Introduction to Operating Systems

CS-2301 B-term 2008 1

Introduction to Operating Systems

CS-2301, System Programming for Non-majors

(Slides include materials from The C Programming Language, 2nd ed., by Kernighan and Ritchie and from C: How to Program, 5th ed., by Deitel and Deitel)

Introduction to Operating Systems

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Why an Intro to Operating Systems?

• This is a System Programming Course• For people who are not CS majors

• (Nearly) every programming task in real-life includes working with an OS

• Inevitably will have to deal with principle OS features

• Even if not knowledgeable in their designs

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Class Discussion

What is an Operating System?

(Laptops closed, please!)

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What Operating Systemshave you Used?

(Other than Windows, Linux, Mac-OS, Unix)

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What is an Operating System?

• Characteristics– Large, complex set of

programs

– Long-lived, evolving

– Worked on by many people for many years

• Functions– Creates abstractions

– Multiplexes concurrent activities

– Manages resources

– Mediates access to hardware devices

– Provides a variety of services to users and applications

– …

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What is an Operating System?

• Characteristics– Large, complex set of

programs

– Long-lived, evolving

– Worked on by many people for many years

• Functions– Creates abstractions

– Multiplexes concurrent activities

– Manages resources

– Mediates access to hardware devices

– Provides a variety of services to users and applications

– …

Large = 108–109 lines of code

(Windows and Linux)

107 line of code for a real-time OS.

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Definition – Abstraction

• The distillation of a complex mechanism into a simple, conceptual model

• User of abstraction does not need to worry about details

• Implementer of abstraction does not need to worry about how user will use it (within limits)

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Operating Systems

• Typically– Long-lived– Frequently extended and updated– Worked on by many developers– Used and, maybe, abused by a variety of users with

varying expertise and expectations

• Essential to create an acceptable computing environment to create and execute other programs that achieve business or personal goals

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Kinds of operating systems

• Mainframe Operating Systems• Server Operating Systems• Multiprocessor Operating Systems• Personal Computer Operating Systems• Handheld Computer Operating Systems• Embedded Operating Systems• Sensor Node Operating Systems• Real-time Operating Systems• Smart-card Operating Systems• …

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OS and Hardware

• OS mediates programs’ access to hardware– Computation – CPU– Storage – volatile (memory) and persistent

(disk)– Networks – NIC, protocols– I/O devices – sound cards, keyboards, displays

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Four fundamental Abstractions

• Processes & threads• Multiplexing of processor(s) to create the illusion of

many of them

• Virtual memory• Multiplexing of physical memory and disk blocks to

create illusion of own memory per process

• Files & persistent storage• Organizing principles about long-term data storage

• Sockets & connections• Organizing principles about network communication

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Definition – Process

• A particular execution of a program• Different from all other executions of that program

• Different from executions of other programs

• The OS uses one or more CPUs to make it look like each process has own CPU

• Can execute at same time!

• Uses interrupts to manage and enforce multiplexing of CPU

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Why Processes?

• Enables programmers – to completely disengage from issues of

concurrent execution of independent programs– to build applications with more than one

concurrent activity

• Enables independent applications to share a computing system

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Why Processes (continued)?

• Exploit modern processors– Capable of executing multiple threads of

execution simultaneously– Interleaved at instruction level or even memory

access level

• Moore’s Law:–– Integrated circuit components shrink in size by

50% every 18 months– Double in speed every 18 months

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Why Processes (continued)?

• Exploit modern processors– Capable of executing multiple threads of

execution simultaneously– Interleaved at instruction level or even memory

access level

• Moore’s Law:–– Integrated circuit components shrink in size by

50% every 18 months– Double in speed every 18 months

Modern limitation due to power

dissipation.

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Resources Assigned to a Process

• Memory• Virtual or real

• Processor time• Priorities• Deadlines for real-time activities

• Privileges• Security, authentication, etc.

• Files and file space• For long-term storage, temporary storage

• Devices• For input and output activity, sensors, etc.

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Resources (continued)

• Managed by OS

• Protection and isolation from other processes

• Allocation according to defined policies

• Enforcement of limits, etc.

• …

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Threads

• A refinement of concept of process• Short for “thread of control”

• An concurrent execution of a function within the context of a process

• Including all functions it calls

• Needs own stack

• Shares heap with other threads of same process

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Threads (continued)

• Linux:–• A thread is a special kind of process that shares all

resources with other threads

• A Process group is the collection of threads making up a process

• Windows:–• Threads are fundamental objects

• Process is a group of threads plus memory plus other resources

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Why Threads?

• To enable development of applications with concurrent activities inside them

• Need to share data (difficult with separate processes)

• Examples• Web server over common data pages

• Transaction processor over common data base

• Applications within a mobile phone or PDA

• Applications with different speeds of devices

• …

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Virtual Memory

• Definition:– the illusion that a process has its own, isolated memory

• (Often) more memory than machine has installed

• May be implemented using interrupts, pages, and disk blocks

• Swapping fast enough so process is unaware

• May be implemented by partitioning• Swapping not necessary for real-time activities

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Typical Virtual Memory for Process(Windows & Linux)

0x00000000

0xFFFFFFFF

address space

program code(text)

static data

heap(dynamically allocated)

stack(dynamically allocated)

PC

SP

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Typical Virtual Memory for Process(Windows & Linux)

0x00000000

0xFFFFFFFF

address space

program code(text)

static data

heap(dynamically allocated)

stack(dynamically allocated)

PC

SP

Every process has one of these.

Separate from every other process.

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Virtual Memories in Embedded System

OS Kernel

stack

Process 1

stack

Process 2

0x00000000

0x0000FFFF

Physical

memory

stack

Process 3

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Virtual Memory for Multiple Threads

0x00000000

0xFFFFFFFF

Virtual

address space

code(text)

static data

heap

thread 1 stack

PC (T2)

SP (T2)thread 2 stack

thread 3 stack

SP (T1)

SP (T3)

PC (T1)

PC (T3)

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Note on Threads

• Most embedded system programmers will probably have to design with multiple threads in the future

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Questions?