CPU) → Waiting (waiting for I/O)

Context switching — OS saves current process state and restores another; enables multitasking

File Management

File System Management

OS provides a file system — organises files in directories/folders (hierarchical structure)

Manages: creating, reading, writing, deleting, renaming, moving files

Permissions — read (r), write (w), execute (x) for owner, group, others

File allocation — tracks which disk blocks belong to which file (FAT, NTFS, ext4)

Buffering — temporarily holds I/O data in memory to speed up transfers

Device Management

Device Drivers & I/O

A device driver is software that allows the OS to communicate with specific hardware. Each device (printer, graphics card, USB) needs its own driver.

Plug-and-play — OS detects new hardware and installs appropriate driver automatically

Spooling — queueing documents for a printer; OS manages the queue so users aren't blocked

Interrupt handling — CPU processes hardware interrupts (keyboard press, disk read complete) via OS interrupt handler

User Interface

CLI vs GUI

CLI — Command Line Interface

Text commands typed by user. Fast, scripting, low resources. Used by developers, sysadmins. Examples: bash, PowerShell, cmd

GUI — Graphical User Interface

Windows, icons, menus, mouse. Easy for non-technical users. More RAM/processing needed. Examples: Windows, macOS, Android

Security

OS Security Features

Authentication — username/password, biometrics, two-factor authentication to verify identity

Access control — users only access files/resources they have permission for (principle of least privilege)

Encryption — BitLocker (Windows), FileVault (Mac) encrypt entire drives

Audit logs — records who accessed what and when; detects breaches

Firewall — OS built-in firewall monitors network traffic (Windows Defender Firewall)

AQA Exam Style

Practice Question

AQA 7517 — Paper 2 Style

(a) Explain what is meant by 'virtual memory' and describe ONE disadvantage of using it. [3]
(b) Describe what happens during a context switch in a process scheduler. [2]
(c) Give TWO reasons why a developer might prefer a CLI over a GUI. [2]

[7 marks]

3 marks

(a) Virtual memory uses secondary storage (hard drive) as an extension of RAM [1], allowing programs larger than physical RAM to run [1]. Disadvantage: accessing disk is much slower than RAM, causing performance degradation/thrashing [1]

2 marks

(b) OS saves the state (registers, program counter, etc.) of the current process to memory [1], loads the saved state of the next process to be run [1]

2 marks

(c) Any 2: faster for repetitive tasks / can be scripted/automated / uses less memory / more precise control / can be used over SSH

Summary

Key Points to Remember

Memory management — RAM allocation, paging, segmentation, virtual memory (uses disk)

Process scheduling — Round Robin, priority; process states: running/ready/waiting

File management — hierarchical directories, permissions, file allocation tables

Device drivers — bridge between OS and hardware; spooling for printers

CLI: powerful/scriptable; GUI: easy/visual; OS provides both options

🎉 Lesson complete — move to the quiz!