The Operating System: Files, Peripherals & Multitasking

🎯 Syllabus & Goals 3 min

Cambridge 4.1 · The role and functions of an operating system Paper 1 · Computer Systems

By the end of this lesson you can:

1. Describe how the OS manages peripherals using drivers, queues and buffers.
2. List the tasks of file management and user account management.
3. Describe how the OS manages multitasking using time limits, interrupts and priorities.

Textbook: Chapter 4, §4.1.2 (pp. 158–160)

Recap / Warm-Up 5 min

Last lesson covered three OS functions: the HCI, memory management and security management. Today covers four more.

Quick starter

Give one advantage of a CLI and one advantage of a GUI.

🧠 Key Concept 14 min

1 · Hardware peripheral management

• Communicates with every input and output device through its device driver.
• Uses the driver to translate data from a file into a format the device understands.
• Gives each hardware resource a priority, so it can be used and released as needed.
• Controls queues and buffers for input/output devices.

2 · File management

3 · Multitasking

The processes share the hardware under the OS's control. To stop them clashing, the OS constantly monitors each one:

• resources are given to a process for a set time limit;
• a running process can be interrupted;
• each process has a priority, and gets resources according to it.

These three rules are called pre-emptive multitasking. The risk is that a low-priority process can be starved of resources. Done well, multitasking makes the best use of CPU time, main memory, storage and virtual memory.

4 · Management of user accounts

Several people can log on to one computer. Each has a user account, protected by a user name and password. The OS keeps each user's data separate. Each user can:

• customise their screen layout and settings;
• use their own folders and files and manage them.

An administrator creates and deletes accounts and restricts what each one can do. On large systems, multi-access levels control who sees what.

Worked Example 12 min

(a) Multitasking three processes on one CPU core

Three processes are running: a music player (high priority), a web browser (normal) and a virus scan (low). The OS gives out the CPU in short time slices.

1. Slice 1: the OS gives the CPU to the music player.It has the highest priority, and must fill its sound buffer so playback never stops.
2. Slices 2–3: the browser runs, until its time limit ends.A time limit stops any one process holding the CPU for ever.
3. Slice 4: the music player interrupts and runs again.Running processes can be pre-empted when a higher-priority one needs the CPU.
4. Slice 6: the virus scan finally gets a slice.Low priority means it waits. If high-priority work never stopped, it would be starved.
5. The switching is so fast the user notices no pauses.This is why multitasking feels simultaneous, even on a single core.

(b) Printing three documents

Sam sends three documents to a shared printer. Document 3 is a low-priority job. The printer is already printing Document 1.

1. The OS locates the printer driver and loads it into memory.Without the driver the data cannot be translated for this printer.
2. Document 1's data is sent to the printer buffer.The buffer holds data ready for the slow printer, so the CPU can move on.
3. Documents 2 and 3 go into the printer queue.The printer is busy; a queue keeps them in order until the buffer is free.
4. Document 2 moves to the buffer before Document 3.Hardware management uses priority to decide the order.
5. The printer runs out of paper and sends an interrupt; the OS shows an error message.The OS receives and handles error messages and interrupts from the printer.

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

The OS drives peripherals through drivers, queues and buffers. File management names, organises and protects files. Multitasking shares the CPU using time limits, interrupts and priorities. User accounts keep each user's data separate.

Hardware management

The part of the OS that controls all input and output devices, e.g. printer management.

Buffer

A memory area used to store data temporarily.

File management

The part of the OS that manages files, e.g. create, delete, copy, open, close and rename.

Multitasking

A function that allows a computer to process more than one task/process at a time.

User account

An agreement allowing an individual to use a computer, entered with a user name and password.

Administrator

The person responsible for the upkeep of a multi-user system, who can create, delete and restrict accounts.

Homework 1 min

Task (≤ 15 min): Choose four functions of an operating system from Lessons 3 and 4. For each, name the function and describe what it does in one or two sentences. [8]