Purposes of an Operating System (OS)

Cambridge (CIE) A Level Computer Science

Resource Management

System loading & kernel
CPU scheduling
Memory allocation
I/O management & DMA
Hiding hardware complexity

Process Management

Multitasking & process states
Scheduling routines
Scheduling algorithms
Interrupt handling
Kernel operations

Memory Management

Paging & segmentation
Virtual memory
Memory allocation techniques
Fragmentation issues
Performance optimization

Understand Operating System Purpose

Identify key functions and resource management techniques

理解操作系统的目的和资源管理技术

Analyze Process Management

Explain multitasking and different scheduling algorithms

分析进程管理和多任务调度算法

Compare Memory Management

Evaluate paging, segmentation, and virtual memory techniques

比较分页、分段和虚拟内存管理技术

List Your Devices

List all the devices you use that have an operating system

列出你使用的所有有操作系统的设备

Computer

Smartphone

Tablet

Smartwatch

Smart TV

Router

Think About This

What would happen if these devices didn't have an operating system?

如果这些设备没有操作系统会发生什么？

What is Resource Management?

The Operating System (OS) is responsible for managing the computer's hardware efficiently to ensure the system runs smoothly

Maximising performance of the system
Reducing bottlenecks in resource allocation
Ensuring multitasking works correctly

Types of Resources

CPU

Processing power and execution time

Memory

RAM allocation and management

I/O Devices

Input/output operations

Storage

File system management

Boot Process

BIOS Activation

When computer is switched on, BIOS (stored in ROM) runs a bootstrap program

Kernel Loading

Bootstrap loads the kernel and essential parts of the OS from storage into main memory (RAM)

System Initialization

OS initializes hardware components and prepares the system for user interaction

Device Differences

Traditional Computers

Standard boot process with BIOS loading OS from hard disk or flash storage

Tablets & Smartphones

Flash memory contains read-only section for the OS and a second section for apps and user data

What is the Kernel?

The core of the Operating System that manages all hardware and software resources

操作系统的核心，管理所有硬件和软件资源

Acts as a bridge between applications and hardware, providing essential services

作为应用程序和硬件之间的桥梁，提供基本服务

Key Responsibilities

主要职责

Process Management

进程管理

Schedules processes, allocates CPU time, handles multitasking

Memory Management

内存管理

Allocates RAM, handles virtual memory, prevents clashes

Device Management

设备管理

Controls I/O devices using device drivers

Interrupt Handling

中断处理

Deals with interrupts from hardware components

File Management

文件管理

Handles reading/writing from files and file systems

Maximizing CPU Usage

The OS maximizes CPU usage through scheduling, which allows multiple processes to be managed efficiently

Multitasking ensures rapid CPU switching between processes
Different scheduling algorithms share CPU time fairly
Optimizes system performance and responsiveness

Scheduling Algorithms

Different algorithms are used to allocate CPU time based on system requirements:

Round Robin

Equal time slices for all processes

Priority-based

Higher priority processes get CPU time first

First Come First Served

Simple queue-based approach

Shortest Job First

Prioritizes processes with shortest execution time

Dynamic Memory Allocation

RAM is dynamically allocated to active programs and system processes

Memory shifts between active and inactive applications
Prevents memory conflicts between processes
Optimizes system performance for multitasking

Virtual Memory Process

When RAM is full, the OS uses virtual memory on disk to simulate extra memory

RAM fills with active processes

OS moves less-used data to disk

More programs can run simultaneously

I/O Device Management

I/O devices are much slower than the CPU, so the OS optimizes their use:

I/O operations managed through device drivers and interrupts
Buffering techniques to handle speed differences

Device	Typical Data Rate
Keyboard	~50 bps
Mouse	~120 bps
Laser printer	~1 Mbps
Hard disk	~100 Mbps

Direct Memory Access (DMA)

The DMA controller allows data transfer between memory and devices without CPU involvement

DMA transfers data directly between device and memory

CPU is freed to perform other tasks

DMA sends interrupt when transfer is complete

Dramatically improves system efficiency
Enables true multitasking capability

Abstraction Layers

The OS provides a user-friendly interface and handles the complexity of interacting with hardware

Graphical User Interface (GUI)

Makes tasks like file transfers easy (e.g. drag-and-drop instead of command-line)

Device Drivers

Handle communication with specific hardware components

APIs & System Calls

Provide standardized interfaces for software to access hardware

Simplifying User Experience

Users don't need to know technical commands, the OS abstracts this complexity

Without OS

Direct hardware interaction, complex commands, specialized knowledge required

With OS

Intuitive interfaces, simplified operations, accessible to all users

Creates a consistent experience across different hardware
Enables plug-and-play functionality for new devices
Provides security layers between users and hardware

Resource Management Techniques

Resource	Technique Used
CPU	Scheduling algorithms (Round Robin, Priority-based) Multitasking through rapid context switching Process control and priority management
Memory	Allocation to active processes Paging and segmentation techniques Virtual memory to extend physical RAM
I/O	Interrupts to signal completion DMA for efficient data transfer Buffering and driver management
User Interface	Abstracts complexity through GUI System utilities for user interaction
Storage	File system management Read/write optimization via caching and buffering

What is Multitasking?

Multitasking is the ability of the OS to manage system resources to give the illusion that multiple programs run simultaneously

CPU executes one instruction at a time
Processes billions of instructions per second
Rapid switching between tasks creates the illusion

Task 1

CPU executes for a time slice

Context Switch

State saved, next task loaded

Task 2

CPU executes for a time slice

What is a Process?

A process is a program in execution with its own allocated resources

Program Code

Instructions to be executed

Current Data

Variables and values in use

Register Values

CPU state information

Memory Space

Allocated RAM for the process

Each running application is a separate process
Processes change state based on resource availability

A process does not always run continuously. It changes state depending on what it's doing and whether it has access to resources.

进程并不总是持续运行。它会根据其活动和资源可用性改变状态。

Running

运行中

The process is actively being executed by the CPU

进程正在被CPU主动执行

Example

A word processor processing user input as they type

文字处理器处理用户输入

Ready

就绪

The process is prepared to run but is waiting for the CPU to be available

进程准备运行但等待CPU可用

Example

A background application waiting for CPU time to process data

后台应用程序等待CPU时间处理数据

Blocked

阻塞

The process is waiting for an event or resource, such as I/O completion

进程正在等待事件或资源，如I/O完成

Example

A browser waiting for a webpage to download from the internet

浏览器等待网页从互联网下载

What is Scheduling?

Deciding which tasks to process, for how long, and in what order through scheduling algorithms

CPU processes tasks as fast as possible
Different algorithms for different scenarios
Each has unique benefits and drawbacks

Scheduling Categories

Pre-emptive

Allocates CPU for time-limited slots

Can interrupt current processes
May neglect low-priority tasks

Round Robin

Shortest Remaining Time

Non-pre-emptive

Allocates CPU for unlimited time

Process holds CPU until completion
Long processes block shorter ones

First Come First Serve

Shortest Job First

Pre-emptive

Allocates CPU for time-limited slots

Specific time quantum for each process
Allows interruption of running processes
May neglect low-priority processes

Example Algorithms

Round Robin

Shortest Remaining Time

Non-pre-emptive

Allocates CPU for unlimited time slots

Process holds CPU until completion
Cannot be interrupted until finished
Long processes block shorter ones

Example Algorithms

First Come First Serve

Shortest Job First

Round Robin

Pre-emptive

Equally distributes processor time among all processes

Each process gets a time quantum to execute
Unfinished processes move to back of queue
Fair distribution of CPU time

Example

P1

P2

P3

P1

Time Quantum

Fixed time for each process

First-Come-First-Served

Non-pre-emptive

Prioritizes processes that arrive at the queue first

Process runs until completion
Current process blocks all others
New tasks join the back of queue

Example

P1

P2

P3

Arrival Order

Processes served in order they arrive

Multi-Level Feedback Queue

Pre-emptive

Priority algorithm where shorter and more critical tasks are processed first

Multiple queues group similar-sized tasks
Processes trickle down to lower priority queues
Creates prioritization system

Example

Q1: High

Q2: Med

Q3: Low

Shortest Job First

Non-pre-emptive

Processes continuously sorted by burst time from shortest to longest

Shorter jobs placed at front of queue
Longer jobs have lower priority
Minimizes waiting time

Example

P1: 3ms

P2: 5ms

P3: 8ms

Shortest Remaining Time First

Pre-emptive

Pre-emptive version of SJF where processes with shortest remaining time have higher priority

Time quantum set for each task
Unfinished tasks re-queued for further processing
High context switching overhead

Example

P1: 2ms

P2: 4ms

P1: 1ms

Algorithm Comparison

Algorithm	Benefits	Drawbacks
Round Robin	Fair CPU share for all processes Good for time-sharing systems Predictable execution time	Difficult to choose optimal time quantum High turnaround time for long processes
First Come First Served	Simple and easy to implement Fair based on arrival order	Poor performance with long processes High-priority tasks wait in queue
Multi-Level Feedback Queue	Prioritizes smaller tasks Groups similar-sized tasks together	More complex than other algorithms Setting correct parameters is difficult
Shortest Job First	Minimizes waiting time Efficient for short processes	Requires knowing burst time in advance Long processes may starve
Shortest Time Remaining	Ideal for jobs with short burst times Pre-emptive for optimal performance	Requires knowing burst time in advance High context switching overhead

What is Memory Management?

Fundamental role of the OS dealing with allocation and deallocation of the computer's primary memory

Application data loaded from storage into active memory for smooth execution
File data loaded alongside application data when opening files
Primary memory is a limited resource requiring careful management

Benefits & Techniques

Efficient allocation enables multitasking

Maintains security between processes

Key Techniques

Paging

Divides memory into fixed-size blocks

Segmentation

Divides memory into variable-sized segments

Virtual Memory

Uses disk space as extension of RAM

Paging

Divides memory into fixed-sized blocks (pages)

Facilitates efficient memory management
Enables use of virtual memory
Can lead to internal fragmentation

Example: 200KB file in 64KB pages

64KB

8KB used

56KB unused

Unused space in a page is wasteful as other data cannot be stored there

Segmentation

Divides memory into variable-sized segments based on logical parts

Allows intuitive memory access
Space-efficient allocation
Can result in external fragmentation

Example: Video editing application

Video Data

Audio Data

Effects

UI Elements

Physical gaps reduce maximum size of new segments that can be allocated

Virtual Memory

虚拟内存

When computer runs low on primary memory, it can make secondary storage act as an extension of main memory

当计算机主内存不足时，它可以使辅助存储作为主内存的扩展

OS offloads data from RAM to virtual memory
操作系统将数据从RAM卸载到虚拟内存
Creates illusion of larger memory
创建更大内存的错觉
Accessing virtual memory is considerably slower than RAM
访问虚拟内存比RAM慢得多

RAM

随机存取存储器

Fast, limited capacity

快速，容量有限

Virtual Memory

虚拟内存

Slower, larger capacity

较慢，容量更大

Memory Management Summary

内存管理总结

Technique	Benefits	Drawbacks
Paging 分页	Efficient memory management Enables virtual memory	Internal fragmentation
Segmentation 分段	Intuitive memory access Space-efficient	External fragmentation
Virtual Memory 虚拟内存	Run larger programs Effective multitasking	Slower than physical memory Performance issues if overused

Worked Example

例题解析

(a) Explain what is meant by virtual memory.

(a) 解释什么是虚拟内存。

Virtual memory, paging and segmentation are used in memory management.

虚拟内存、分页和分段用于内存管理。

Answer

答案

Disk/secondary storage extends RAM

More memory space than available RAM

Data swapped between RAM and virtual memory

磁盘/辅助存储用于扩展RAM，使CPU看起来能访问比可用RAM更多的内存空间。只有使用中的数据需要在主内存中，因此数据可以根据需要在RAM和虚拟内存之间交换。

(b) State one difference between paging and segmentation in the way memory is divided.

(b) 说明分页和分段在内存划分方式上的一个区别。

Answer

答案

Paging: fixed-size blocks

Segmentation: variable-sized blocks

Paging: faster access times

分页允许内存被划分为固定大小的块，而分段将内存划分为可变大小的块。操作系统将内存划分为页，编译器负责计算段的大小。分页的访问时间比分段快。

Quick Assessment

快速评估

Test your understanding of operating systems

测试你对操作系统的理解

1

What is the main purpose of an operating system?

操作系统的主要目的是什么？

Hint: Think about resource management

提示：考虑资源管理

2

Name two resource management techniques

说出两种资源管理技术

Hint: Consider CPU, memory, and I/O

提示：考虑CPU、内存和I/O

3

Explain the difference between pre-emptive and non-pre-emptive scheduling

解释抢占式和非抢占式调度的区别

Hint: Think about time allocation

提示：考虑时间分配

4

What is the main difference between paging and segmentation?

分页和分段的主要区别是什么？

Hint: Consider block sizes

提示：考虑块大小

Complete these questions on a separate sheet and submit before leaving. This will help assess your understanding of today's lesson.

在单独的纸上完成这些问题并在离开前提交。这将有助于评估你对今天课程的理解。

Purposes of an Operating System (OS)

Cambridge (CIE) A Level Computer Science

Contents

Resource Management

Process Management

Memory Management

Lesson Objectives

Understand Operating System Purpose

Analyze Process Management

Compare Memory Management

Starter Activity

List Your Devices

help_outline Think About This

Resource Management

settings What is Resource Management?

category Types of Resources

CPU

Memory

I/O Devices

Storage

Start-up and System Loading

power_settings_new Boot Process

BIOS Activation

Kernel Loading

System Initialization

devices Device Differences

Traditional Computers

Tablets & Smartphones

Kernel

memory What is the Kernel?

settings Key Responsibilities

Process Management

Memory Management

Device Management

Interrupt Handling

File Management

CPU Resource Management

speed Maximizing CPU Usage

settings Scheduling Algorithms

loop Round Robin

low_priority Priority-based

first_page First Come First Served

timer Shortest Job First

Memory Resource Management

memory Dynamic Memory Allocation

compare_arrows Virtual Memory Process

Input/Output Management and DMA

devices I/O Device Management

swap_vert Direct Memory Access (DMA)

Hiding Hardware Complexity

layers Abstraction Layers

Graphical User Interface (GUI)

Device Drivers

APIs & System Calls

compare_arrows Simplifying User Experience

Without OS

With OS

Resource Management Summary

summarize Resource Management Techniques

Multitasking & Process States

swap_horiz What is Multitasking?

Task 1

Context Switch

Task 2

memory What is a Process?

code Program Code

storage Current Data

settings Register Values

sd_storage Memory Space

Process States

Running

lightbulb Example

Ready

lightbulb Example

Blocked

lightbulb Example

Scheduling Routines

schedule What is Scheduling?

category Scheduling Categories

timer Pre-emptive

Think About This

What is Resource Management?

Types of Resources

Boot Process

Device Differences

What is the Kernel?

Key Responsibilities

Maximizing CPU Usage

Scheduling Algorithms

Round Robin

Priority-based

First Come First Served

Shortest Job First

Dynamic Memory Allocation

Virtual Memory Process

I/O Device Management

Direct Memory Access (DMA)

Abstraction Layers

Simplifying User Experience

Resource Management Techniques

What is Multitasking?

What is a Process?

Program Code

Current Data

Register Values

Memory Space

Example

Example

Example

What is Scheduling?

Scheduling Categories

Pre-emptive

Non-pre-emptive

Example Algorithms

Example Algorithms

Example

Example

Example

Example

Example

Algorithm Comparison

What is Memory Management?

Benefits & Techniques

Example: 200KB file in 64KB pages

Example: Video editing application

Virtual Memory

Memory Management Summary