Computer Systems 2009-2010 Week 12: System Software Alma Whitfield.

Computer Systems

2009-2010

Week 12: System SoftwareAlma Whitfield

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Session Aims Understand what System Software

is Operating System as an example

of System Software Functions of an operating system Study of operating system process

management function

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Need for system software Computers very complex Come with large bundles of software

to help the non-specialist This software is called system

software

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Software types System software

operating systems language translators (e.g. Java

language)

Application software programs we buy (sometimes and use)

e.g. Microsoft Word, Powerpoint, web browsers, games.

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Operating systems In this session we focus on the most

important kind of system software – the operating system

Examples of operating systems include MS-DOS Windows Linux

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What an operating system does

Just like mother

Teenager does not know how to operate toaster etc ..

… but mother does

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What a mother does Teenager makes

service call Mother responds

by controlling the device needed

When device done it interrupts mother

Mother responds by delivering output to teenager

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What an operating system does

Alternative would be to have the teenager understand how to work all the hardware

Not regarded by teenager as efficient!

Computers are invented by teenagers

Can you see where this is heading?

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What an operating system does

Who is mother Operating

system Who is the

teenager Application

program

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Tasks carried out by an operating system Running programs Managing files Managing main memory Controlling peripherals Utility programs

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Running programs User requests a program

mouse click program name at command line

This triggers a sequence of actions ensure program is in main memory start fetch execute manage program’s file access manage program’s I/O device access

Operating system can detect and respond to errors

end a program after illegal attempt to access memory retry an operation when device not ready recover after power failure

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Managing files Programs and data saved on disc as

files Operating System file manager

organises files enables user to add, update delete files controls file ownership in shared

system e.g. network, mainframe

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Managing main memory Managing memory space Simultaneous programs

must not interfere with each other’s space

Large programs possible to load only required parts may use overlays

e.g. Word with little used menu options

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Controlling peripherals Operating

system includes

kernel drivers

Drivers plug and play

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Kernel The kernel is a key portion of the

Operating System that is loaded into memory when the machine is switched on and remains constantly in memory.

Contains the programs to handle the most frequent commands

The kernel is also know as the nucleus Good performance by the kernel is very

important for good performance by the machine

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Utility programs Editors Clock Calculator Calendar Whatever else those nice guys at

Microsoft can think of

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Issues over System Software What constitutes an operating system?

e.g. should it include Web browser, Java Virtual Machine and media player?

Microsoft was taken to court in the United States for anti-competitive conduct in making Internet Explorer 4.0 (and a non-standard JVM) non-removable parts of Windows 98

In 2004 the European Commission ruled that Microsoft broke EU competition law in tying Windows Media Player to its operating system

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Do we need an Operating System?

Is the statement below True or False ?

You can’t use a computer without an Operating System!

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False You can use a computer without an

operating system. However, you must be highly skilled

in Electrical and Mechanical Engineering

Some of the very first computers did not have Operating Systems

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Studying the tasks carried out by an operating system Running programs Managing files Managing main memory Controlling peripherals Utility programs

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Studying the tasks carried out by an operating system Running programs

today (Week 12) Managing files

done in Week 10, 11 Managing main memory

Week 14 Controlling peripherals

Week 13 (hard disk) Utility programs

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Running programs

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Running programs First machines

single user single program running at a time uniprogramming

More modern machines multiple users

e.g. bank system each user perhaps running several programs

at same time e.g. Windows PC

multiprogramming

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Uniprogramming No Operating System Machines run from a console with

display lights and toggle switches, input device and printer

Scheduling time was an issue because one job had to finish before another could start

CPU must wait for I/O instructions to complete before proceeding

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Programming with a femaleoperating system

http://www.berkeleyprep.org/webhouse/bh/html/histpic.htm

The girls organise the loading and running of programs.

They are effectively the 1940s version of the modern operating system

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ENIAC (Electronic NumericalIntegrator and Computer) 1946

Men occasionally involved as well!

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ENIAC (Electronic NumericalIntegrator and Computer) 1946 ... was the first large-scale,

electronic, digital computer capable of being reprogrammed

cost $500,000 only operational for 5 months before

it had to be shut down for refurbishment and a memory upgrade

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Uniprogramming

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Multiprogramming Multiple users accessing the same

data e.g. bank system

Single user running several programs at the same time e.g. Windows PC

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Multiprogramming – two main issues Sharing the processor

how can two programs be fetching and executing at the same time in the same CPU?

Sharing the main memory how can we avoid the different sets of

program instructions and data in main memory getting mixed up?

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Multiprogramming – two main issues Sharing the processor

how can two programs be fetching and executing at the same time in the same CPU?

Sharing the main memory how can we avoid the different sets of

program instructions and data in main memory getting mixed up?

Today

Week 14

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Sharing the processor Original problem:

expensive , fast CPU waiting for slow peripherals – uneconomic

Solution have CPU run another program while

waiting

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Other advantages Carry out tasks simultaneously

E.g. control central heating and word process Share peripherals

Mainframe and minis can centralise printers etc

Dumb terminals are cheap Share data

Users can access same files Concurrent programs on same machine

This is how we use Windows

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Disadvantages Breakdowns affect everybody Portability – need link to central

machine Expense – but getting cheaper Security risks

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Switching between programs Done by the kernel (part of

operating system) Known as a context switch Each program needs a control

block Preserves program’s state

CPU register contents Which data files are opened

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Different strategies for switching When program finished or blocked

Simple to implement But, programs can hog the processor

At regular intervals (time slicing) Programs cannot hog processor But, complex to control so time

overhead This is the strategy in common use

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Time slicing

Each interval is a time slice

Blocked programs (waiting for a peripheral) are not

allocated time slices

Scheduling – deciding which program runs next

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Running programs are “processes” Processes consist of:

The program (set of instructions) Current state of data structures Current state of CPU registers Which I/O devices currently open Which files are currently open

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Process Creation Parent processes create children

processes, which, in turn create other processes, forming a tree of processes

Resource sharing alternatives Parent and children share resources Children share subset of parent’s resources Parent and child share no resources

Execution alternatives Parent and children execute concurrently Parent waits until children terminate

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Process states Running

During its timeslice Ready

While some other process is running Blocked

Unable to proceed e.g. waiting for input device

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Changing Process States ready/waiting running blocked

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Context switching with interrupts

Interrupt Signal to CPU to

stop what it is doing and execute an interrupt routine

Clock interrupt generated by clock at fixed intervals

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Scheduling Algorithms A scheduling algorithm is the

method by which processes are given access to processor time.

e.g. Round Robin Fair Queuing FIFO

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Lots of Scheduling Algorithms A

Anticipatory scheduling Atropos scheduler

C CFQ Critical path method

D Deadline-monotonic scheduli

ng Deficit round robin

E Earliest deadline first schedul

ing Elevator algorithm

F FIFO FINO Fair Queuing Fair-share scheduling

And so forth

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What have we covered? Characteristics of System Software The responsibilities of Operating

System The KERNEL Uniprogramming and Multiprogramming Context Switching What makes a process How Processes are managed CPU allocation and Scheduling

Algorithms

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Round 1Which of the following best describe

the purpose of an Operating System?

a) To make the computer easy to useb) To ensure hardware is used efficientlyc) To store data.d) To execute program instructions.

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Round 1Which of the following best describe

the purpose of an Operating System?

a) To make the computer easy to use

b) To ensure hardware is used efficiently

c) To store data.d) To execute program instructions.

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Round 2 Which of the following are

operating systems:1. Unix2. Visual Basic3. Pascal4. Pie Eater5. Windows6. Linux

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Round 2 Which of the following are

operating systems:1. Unix2. Visual Basic3. Pascal4. Pie Eater5. Windows6. Linux

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Round 3Which of the following best describe

the responsibilities of an Operating System?

a) To Manage Resourcesb) To store data.c) To execute program instructions.

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Round 3Which of the following best describe

the responsibilities of an Operating System?

a) To Manage Resourcesb) To store data.c) To execute program instructions.

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Round 4Which of the following best describes

the KERNEL?

a) The centre of the Operating Systemb) The part of the Operating System that

remains resident in memoryc) The execution element of the

Operating System

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Round 4Which of the following best describes

the KERNEL?

a) The centre of the Operating Systemb) The part of the Operating System

that remains resident in memoryc) The execution element of the

Operating System

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Round 5Which best describes

Multiprogramming Operating Systems?

a) The ability to process multiple applications at once

b) The ability to give the impression that it is processing multiple applications at once

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Round 5Which best describes

Multiprogramming Operating Systems?

a) The ability to process multiple applications at once

b) The ability to give the impression that it is processing multiple applications at once

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Round 6 Context Switching can best be

described as:a) Switching from one CPU to another in a Dual Core Processor

b) Saving the state of a process and loading the state of another process

c) Switching from one Multiple Simulation to the next

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Round 6 Context Switching can best be

described as:a) Switching from one CPU to another in a Dual Core Processor

b) Saving the state of a process and loading the state of another process

c) Switching from one Multiple Simulation to the next

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Round 7 Windows will work on an Apple

Macintosh because the computer architecture is similar.

TRUE or FALSE ?

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Round 7 Windows will work on an Apple

Macintosh because the computer architecture is similar.

FALSE

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Next Week Hard disc management Quiz in your seminar which is

TIMED.

You should also be working on the research in relation to your presentation assignment

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Additional Reading O’Gorman J (2000) Operating

Systems

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ReferencesCoustan D, Franklin C How Operating Systems Work [on-line] http://Computer.Howstuffworks.com [Accessed 10-1-2011]