Operating Systems Brian Farrimond. Topics Introduction Overview of operating system tasks Operating systems in detail

Operating Systems

Brian Farrimond

Topics

• Introduction

• Overview of operating system tasks

• Operating systems in detail

Topics

• Introduction



Computers run software

• Computers– built to run software

• Software– consists of sets of instructions (programs)

Need for system software

• Computers very complex

• Come with large bundles of software to help the non-specialist

• this software is called system software

Types of software

• System software– operating systems– language translators

• Applications software– special purpose programs– e.g. Word, Paintbrush, Internet Explorer

Focus of this course

Topics

• Introduction



Operating system tasks

• managing files

• running programs

• memory management

• control of peripherals

• utility programs


• managing files





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


• managing files





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


• managing files





Memory management

• Managing space

• Simultaneous programs – must not interfere with each other’s space

• Large programs– possible to only load required parts– may use use overlays

• e.g. Word with little used menu options


• managing files





Control of peripherals

• Operating system– consists of

• kernel• drivers

• Drivers– plug and

play


• managing files





Utility programs

• Editors

• Compilers

• Clock

• Calculator

• Calendar

Topics

• Introduction



Operating systems in detail

• MS-DOS filing system

• Managing disc storage

• Multiprogramming– sharing the processor– sharing memory





MS-DOS Filing System

• Micro Soft Disc Operating System– operating system for first PCs– Windows was a layer on top of the MS-DOS– Now Windows replaces MS-DOS

• Shares characteristics of Unix (Linux)– directory structure– command line

MS-DOS Directories

• Access to files is through a directory– Example: – LETTER.TXT, POEM.TXT, JOKE.TXT

Directory also records: file size, date last changed

Directory is same as a Windows folder

Organise with directory structure

• directory can hold:– file names– sub-directory names

root is top level directory

File names

• Local name– CHAPTR1

• Full name– \BOOKS\LOSTLOVE\CHAPTR1

Path Local name

Current directory

• User is always “in a directory”

• This is the Current Directory

• Example– opening a file– file dialog’s starting directory

Changing directory

• cd

• Example using full path– cd \LETTERS\LOVELETT

Using short cuts with cd

• cd ..– .. refers to parent directory





Managing Disc Storage

• Two main types of secondary (permanent) storage– magnetic disc

• floppy disc

• hard disc

– optical disc• CD-ROM

• DVD

Magnetic disc

• flat circular disc (platter)

• coated with magnetizable material

• same technology as– audio cassettes– video tape

Disc pack• Several platters on same spindle

Formatted magnetic disc

• concentric tracks

• each track divided into sectors

Old Disc statistics

Format Surfaces Tracks Sectors / track

Capacity

DD (floppy) 2 80 9 720 Kb

HD (floppy) 2 80 18 1.44 Mb

IBM PS/2 7 583 36 70 Mb

Storing files on disc

• Two alternative strategies for allocating sectors to a file– contiguous allocation

• file stored on adjacent sectors

– non-contiguous allocation• file scattered across disc

Contiguous allocation• Advantages

– simple to implement– fast

• Problems– need to find large

enough slot on disc– difficult to add to file– fragmentation on

deletion

Non-contiguous allocation

• The most common approach

• Method:– files chopped up into fixed size blocks– blocks need not be stored in adjacent sectors

Non-contiguous allocation

• Disadvantages– more complex to implement– increased access time– most files waste space in incompletely filled

blocks

• Advantage– increased flexibility

Block size

• How big should blocks be ?– too big then too much space wasted– too small then considerable effort needed to

retrieve file

• MS-DOS– block = 2 x 512 byte sectors– known also as a cluster

Storing a file

• operating system ...– obtains a free block– starts filling it with data– when full, obtains another block– continue until all file is stored

• Problems to solve:– recording sequence of blocks making up a file– keeping track of free blocks

The MS-DOS Solution

File Allocation Table (FAT)Directory Entry

A directory





Multiprogramming

• First machines– single user

• More modern machines– multiple users

• e.g. bank system

– each user perhaps running several programs at same time

• e.g. Windows PC

Achieving multiprogramming

• Single CPU– Concurrent execution– CPU continually switches between programs– C.f. chess grandmaster playing lots of club

players “simultaneously”

• Multiple CPUs– Parallel execution– Each program runs on a separate CPU

Virtual machines

• Each user sharing a machine appears to have machine to themselves

• One real machine

-> many virtual machines

• Two main issues:– Sharing the processor– Sharing main memory

Sharing the processor

• Original problem:– expensive , fast CPU waiting for slow

peripherals – uneconomic

• Solution– have CPU run another program while waiting

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

Disadvantages

• Breakdowns affect everybody

• Portability – need link to central machine

• Expense – but getting cheaper

• Security risks

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

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

Time slicing

Each interval is a time sliceBlocked programs (waiting

for a peripheral) are not allocated time slices

Scheduling – deciding which program runs next

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

Process states

• Running– During its timeslice

• Ready– While some other process is running

• Blocked– Unable to proceed e.g. waiting for input device

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





Sharing the Memory

• Programs need to be in main memory when executing

• May not all fit in

Sharing by swapping

• Program and data– copied into memory at beginning of each time slice

– copied back to disc at end of time slice

• All main memory available to program• But, high overhead of swapping time

Sharing by paging

• Divide program and data into pages

• Only swap in the pages needed during time slice

• Memory divided into pageframes– One page per pageframe

Loading pages

• Pages can be loaded in any order• Use a page table to keep track

Translating addresses

• Given an instruction like JMP 314

• 314 means 314 locations from beginning of program

• Assume each page 100 locations long

• JMP 314 means– page 3 offset 14

• Where is the location in real memory ?

Carrying out the translation

JMP 314

Page number

Real memory address: 2514

Page fault

• Interrupt generated when required page not in main memory

• Causes– Required page to be fetched from disc– Page table updated– Program continues

Thrashing

• Page faults are too frequent

• Most time spent swapping pages

• Caused by:– Bad program design– Too many programs attempting to run

concurrently

Paging strategies

• Fetch strategy– When to fetch a page

• Replacement strategy– When to swap page out

Fetch strategies

• Demand fetch

• Anticipatory fetch

Replacement strategies

• Optimal page replacement

• Longest resident

• Least recently used

• Least frequently used

Documents

Operating Systems Brian Farrimond. Topics Introduction Overview of operating system tasks Operating systems in detail