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1 ComputerSystemenenApplicaties CSA deel#1vanhetcollegedictaat PatrickvanderSmagt September1995 Secondedition

2 2 thenamesoftheauthorsandtheuniversityofamsterdamarementioned.permissionisalso grantedtousethisbookfornon-commercialcourses,providedtheauthorsarenotiedofthis thisbookfornon-commercialuse,aslongasitisdistributedasawholeinitsoriginalform,and c1994,1995theuniversityofamsterdam.permissionisgrantedtodistributesinglecopiesof beforehand. Theauthorscanbereachedat Kruislaan403,NL{1098SJAmsterdam UniversityofAmsterdam FacultyofMathematics&ComputerScience THENETHERLANDS Phone: Fax: September1994 ComputerSystemenenApplicaties

3 Contents 1Introduction 1.2Structure1:Layersofcomputation:::::::::::::::::::::::::: 1.1Howtounderstandacomputer?::::::::::::::::::::::::::: 2Operatingsystems 1.3Overviewofthisbook::::::::::::::::::::::::::::::::: Tasksofanoperatingsystem::::::::::::::::::::::::::::: Memorymanagement::::::::::::::::::::::::::::: Processmanagement::::::::::::::::::::::::::::::15 2.2Operatingsystemstructure:::::::::::::::::::::::::::::: Themonolithicstructure::::::::::::::::::::::::::: Filemanagement:::::::::::::::::::::::::::::::: Theclient-serverstructure::::::::::::::::::::::::::: Communication:::::::::::::::::::::::::::::::::22 3Networks 3.1Introduction::::::::::::::::::::::::::::::::::::::: Goalsofcomputernetworks:::::::::::::::::::::::::: Buildingblocksofanetwork::::::::::::::::::::::::: TheISO/OSImodel:::::::::::::::::::::::::::::::::: Typesofnetworks::::::::::::::::::::::::::::::: Thesessionandtransportlayers::::::::::::::::::::::: Thepresentationlayer::::::::::::::::::::::::::::: Theapplicationlayer:::::::::::::::::::::::::::::: Thenetworklayer::::::::::::::::::::::::::::::: Thephysicallayer::::::::::::::::::::::::::::::: Thedatalinklayer:::::::::::::::::::::::::::::::37 4Computerarchitecture 3.2.7Summingup:Thereality:::::::::::::::::::::::::::37 4.2Thebus:::::::::::::::::::::::::::::::::::::::::43 4.1Generations::::::::::::::::::::::::::::::::::::::: Architectureofacomputer::::::::::::::::::::::::::41 4.4Processors::::::::::::::::::::::::::::::::::::::::46 4.3Memory::::::::::::::::::::::::::::::::::::::::: Bits,bytes,sizes:::::::::::::::::::::::::::::::: CISCvs.RISCprocessors::::::::::::::::::::::::::: Measuringprocessorspeed:::::::::::::::::::::::::: Memorytypes::::::::::::::::::::::::::::::::: Pipelining,superscalarandvectorprocessors::::::::::::::::50 3

4 Oddmanout::::::::::::::::::::::::::::::::::51 CONTENTS ATechnologietrendsindeinformatica 4.5I/O:::::::::::::::::::::::::::::::::::::::::::: Videodisplays:::::::::::::::::::::::::::::::::52 A.3Mijlpalen::::::::::::::::::::::::::::::::::::::::58 A.2Watiseencomputer?:::::::::::::::::::::::::::::::::56 A.1Inleiding:::::::::::::::::::::::::::::::::::::::::55 A.4Hetexponentieletechnologie-groeimodel:::::::::::::::::::::::60 A.3.3Bedrijfssystemen::::::::::::::::::::::::::::::::60 A.3.1Hardware::::::::::::::::::::::::::::::::::::58 A.4.1Voorbeeldvanexponentieleontwikkeling:geheugenelementen:::::::60 A.3.2Programmeertalenen-methoden:::::::::::::::::::::::59 A.4.2Waarvoorwordtdierekenkrachtgebruikt?::::::::::::::::::61 A.4.3Waaromsteedskleiner?::::::::::::::::::::::::::::61 A.6Toekomstigeontwikkelingen::::::::::::::::::::::::::::::64 A.5Delevensloopvaneencomputer:::::::::::::::::::::::::::63 A.4.5Andereaspecten::::::::::::::::::::::::::::::::63 A.4.4Chipsvandichtbijbekeken::::::::::::::::::::::::::62 Bibliography BGlossary September1994 ComputerSystemenenApplicaties

5 ListofFigures 1.1Threeexamplesoflogicalgates:OR,AND,andNOT.::::::::::::::11 2.2Virtualvs.realmemoryspace.::::::::::::::::::::::::::::19 2.3AnexamplelesystemonaUnixcomputer.::::::::::::::::::::20 2.4Theexecutionofasystemcallinamonolithicoperatingsystem.:::::::::25 2.1Anexampleprocesstree.:::::::::::::::::::::::::::::::17 2.6Theclient-serveroperatingsystemlendsitselfperfectlyforatransparentlydistributedarchitecture.::::::::::::::::::::::::::::::::::27 2.5Theclient-serverstructureoftheMINIXoperatingsystem.::::::::::::26 3.1Alocalareanetwork,consistingofanethernetconnectingvariousdevicestogether 3.2TwohostsAandBcommunicatingoveranOSInetwork.:::::::::::::37 4.1Apictureoftheinsideofa1994workstation.::::::::::::::::::::42 andwiththeoutsideworld.::::::::::::::::::::::::::::::32 4.4Pipelininginaprocessor.:::::::::::::::::::::::::::::::51 4.3TheinsideofaCPU.::::::::::::::::::::::::::::::::::47 4.2Schematicdrawingof(partof)theinsideofacomputer.::::::::::::::42 B.1Virtualmachinesstackedontopofeachother.:::::::::::::::::::67 B.2Atypicaldiskstructure(yetthetrackwidthismuchenlarged).::::::::::68 5

6 6 LISTOFFIGURES September1994 ComputerSystemenenApplicaties

7 ListofTables 1.1Apossibleanatomyofthemoderndigitalcomputer(after(Tanenbaum,1984)).: 2.1BSDandSysVimplementationsofUnixoperatingsystems.::::::::::::14 3.1Namesofthehighest-levelInternetdomains.:::::::::::::::::::: Thehistoryofcomputerarchitecturegenerations.::::::::::::::::::40 3.2TheISO/OSIreferencemodelfornetworking.::::::::::::::::::::34 4.2Comparisonoftypesofmemory.:::::::::::::::::::::::::::46 4.4Numberofdhrystonespersecondforvariouscomputers.::::::::::::::50 4.3Variousprocessorsthroughouttheages.:::::::::::::::::::::::49 B.1TypicaldataforaSunSCSIharddisk.::::::::::::::::::::::::68 7

8 8 LISTOFTABLES September1994 ComputerSystemenenApplicaties

9 Chapter1 Introduction Whatmakesacomputerrun?Therearemanypossibleanswerstothisquestion,dependingon 1.1 thepointofviewofthequestioner.forinstance,anelectricalengineerwho'susedtobuilding Howtounderstandacomputer? circuitshimselfmayhaveatotallydierentviewthanasecretarytypinglettersforhisbosson amaccomputer. systems:thehierarchical,layeredstructure.thislayeredstructuredescribesincreasinglevels andmanyauthorssincethen. ofcomplexityofviewingacomputersystem.inthiswefollowtanenbaum(tanenbaum,1984) Inthischapter,andthroughoutthisbook,we'lladoptanot-so-shockingviewoncomputer 1.2Owego. intable1.1isusefultobreaksacomputersystemdownintosmallerparts.althoughthe Togetabetterunderstandingofthecomputer'sinnerworkings,thelayeredstructureasshown Structure1:Layersofcomputation LevelAbstraction 2546 high-levelprogramming 13 imperativeprogramming assemblylanguage operatingsystemkernel machinelanguage Table1.1:Apossibleanatomyofthemoderndigitalcomputer(after(Tanenbaum,1984)). 0 microprogram digitallogic dissection,aspresentedinthetable,willnotbeunfamiliarorcontroversialtotheinitiated, thedistinctionbetweenthelevelsisnotasclear-cutasthetablemaysuggest.thereisone importantdierence,however,whichmayberegardedasaguideline:ingeneral,thetranslation betweenthehigherlevelsoccursthroughcompilation;thetranslationamongthelowerlevels throughinterpretation.thatistosay,apascalprogramatlevel5will,ingeneral,becompiled thatcomputerwhich,ontheonehand,considersitlevel-by-level,butontheother,alsoexamines isactuallyinterpretedinthemicroprogramatlevel1,insteadofcompiled. tolevel4beforeitisexecuted.ontheotherhand,aprogramatlevel2(inmachinelanguage) Bydissectingadigitalcomputerinsuchaway,thistextwillattempttopresentaviewof 9

10 10 CHAPTER1.INTRODUCTION level-to-level.let'sconsiderthelevelsinturn. 6:Highlevelprogramming.Programminglanguageshavedevelopedalongseverallines.At thetoplevelwethereforedistinguishthree(mutuallyincomparable)programmingstyles: visualprogramming,declarativeprogrammingandobject-orientedprogramming.alanguagemaybelongtoseveralofthesecategories.eachofthethreecategoriescanbe explainedbestbycomparingittoimperativeprogrammingatlevel5(whereyouwillread: aprograminanimperativelanguageisanorderedsequenceofstatementsandprocedures callsspecifyingthestepsthecomputermustfollow). Visualprogrammingallowstheusertousegraphicstocreateaprogram.Sometypes ofcomplexprograms,suchasthosethatuseconcurrentprocessesordealwithreal-time systems,arediculttodescribewithtextuallanguages;graphicalspecicationsmaybe moreappropriate.forinstance,partoftheexspectlanguagethatweuseintheprogramminglabofthiscourseisgraphicalandistypicallyusedtospecifynetworksofinteracting components. Aprogramindeclarative(non-procedural)styletellsthecomputeronlywhatistobe accomplished,butnothowtodoit.logicallanguageslikeprolog,functionallanguages, andalgebraicspecicationlanguagesformsubclassesofdeclarativelanguages. Whereasprogramsinanimperativestyleareprocess-oriented,object-orientedprogrammingfocusesonthedescriptionofobjects:data-structureswiththerelatedoperations.Object-orientedprogrammingnaturallyleadstomodularstyleofprogramming. Somelanguageswithfeaturessupportingtheobject-orientedprogrammingparadigmare Simula,Smalltalk,C++andEiel. 5:Imperativeprogramming.Imperativeorproceduralprogrammingisstilltheprevalent style.aprograminanimperativelanguageisanorderedsequenceofstatementsand procedurescallsspecifyingthestepsthecomputermustfollow.thestatementsreect basicmachineoperations.thenumberandsizeoftherequiredstepsvarywithvarious imperativelanguages.wellknownlanguagesinthisclassarepascal,fortran,algol, COBOL,BASICandC. 4:Assemblylanguage.Thisisthelowestlevelthattheuserusuallysees,ifatall.Assembly languageprogramsconsistoflongsequencesofcodedinstructionssuchasadd,jump,load. Eachinstructiondirectlycorrespondswithaprocessorinstruction.Thereforeassembly languagesarelow-levellanguages:theyaretotallymachinedependent.thisincontrast withthelanguagesatlevels5{8,calledhigh-levellanguages,whichcanusuallybeported fromonemachinetoanotherwithoutmuch(orany)adaptation. Inassemblylanguages,thereareonlyafewdatatypesavailable(forinstance,oating point,character,andinteger),andnoinstructionstobuildnewtypesfromtheexisting ones,suchasispossibleinallhigh-levellanguages.secondly,thereareinstructionsfor jumpingtootherinstructionsand,often,thepossibilitytocallsubroutines(procedures, functions).itisalsopossibletomakesystemcallsbyjumpingtosystemsubroutines:read akeyboardcharacter,readablockfromdisk,etc.suchcallsarehandledbytheoperating system. 3:Operatingsystemkernel.Thislevelcanbeseenasanextensiontolevel2.Thelanguage availableatthatlevelisstillavailablehere,butiscomplementedwiththeoperatingsystem, whichhidesmuchofthelargeintestineofthecomputer.chapter2discussesoperating systems. 2:Machinelanguage.Machinelanguageislikelevel4(assembly),butdoesnotprovidecalls totheoperatingsystem.thisisbare-boneassembly,andgivesthefullpowerofthe September1994 ComputerSystemenenApplicaties

11 1.2.STRUCTURE1:LAYERSOFCOMPUTATION machinetotheuser.prettycumbersometoprogram,this,andwewillnotgointothis 11 1:Microprogram.Onsomecomputers,instructionsonlevel2areinterpretedbythemicroprogramintomicrocodeonlevel1.Considerthefollowingexample.Aninstructionto levelanyfurther. addtonumbers,oneinmemorylocationaandtheotherinmemorylocationb,together andstoretheresultina,mightbeinterpretedasfollows: 2.decodetheinstruction;inthiscase,itisfoundthatthevaluesofaandbmustalso 1.readthenextinstructionstoredatthememoryaddressindicatedbytheprogram befetched. counter. 3.fetchthevalueofa. 4.fetchthevalueofb. 5.getcontentsofmemorylocationa. 6.getcontentsofmemorylocationb. 9.incrementtheprogramcountertoskiptothenextinstruction. 8.storetheresultbackinmemorylocationa. 7.addtwonumberstogetherintheprocessor. tion4.4),theallowableinstructionsaresocomplexthattheycannotbedirectlyexecuted Insomeprocessordesigns(tobeprecise,CISCprocessorswhicharediscussedinsecstructionstoredinmemoryiskept. Theprogramcounterisaregisterintheprocessorinwhichtheaddressofthenextin- 0:Digitallogic.Thenallevel,thatofthedigitallogic,representsthecomputerprogram hardware.nomoreonthislevel,too. inhardwarebutarebrokenintosmaller,simpler,instructionsthatcanbeexecutedin anddataby0and1valuesontheso-calledlogicalgates.thesegatesperformboolean operationssuchasandandorontheirinputs.someexamplesareshowningure1.1. symbol: x1 x2 x1 NOT y=x1jx2 y x2y=x1&x2y zy=xy truthtable: Cnotation: standardnotation: y=x1+x2 00 x1x2y y=x1x2 00 x1x2y y=x 01x y10 can'tbuildaclock,though,withoutacrystal).indeed,computersconsistforalargepart TheAND,OR,andNOTgatestogethersuce(inprinciple)tobuildacomputer(you Figure1.1:Threeexamplesoflogicalgates:OR,AND,andNOT. ComputerSystemenenApplicaties September1994

12 12 fromtheselogicalgates.bycombiningseveraltypesofgates,youcanbuildadders(i.e., CHAPTER1.INTRODUCTION circuitsthataddtwonumbers)andmemory. allnumbersareeither0or1,isnamedaftergeorgeboole(1815{1864)whorstpublished onthesubject.thegatesarefunctionsinthatalgebra,andeachfunctioncanbedescribed Todesigncircuitryusingthesegates,Booleanalgebraisused..Thisalgebra,inwhich byatruthtableasshowninthegure.thesetruthtablesgivetheoutputofthegate Example1Showhowasinglememorycell(i.e.,apieceofcircuitrythatcanstorea0 or1)canbedesignedfromtwonand's(anandisanandfollowedbyanot,which foreachinput;thisispossibleduetothefactthattheinputsarebinary. Letustakeanotherview,somewhatmorehigh-level,atcomputersystems. thereforeoutputsa0whenbothinputsare1,and1otherwise.insymbolsy=x1+x2). Clearly,theauthorstrytocramawholecurriculumintoonecourse.Firstofallwewanttolook 1.3 thelevelsofcomputationinmoredetail.chapter2,forinstance,looksattheoperatingsystem, Overviewofthisbook aroundlevel3andgropingupwardsanddownwards.chapter3makes,literallyandguratively, howtheyconnect.thisleadstoanewlayeredsystemknownasosi.backtobasicsinchapter4, architectures.nextsetoflevels:theprogramminglanguages.moretobewrittenonthatin aside-stepbyplacingafewofthelevel-hierarchiesofcomputersystemsinanetwork,andsee whichconcentratesondesignissuesoccurringbelowlevel3:thehow,what,andwhyofcomputer thisintroduction. underlyingtheoreticalconcepts.howdowemodeltheconnectionbetweentoprocessesrunning ononecomputer?howtopreventdeadlocks?toensuresecurecomputation(nicewhenyou're operatinganuclearplantorasubmarine)?eachchapterwillshowsidestepstosuchtopics. So,apartfromwalkingalongtheroadofboringfactafterfact,wealsowanttoelucidatethe Thuswewillnotonlygointothewhatofcomputerscience,butalsothewhy. toofartoexplainindetail,butareinterestingtoknowanyway. And,thewheniscoveredinappendixA.AppendixBgivesaglossaryoftopicswhichgo September1994 ComputerSystemenenApplicaties

13 Chapter2 Operatingsystems theoperatorprogrammedthemachinebyprovidingitwithasetofinstructions,whichwere Earlycomputing1inthe1940'sand1950'swasabitlikeworkingonapowerfulpocketcalculator: directlyexecutedbythehardware.programmingoccurredinmachinelanguage,codedasbinary orhexadecimalnumbers.withthedevelopmentoftechnologyduringthe1960's,computers becamelarger,faster,andmorecomplex.soonitbecameinfeasibletohaveasingleperson controlallpartsofthecomputersystembyhand.partbypartthesetaskswereautomatedto easethejoboftheoperator:therstoperatingsystemswereborn.inthe1960'scomputers becamepowerfulenoughtoservemultipleusersatthesametime,inordertodecreasethe alsoknownasmultiprogramming,wasborn.mit,belllabs,andgeneralelectricmadean timethatsomeonehadtowaitbeforehecouldgetaccesstothecomputer.thustime-sharing, ofuserstoconnectsimultaneouslytoasinglecomputer. attemptatwritinganoperatingsystemtosupporttime-sharing.thisoperatingsystem,called MULTICS(MULTiplexedInformationandComputingService),wastoallowseveralhundreds Indeed,acurrent-daycomputerwithoutoperatingsystemisunthinkableandvirtuallyunusable. ity,too,inordertokeepthecomputerusableandunderstandableforalargevarietyofusers. Inthelayeredstructureofacomputersystemshownintable1.1,themiddlelayerisrepresentedbytheoperatingsystem.Thismiddlelayerisveryimportant,sinceitshieldsthe particularitiesoftheunderlyinghardwarefromtheabovelayersasmuchaspossible(andde- Asthecomplexityofcomputersincreased,operatingsystemsincreasedinsizeandcomplex- whatevermachinewithoutmuchextrawork;or,thatyoucanupdatetoanext,faster,version thisusercandothesameoperationsonverydierentcomputers.thisasksfortheexistence sired).theoperatingsystempresentstheuserwithavirtualmachine(glossaryp.67)suchthat ofstandards,suchthatyourprogramscanbeusedonasun,silicongraphics,next,pc,or anditwillbeusedeverafter.withpatches,extensions,andalterationstillkingdomcome, ofthesamecomputerwithouthavinganysideeects. ormakesacomputerarchitecturewhichissubsequentlywell-marketed,itspopularitybooms, tokeepupwithnewtrends.unixisaverygoodexample(see(goodheart&cox,1994)fora Unfortunately,standardsareusuallydefacto.Thatistosay,someonewritesaprogram camemuchfurtherthanrunningonafewcomputersatmit.however,kenthompsonfrom toocomplexaproject,belllabsandgeneralelectricbailedout,andtheoperatingsystemnever wonderfuldescriptionofthehistoryofunix).atthetimethatmulticswaswritten,itwas BellLabs,whohadbeenworkingontheMULTICSproject,simpliedMULTICStoprovide accesstoacomputerforoneuser(infact,hehimselfrunningapdp{7),leadingtoanoperating systemwhichhedubbedunics(uniplexedinformationandcomputingservice).together AppendixAislessconcise. withdennisritchie,alsofrombelllabs,theyrewrote,redesigned,andrespelledtheoperating 1Therstsectioninchapter4alsogivesaconcisehistoryofcomputing,notverydierentfromthisone. 13

14 systemtounixinthehigh-levelprogramminglanguagecdesignedbyritchie.whyisunixso 14 CHAPTER2.OPERATINGSYSTEMS popular?well,whenthompsonandritchiehadwrittenunix,at&tlicensedtheoperating systemtouniversitiesalmostforfree,andthusitgainedverymuchinpopularity.sincethen manyavoursofunixsawthelight,andafewattemptsforstandardisationleadtoanincreasing 4;see(Goodheart&Cox,1994)). mess. (SystemFive)wasdesignedatAT&TBellLabs,andintendedasastandardisationofUnix isaavourofunixthathasbeendevisedattheuniversityofcaliforniaatberkeley.sysv ThetwomainstandardisationsareBSDandSysV(alsoknownasSVR4forSysVRevision whichwouldsurpassprevioushacks ataskthathasmostlybeenreached.oneofthemost BSDstandsforBerkeleyStandardDistributionand versions,takingthebestfromallworlds.table2.1showssomeuniximplementations,and importantfeaturesofsysvunixisthatitcombinesbsd,svr3,sunos,andotherunix whetherthey'rebsdorsysv.so,whatisthedierencebetweenthesetwoavours?well,it Table2.1:BSDandSysVimplementationsofUnixoperatingsystems. SunOS(=Solaris1) Solaris(v.2andhigher)Sun Irix NeXTStep OSname machine NeXT,486,Sun,:::BSD SiliconGraphics type Tondoutwhichoperatingsystemyou'reusing,typeuname-a.IfitsaysSunOS5.2orhigher,thenit'sSolaris. Linux PC SysV wouldbepossibletoprovidealistofdetailswheretheonediersfromtheother(wewon't; don'tworry.ifyoureallyneedsuchalist,consult(frisch,1991)and(goodheart&cox,1994)). But:::fundamentaldierences?AccordingtoPerryHutchison, fromstrenuousdisagreementastowhichisthebrokenone. Thefundamentaldierenceisthatoneisbroken,theotherisnot.Theamesarise thesunos4.1.3bsdenvironmentmaybebroughttolight. But,ofcourse,manypeopledisagree. areforthisenvironment,butinafewcasesdierencesbetweensuchasysvenvironmentand Asthetableshows,Solaris2.3isSysVbased.Mostoftheexamplespresentedinthisbook Thematerialinthischapterisforalargepartbasedon(Tanenbaum,1987)and(Hwang,1993), amodulardescriptionofthetasksthathavetobeperformedbyanoperatingsystem. useful. butalsoderivedfromdiscussionswithvariousknow-it-all's.forthis,theinternetisextremely partsarecombinedinsection2.2,whichshowsagoodandanot-so-goodwayofsettingupan operatingsystem. Thestructureofthischapterisasfollows.Insection2.1,therealworkisbegun:itgives Morepracticalissuesarecoveredintheprogramminglabmanual,whichalsocoverstheshell All September1994 indetail.sincetheshellisactuallynotapartoftheoperatingsystembutratheraninterface towardsit,itisnotcoveredhere. ComputerSystemenenApplicaties

15 2.1.TASKSOFANOPERATINGSYSTEM Tasksofanoperatingsystem Aswehaveseeninsection1.2,acomputersystemcanbedissectedinanumberoflayers.One ofthelayersistheoperatingsystem,whichagainconsistsofafewlayers2.inthischapterwe willstudythelayeredstructureofanoperatingsystem. Traditionallythe`layeredness'ofoperatingsystemswasmoreaprinciplethanafact.After all,ifyoureallywanttoseparatepartsofthefunctionalityofanoperatingsystem,youwillalso havetoprovideameansofcouplingthosepartstogether.inatrulymodularsystemthatmeans communication,andcommunicationmeansoverhead:itsimplycoststimetocommunicate,and ifthattimebecomesnoticeable,userswilltendtochooseafasteroperatingsystem.yet,with theincreaseofmemorysize(seesection4.3)andcomputingpower(seesection4.4)ofcomputers, modernoperatingsystemscanindeedassumealayeredstructure,wherethepartsofthesystem arecompletelyseparatedandcooperatebypassingmessages. Theintroductionofthischapterattemptedtoshowwhyyoureallyneedanoperatingsystem. Inthesubsequentsectionswewillstudysometasksanoperatingsystemhastoperform.In ordertosimplifytheunderstandingofanoperatingsystem,thetasksithastoperformcanbe dissectedinanumberofclasses.theseclassesarerelatedtopartsofthecomputer:thecpu (processmanagement),i/o(lemanagement),andthememory(memorymanagement).we willdiscussthesethreepartsinturn.notethatthistrisectionisonlyexemplar;forexample,a textdescribingacomputerdedicatedtodatabasesmighthaveincludedasectionondatabase management;thiscouldalsobeincludedasapartoftheoperatingsystem.or,asystemusedfor imageprocessingmayhaveaspecialpartoftheoperatingsystemdedicatedtospecicoperations forimageprocessing.putallpartstogetherandyouhaveanoperatingsystem Processmanagement Inthemostsimpleoperatingsystems,suchasMSDos,thereisalwaysoneprogramrunning atanytime.forexample,theuserstartstoeditaprogram,andwhennishedquitsthe editor.next,thecompilercanbestartedtocompiletheprogram.thentheuserrunsit.this schememaybesucientforsuchsimpleuse,butisveryinecientandannoyingwhenmore complicatedproblemshavetobetackled.forinstance,aminimumrequirementwouldbeto useacomputerwhileatextisbeingprinted.thisexampleiseasilyextendedtothedesireto runatime-consumingcomputation,whilethecomputercanstillbeusedforediting,compiling, orwhatever.thisalsoleadstoamuchmoreecientuseofacomputer:forinstance,during editingtheeditorspendsintheorderof99.6%3ofitsrunningtimewaitingfortheusertopress thenextkey valuabletimewhichcanbeusedforotherthings. Perhapsthemostimportantsingletaskofanoperatingsystemistoprovidetheuserwith processmanagement.belowtheoperating-systemlevel,adigitalcomputerconsistsofa`stream' ofsimplecommandsinmachinelanguage.thesesimplecommandswillberecognisedbythe processorandleadtosomekindofaction:jump(conditional)toapartoftheprogramstored inmemory,orcallaprocedure(i.e.,retainthereturnaddress);addormultiply;assignvalues. Clearly,atthislevelitwouldnotbeeasytomakeaprogramwhichallowsausertorunmultiple programsatthesametime. Thereforeanoperatingsystemoersprocessmanagement.Aprocess,withincomputer scienceterminology,isaprograminaction.eventhoughwewouldliketorunmultipleprocesses, weoftenhaveonlyoneprocessoratourdisposal.thisproblemissolvedwithtimeslicing. Therunningtimeoftheprocessorisslicedintoverysmallparts,intheorderof10{500ms. 2Andtheshell?Well,theshellactuallyisaninterpreterofadeclarativelanguage.Itmaythereforebeplaced atlevel5,althoughthisisnotreallywhatismeantbythatlevel. 3ThiswasmeasuredbymonitoringvariousviprocessesrunningonSun's. ComputerSystemenenApplicaties September1994

16 16 CHAPTER2.OPERATINGSYSTEMS OnaUnixmachine,theprogrampsshowsalistofprocessesrunningunderthatshell.To getacompletelistofprocesses,typeps-waxu(bsd)orps-edf(sysv).thetopprogram continuouslyshowstherunningprocesses.mostimportantaretheowner,name,processid, andstatusthatareshown. Aprocesscanbecreatedby,forexample,startingaprogramfromtheshell.E.g.,whenyou typetop,youwillalsoseetopasanactiveprocess.youcansendthis(oranyotherprocess ofwhichyouaretheowner)asignal.e.g.,youcankillthetop-processbytyping,inanother window,kill-killprocnr,withprocnrtheprocessidoftop. Duringeachtimeslice,aprocessischosenbytheschedulerandsubsequentlyexecuted.Atthe endoftheslice,theprocessisputinthewaitingstate,andanextprocessischosen.switching fromoneprocesstoanextoneiscalledacontextswitchorprocessswitchandmaytake intheorderof100sonasunsparc.sotheprocessorcontinuouslyswitchesfromrunningone processtoanotherone.thetimethatoneprocessisrundependsonvariousfactors:howmany processesarerunning,howimportantistheprocess,howlongdoesacontextswitchtake.for instance,when100processesarerunningandeachprocessisaallowedtorun100ms,thenit willtake10sbeforeyoureditorwilldisplaythecharacteryoutyped wellabovetheirritation limit. Anoperatingsystemwheretheschedulercontinuouslyswitchesfromonerunnableprocess toanotherissaidtoimplementpreemptivescheduling.thisasopposedtoaso-calledrun tocompletionsystemwhereeachprocessiscompletedbeforeanotheroneisstarted.aswe shallseebelow,althoughpreemptiveschedulingisanecessityonmodernoperatingsystems, itcanleadtoraceconditionswhenmorethanoneprocessneedtoshareresources(e.g.,the disk).aconsiderableeortneedstobeputintheoperatingsystemtosolvethesefacts-of-life. Anotherdisadvantageofpreemptiveschedulingisthatthecomputercannotbeusedforprocess controlanymore.therunningtimeofanyprocesscannotbepredicted,sinceitdependsonother processes.thesystemisnotreal-timeanymore;whenyoutrytocontrolanuclearplantora robotwithsuchasystemyoumayendupwithnastysurpriseswhensomeimportantprocess hadtowaitafewmsbecausenewmailarrived!4 Aprocessisalwaysstartedbyanotherprocessusingoneofvarioussystemcalls.Asystem callislikeanordinaryfunctioncall,butthefunctioniscodedintheoperatingsystem.when processacreatesprocessb,aiscalledtheparentprocessandbthechild.thisleadstoa processtreeasshowningure2.1. Foreachprocessthatisrunningonasinglemachineanentryintheprocesstableis maintained.eachprocesshasaprocessnumber,aprocessstate,andaparentprocess number.theprocessnumberisauniqueidenticationofthatprocess,andisusedtochange therunningpriorityoftheprocess,killaprocessorsenditanothersignal,andsoon.the processstate(seebelow)indicateswhetheraprocessisrunning,sleeping,etc.theparent processnumberindicateswhichprocessstartedthisone;forinstance,forprocessesthatwere startedbytheuser(e.g.,ls,vi)thisistheshellprocessinwhichtheprocesswasinitiated. Theimportanceoftheprocessconceptisthatonecandescribethecomputerintermsofautonomousentities. Processstateandscheduling. Aswasalreadymentioned,anyprocesscanbeinoneofseveralstates.Thesestatesinclude: 4Ofcourse,processescanbegivenaprioritylevel.OnSolarisitisevenpossible(fortrustedusers)tostart real-timeprocesses.theschedulerwillalwaysrunareal-timeprocesstocompletionbeforestartingpreemptive schedulingagain.anyway,caremustbetaken. September1994 ComputerSystemenenApplicaties

17 2.1.TASKSOFANOPERATINGSYSTEM 17 rpcbind sched ttymon init xinit startxpageout keyservfsflushstartedfromkernel ttymonprocessreplacedbyxwindowsession X theuserdecidedtorunxwindows syslogd tcsh xterm xterm tcsh xterm tcsh xclock xcalc OnUnixsystems,initisalwaystherstprocess.Initisthereforetherootofthetree(not Figure2.1:Anexampleprocesstree. xtetris memorymanagerandlesystem).finallytheuserendsupwithxterm,whichinfactstartsashell. runningaprocessisrunning,i.e.,ithasbeenselectedbythescheduler.alsoknownasactive. countingthethreeotherprocessesstartedfromthekernel:thescheduler,andtwoprocessesforthe sleepingasleepingprocessisaprocesswaitingforaninterrupt,orforanotherprocesstoreturn runnablearunnableprocessisabletorun,butcurrentlynotchosenbythescheduler.also controltothisprocess.forinstance,whenaprocessissuesasystemcalltoreaddatafrom knownasreadyorwaiting. Thusthestateofaprocessdependsonwhattheprocessisdoing,andwhetherthescheduler sleepingprocesswillneverbechosenbythescheduler.alsoknownasblocked. disk,theprocesswillbeputinsleepingstateuntiltherequesteddataisavailable.a choseittorunornot.forexample,whenaprocessawantstoreadsomedatafromdisk, theprocesswillbeputintosleepingstatewhileitsrequestisbeinghandled,suchthatother processesmayutilisetheprocessorduringthetimethataiswaitingforitsdata. tocomplywithafewrules.usually,short-runningprocessesareprevalentoverlong-running ones,inordertokeeptheuser,whojustwantstolistadirectory,notwaitingtoolong.secondly, processesmayhaveahigh(0)orlow(127)priority,dependingonhowimportanttheyare,how Whenisaprocesschosenforexecution?Thereareseveralschedulingalgorithms,whichhave longtheyhaverunalready,andothersimilarfactors. Whenmanyprocessesareloaded(eventhoughtheymaybedormant),eachoftheseprocesses 2.1.2Memorymanagement willtakingupapartthecomputer'smemory.notonlyisitwastefulthatnotactiveprocesses isallowedtoreadfromorwritetoeachmemoryaddress,orelseitwouldbeeasytostealor importantmethodsforsolvingthisproblem:swappingandpaging.theissueofsecuritywill usethatmemory,buttheresimplymaynotbeenoughmemorytoholdthemall.therearetwo changesomeoneelse'sdata! notbetreatedhere;however,itbenotedthatthisissueisequallyimportant:noteveryuser ComputerSystemenenApplicaties September1994

18 18 CHAPTER2.OPERATINGSYSTEMS Run,onSolaris(SysV),ps-el(makesureyouhavetherightone.Thispsisstored whichhasthesamefunctionalitybutdierentags(c.q.,-waxu).).everyprocesswillhave /usr/bin/ps.ifnot,specifythefullpath.thealternativemightbethebsd/usr/ucb/ps in/usr/bin;whenyouexecutetheshell-builtincommandwhichpsitshouldshow oneofthefollowingstates: Orunning; Iidle:theprocessisbeingcreated; Ssleeping,i.e.,waitingforaneventtooccur(e.g.,diskinterrupt); Rrunnable,i.e.,itmaybeselectedbytheschedulertorun; Zzombie.Thisoccurswhenaprocesshasnished,butitsparentprocessiskilled.Thechild Ttraced:theprocesshasreceivedaSIGSTOPsignalbecausetheparentistracingit; isstillpresentintheprocesstable. processisnotaliveanymore(thereforecannotbekilled;hencethename`zombie'),but OfeachprocessisalsoshowntheID(processnumber)ofitsparentinthePPIDcolumn.The Xtheprocessiswaitingforamemorypagetobeputintomemory; treehasinitatitsroot. fsflushwritesdiskblocksbacktodiskatregularintervals(`ushing').notethattheprocess pageoutispartofthememorymanager,towritepagesofmemorybacktodiskatintervals, rstfourprocessesare:schedisthescheduler,initistheparentofallfollowingprocesses, InUnixaprocesscanbeassignedanicevalue,whichisanintegerbetween?20and+19 (valueslessthan0canonlybesetbythesuper-user).normally,processesrunatnice-value 0;aprocesswithahighernice-valuehasalowerchancetobepickedbytheschedulerfor execution.atvalue19,aprocessisonlyexecutedifnootherprocess(withalowernice-value) iswaiting.nice-valuescanbecheckedandsetwithtop. September1994 ComputerSystemenenApplicaties

19 2.1.TASKSOFANOPERATINGSYSTEM Swapping 19 WhenaprocessAissleeping,andthememoryitreservesisneededbyanotherprocessB,A canberemovedfromthemainmemory(swappedout)andwrittenonapartitionofthelocal disk,calledtheswapspace.thefreespace,whenbdoesnotneedmorememorythanadid, cannowbeoccupiedbyb. muchswappingmakesacomputerslow.whereasittakesonlyafewmsforacputoswitch fromrunningoneprocesstorunninganother,swappingoutafewmegabytesofmemorytakes tensuptohundredsofmilliseconds. Duetotheharddiskusage(whichareveryslowincomparisonwithmainmemory(table4.2), Paging Butwhatdoyoudowhenasingleprogramistoolargetotintheavailablememory?InMSDos, whenasingleprogramistoolargetotinthemainmemory,usesoverlaysinwhichaprogram issplitup somethingwhichisnotdonebytheoperatingsystembutbytheprogrammer. Firsttherstpartofaprogram(the.exe)isloadedandrun;thispartismainlythecontrol part.itloadstherstoverlay(an.ovlle,usually)fromdiskintomemoryandrunsit.when nished,thesecondoverlayisloadedandrun,andsoon.ofcoursethisisverycumbersomeand inexible;therefore,matureoperatingsystemssupportasystemcalledpaging.theavailable memoryofthecomputerisdividedinanumberofpages.theamountofvirtualmemorythat pagesofthevirtualmemoryareactuallypresentinrealmemory(gure2.2).whenablockof theoperatingsystempresentstotheuserisarbitrarily5larger.atanytime,onlyafewofthe realmemory page1 page2 page3 memoryisrequiredthatisnotinrealmemory,anotherblockwillbemovedoutandreplaced bytherequestedblock.variousmethodsexisttooptimallyimplementapagereplacement Figure2.2:Virtualvs.realmemoryspace. technique(tanenbaum,1987). mainmemoryorram(randomaccessmemory):thememorychipsinyourcomputer.but Withoutexplicitlysayingso,theprevioussectiondiscussedonlyonekindofmemoryknownas 2.1.3Filemanagement thereareothertypesofmemoryfordierentpurposes.thissectiondiscusseshowanothertype ofmemory,viz.diskisused. ComputerSystemenenApplicaties andsegmentation. 5Naturally,thereexistdesignconsiderations.Consult,e.g.,(Tanenbaum,1987)tolearnmoreaboutpaging September1994

20 20Duetothefactthatadiskaveryslowmediumiscomparedtomainmemory(i.e.,the CHAPTER2.OPERATINGSYSTEMS transferofdataismuchslower),itisusedfordierentpurposes.dataondisksisusually accessedthroughles.aleisaprogramorapieceoftextordataonadisk.forexample, alemaybeacprogram,oraletterformattedwithawordprocessor,orindeedthatword processoritself.fileshavenamesandcontents. doesnotstorelesbutblocksofdata;itisthetaskoftheoperatingsystemtolettheuser transfersofblocksofdata. accessthedataonthediskthroughles.theoperatingsystemtranslatestheseletransfersto However,alehasnothingtodowithhowadisk(glossaryp.68)actuallyoperates;adisk les.adirectoryisanodeinthedirectorytreeandmaycontainvariouslesanddirectories. Therootdirectoryingure(2.3)iscalled/inUnix.Subdirectoriescanbeaccessedthrough Filesaregroupedtogetherwithdirectories.Directoriescanbeseenasaspecialkindof opt devices home share man include adam includeles.thedirectory/homeisusedforstoringtheso-calledhomedirectoriesoftheusers optionalsoftwareisstored.thesubdirectorymanstoresmanpages,andincludeisusedforc Therootdirectoryiscalled/.OnSolaris,itcontains,forinstance,thedirectoryoptinwhich Figure2.3:AnexamplelesystemonaUnixcomputer. theirabsolutepath,e.g.,thegamesdirectoryofadamcanbeaccessedas/home/adam/games. onthatcomputer.eachuseragainmayhaveherorhistreeofdirectoriestostoreprogramsand However,whenAdamhasloggedinonthesystem,hiscurrentworkingdirectory(CWD) data. willmostlikelybe/home/adam(whichisknownasadam'shomedirectory).adamcanthen movetohisgamesdirectorybytypingcdgamesinsteadofcd/home/adam/games;heneedonly I/Omanagement specifytherelativepath. terminology.buttwoothertypesofleexist:characterspeciallesandblockspecial Apartfromthedirectories,theleswehaveconsideredsofarareknownasnormallesinUnix les.thesespeciallesareassociatedwithi/o(input/output)devices.forinstance,whereas thekeyboardonacomputerisadevice,itcanbetreatedasifitwereale,anditispossible toreadfromitandwritetoitjustasifitwerealestoredonadisk;nospecialprogramsor systemcallsarerequired.thekeyboardisanexampleofacharacterspecialle:transfergoes September1994 thesedevicesgoesperblockoftypically512or1,024bytes. onecharacteratatime.blockspeciallesaretypicallydisksortapes.transfertoandfrom ComputerSystemenenApplicaties

21 2.1.TASKSOFANOPERATINGSYSTEM Theprogrampwd(printworkingdirectory)descendsthedirectorytree,startingfromthe21 currentworkingdirectory,uptotherootandprintsthepathitdescended.thereisalsoa followingchangedirectory(cd)commands. shellbuilt-incommanddirswhichprintsthecurrentworkingdirectorywhichiskeptfrom Thebasicprogramtoreadfromorwritetoaleiscat.Sincethe`keyboardinput',known asstandardinputorstdin,isalsoale,asis`screenoutput'knownasstandardoutput toprinttheinputofletostdout. promptechoestheinput,linebyline,untiltheend-of-lecharacter^disgiven.typecatle orstdout,thesimplestoperationofcatistocopystdintostdout.typingcatattheshell directoryusingls-l/dev.youwillnoticethatcharacterspeciallesaremarkedbya`c' OnUnixmachines,thespeciallesarekeptinthedirectory/dev.Geta`longlist'ofthe andblockspeciallesbya`b'.mark,forinstance,thele/dev/floppy:thisisthele associatedwiththeoppydisk(providedyourmachinehasone).youwillhavedirectaccess tothediskthroughthisblock-specialle:forinstance,youcanreadfromthediskbytyping, Interrupthandlers.Inaprevioussectionwediscussedthescheduler,whichisapieceof more/dev/floppy. theoperatingsystemwhichrepeatedlychoosesaprocessfromtheprocessqueue,andallowsit Afterall,theschedulerisjustanotherprocess. torunforashortwhile.onequestionwedidnotask,however:whoschedulesthescheduler? aninterrupt.thisinterruptisgeneratedby,inhardware,switchingoneoftheprocessor's aprocessforexecution,itissuesasleep()systemcall,whichallowsittogotosleepfor thespeciedamountoftime.whenthistimehaselapsed,thesystemclockwillgenerate Theanswertothatquestionisthefollowing.Whentheschedulerhasnishedchoosing instructiontellstheprocessorwhichinterrupthandleritwillhavetoexecute. workingon,andreadthenextinstructionfromthedevicethatinitiatedtheinterrupt.this interruptlines`on.'theprocessorwillnowbeinterrupted,stoprunningtheprocessitwasjust interrupthandlerwouldcausetheschedulertoberunagain.whentheschedulerisnished playingaroundwiththeprocesstable,thenextprocesswillbeallowedtorun. handlerfortheharddisk,fortheclock,etc.wouldexist.inthecaseofthescheduler,theclock Thisinterrupthandlerisapieceofcodeintheoperatingsystem.Typically,aninterrupt willstartrunningthedevicedriverforthedisk,andonlyresumecontroltotheinterrupted nishedloadingablockfromthedisktomemory).inthiscase,however,theinterrupthandler programwhenthatdevicedriverisnished. Asimilarthingoccurswhenadiskcontrollergeneratesaninterrupt(e.g.,becauseithas systemonedevicedriverexistsforeachconnectedtypeofdevice;i.e.,onefortheoppydrive, Devicedrivers.WitheachI/Odeviceisassociatedadevicedriver.Ineachoperating oneforthekeyboard,oneforthescreen,etc.thedevicedrivescontainthedevice-dependent softwarenecessarytocontrolthatdevice.forinstance,forascreenthatmaybecodetowrite abytetothescreen,toscrollthescreenonelineup,toclearthescreen,etc. system,andthei/odevices.forinstance,auserprocessmayissuearead()systemcall.the device-independentpartofthei/osoftwareintheoperatingsystemwill,dependingonwhere theissuedreadissupposedtoreadfrom(ale,keyboard,anetwork,:::)forwardthecallto Thedevicedriversprovidetheinterfacebetweenthedevice-independentpartoftheoperating ComputerSystemenenApplicaties thecorrespondingdevicedriver. September1994

22 22 CHAPTER2.OPERATINGSYSTEMS 2.1.4Communication Averyimportantsubjectthatwehavetoconsideriscommunication.Communicationwithin computersystemsconsistsatvariouslevels:communicationbetweenthevariouspartsofthe operatingsystem;communicationbetweentwoprocessesrunningonasingleprocessor;communicationbetweentwoprocessorsinaparallelcomputer;communicationbetweenprocesses runningontwocomputersconnectedtogether;communicationbetweentwoprocessesrunning oncomputersthousandsofmilesapart. Chapter3willstudythistopicinmoredetail.However,inthissectionwewanttodiscuss afewimportantissuesofcommunicationencounteredincomputerscience.inparticular,we willonlydiscussthoseissuesthatareencounteredininter-processcommunicationoripc: thecommunicationbetweenvariousprocessesononecomputer(or,insomecases,betweentwo computersinanetwork). Considerthefollowingcase.SupposetherearetwooperatorsZoe,workinginatravelagency inzierikzee,andzachary,workinginzurich,whobothwanttobookaseatonight521 fortheirrespectivecustomer.thedatabaseforallightbookingsiscentrallymaintainedby Airlines.Asithappens,ZoeandZacharybothmaketheirreservationviaamodematthesame time.accordingtomurphy'slaw6,thefollowingwillhappen.whenzoerequeststheseating availabilityoftheplane,thisdatawillbesenttoher.shepassesittohercustomer,whoneeds afewsecondstodecidehewantstohaveawindoworanaisleseat.whileheisattempting tomakeupismind,zacharygetsthesameinformationfromthecentraldatabase,andhis customerthendecidesshewantstotakeseat11a,whichisstillfree.dittozoe'scustomer!so, Zoewritesbackareservationforseat11a,andsodoesZachary,thusoverwritingZoe'srequest. Who'sgoingtogettheseat? Situationslikethisareknownasraceconditions. Withalittleimaginationitiseasy toseethatthiscaseisalsoverylikelytooccurinanoperatingsystem,forinstancewhentwo processeshavesimultaneousaccesstoadiskblock.whenraceconditionsarenotproperlytaken careof,deadlocksareboundtooccur.adeadlockisaconditionwhereprocessaiswaiting forprocessbwhileprocessbiswaitingfora.inasituationlikethis,neitherprocesswillever continueoperation:adeadlock.naturally,deadlocksshouldalwaysbeprevented. Criticalsections Howcantheseraceconditionsbeavoided?Asolutiontothisproblemistheintroduction ofcriticalsections.acriticalsectionisthatpartofaprogramwheresharedread/write resourcesareaccessed;forexample,accesstosharedmemoryorshareddatabases.whenitcan beguaranteedthatatnotimetwoprocessesaresimultaneouslyintheircriticalsections,race conditionscanbeprevented. Throughouttheyearsofcomputingmanywaysofimplementingcriticalsectionshavebeen invented.mostoperatingsystembookslistmanyoftheseinventions,butwewillkeepthelist downanddiscussonlytwoprincipalmethods. Disablinginterrupts.Theeasiestmethod,whichwouldworkonasingle-usersingle-process machine:disableinterrupts,i.e.,donotallowthecurrentprocesstobeinterruptedbythe scheduler.allcomputershaveaninstructiontothiseect.however,suchapowerfulinstruction couldneverbegiventoauseronatime-sharingsystem:shewouldhavethemachineentirely toherself,andnallythemachinewouldcometoagrindinghalt!fortunately,bettersolutions exist. 6Murphy'srstlawreads:`everythingthatcangowrongwillgowrong.'That'stheonewereferto.His secondlaw,alsonottobeforgotten,maintains,`everythingthatcan'tgowrong,willgowrongnonetheless.' September1994 ComputerSystemenenApplicaties

23 2.1.TASKSOFANOPERATINGSYSTEM Semaphores.Semaphores,introducedbyE.W.Dijkstrain1965,arecounterswhichcan 23 beincrementedanddecrementedbytheinstructionsupanddown,respectively.thedown sleepingonthatsemaphore,oneoftheseprocessesiswokenupandcancompleteitsdown instructiononasemaphorechecksifitsvalueis0.ifitis,theprocessissuingthatdown instruction. Theupinstruction,ontheotherhand,incrementsasemaphore.Ifoneormoreprocesseswere instructionisputtosleep;otherwise,thesemaphoreisdecrementedandtheprocesscontinues. processesthathavetobeexecuted:thebaker'sjob,inwhichacookieisbakedandbroughtto thestore;andthecookie-monster'sjob,inwhichthecookiehastobeboughtandsubsequently computer)whocontinuouslyalternatesinbeingabakerandacookie-monster.therearetwo Considerthefollowingexample.Itconsistsofasingleperson(cf.theprocessorinyour eaten. twovariablesindicatewhetherthestoreisfull(nomorecookiescanbestored)orempty(no don't-interrupt.thecookievariableindicatestheshelfcontainingthecookies.thenext morecookiescanbebought).finally,don't-interrupthastomakesurethatthecookiemonsterdoesnotdecrementthecookiecounter(i.e.,bybuyingacookie)whenthebakerisjust incrementingit.theprogramlookslikethis: processbaker's-job: repeat bake-cookie(cookie); down(store-is-empty); down(don't-interrupt); Thereareafewsharedvariables:cookie,store-is-empty,store-is-full,and forever bring-to-store(cookie); up(don't-interrupt); up(store-is-full); processcookie-monster's-job: repeat down(store-is-full); down(don't-interrupt); buy(cookie); up(don't-interrupt); up(store-is-empty); Notethatthedownandupinstructionsmayneverbeinterrupted!Inanoperatingsystem forever eat-cookie(cookie); theywouldbeimplementedassystemcalls;thesesystemcallsthencouldbewritteninsucha ifthetwodown'sinthebaker'sandcookiemonster'sjobswerereversed,deadlockswouldbe However,youmustbevery,verycarefulwhenprogrammingwithsemaphores.Forinstance, waythattheyneverbeinterruptedbythescheduler. Interprocesscommunication boundtooccur(why?). ComputerSystemenenApplicaties Nowthatwehavelookedintoraceconditionsbetweentwoprocessesrunningononeprocessor, letusshortlylookintosomecommunicationprimitivestobroadentheperspective.theabove September1994

24 solutionwillonlyworkwhentheprocessessharingresourcesrunononeormoreprocessors 24 CHAPTER2.OPERATINGSYSTEMS whichhaveaccesstocommonmemory,i.e.,apieceofmemorytheycanallwritetoandread from.whenyoumovethisideatoadistributedsysteminwhicheachprocessorhasitsownbit Messagepassing.Tothisend,twocommunicationprimitivesareintroduced: betweenremoteprocesses. ofmemory,thatsolutionisnotfeasibleanymore.somehow,theinformationmustbeshared totheoperatingsystem.oneprocesssendsamessagetoanotherprocess,andmayblockuntil send(destination,message)andreceive(source,message)whicharebothsystemcalls thetimeuntilamessagearrives. ananswerarrives.thereceivingprocesscouldbeservingtherstprocess,andmayblockall Remoteprocedurecall.Insomeoperatingsystems,interprocesscommunicationisdone throughremoteprocedurecallorrpc.inrpc,amessageispassedbyaclientprocess sendsamessagetotheserverstubprocedure,whichthencallstherequiredprocedureswithin theservertohandlethemessage.then,amessageissentback,theclientstubwillreceivethe toaserverprocessbycallingaprocedurecalledtheclientstubprocedure.thisprocedure message,andtheclientprocesswillcontinueoperation. getsitsanswer. clientandserver.foralltheycare,thestubprocedurescanbeimplementedusingsemaphores. `Generally,'sincesomethingscangowrong.Forinstance,theservermaycrashbeforetheclient Naturally,thenicepartaboutthisisthatthecommunicationisgenerallytransparenttothe 2.2 Intheprevioussectionsomeofthemoreimportanttasksthatanoperatingsystemhasto performhavebeenlaidout.assumingthatallthesetaskscanbeseparatelysolved,allofthese Operatingsystemstructure discusstwowaysofdoingto:themonolithicstructure,andtheclient-serverstructure. solutionshavetobebroughttogetherintoasingleoperatingsystem.inthissectionwewill 2.2.1Themonolithicstructure Anoperatingsystemwithamonolithicstructurehas,infact,nostructureatall.Allpartsare puttogetherintoonebigchunkofprogram.thisappearstobethemostsimplechoice:make lestogetherintooneexecutable.anyprogrammerwithabitofexperiencewithdesigninglarge separatesourcecodesforthedierentparts,compilethoseintoobjectles,andlinkallobject Unix.Thisisbecauseofthecomputationaleciency:whenallpartsarelinkedtogetherinto fromthepointofviewofmaintenance,portability,clarity,andsoon.yetitisthestructure followedbymanyoperatingsystems,suchasmsdosand,inlesserextent,earlyversionsof programswillimmediatelyrecognisethatthismostobviouschoiceisalsotheworstone,ifonly toandfro.however,withthenecessaryincreasingcomplexityofoperatingsystems,theprice ofthisoverheadtradesverywellagainstthepriceofmaintainingmonolithicoperatingsystems. oneprogram,theycansharedatathroughglobalvariables,insteadofhavingtosendmessages Also,thecostofoverhead(theoperatingsystemrunsslowerbecauseofthetimeneededtopass messagesbetweenitsparts)decreaseswithincreasingcomputerpower. modereal-timemodeexists,whichhasthehighestprecedence).userprocessesnormallyrun berunninginusermodeorkernelmode(insomeoperatingsystemssuchassolaris,athird inusermode.whenauserprocessrequestssomeactionfromtheoperatingsystem,suchas LetusshortlystudyhowamonolithicUnixoperatingsystemworks.Themachinecaneither September1994 openingaleorreadingablock,theuserprocessissuesasystemcall.so,thissystemcall ComputerSystemenenApplicaties

25 2.2.OPERATINGSYSTEMSTRUCTURE userlevel 25 systemcallhandler userprogram kernellevel system le manager memory hardwarelevel hardwarecontrol Figure2.4:Theexecutionofasystemcallinamonolithicoperatingsystem. transferscontroltotheoperatingsystem(seegure2.4),i.e.,theprogramissuingthesystem willbesetinkernelmode:theprocessnowrunningwillhavemanymorepermissions,e.g.,todo callwillbeputinsleepingstateandtheoperatingsysteminrunningstate.next,thecomputer I/Ocontrol,killprocesses,andsoon.Therequestedactionwillbeperformedbytheoperating systemkernel,whichpossiblyhastowaitfori/odevicestocompletearead/writeoperation. Whenthekernelisnishedwithhandlingtherequest,controlisreturnedtotheuserprocess (i.e.,itwillbeputinrunnablestate).induetime,itwillbechosenforexecutionagainbythe scheduler Theclient-serverstructure Thereasonthatthisstructureisonlynowbecomingpopularisthatthecommunicationoverhead ideaisverygeneral:thereisoneserverwhichhandlesrequestsofmanyclients.thisstructureis alsorapidlyincreasingpopularity(or,infact,alreadybecomingstandard)inoperatingsystems. Wehavealreadycomeacrosstheclient-serverstructurebetweencommunicatingprocesses.The computersthatareavailablenowadays.pcownerswhohaveeverexperimentedwithrunning MINIX7ona8088ora80286processorwillquicklylearntoappreciatetheirMSDosoperating system,eventhoughitisuser-unfriendlybeyondcomparison.minix,albeitanoperating (messagepassingoripc)thatitleadstoisonlynowbecomingnegligibleduetothefast aslittleaspossiblecodeintothekernel.thekernel,whichisnowreducedtoamicrokernel, systemwithacrystal-clearorganisation,isjusttooslowforsucharchaiccomputerarchitectures. operatesasaservertohandlerequestsofvariousclients.thetaskthatislefttothismicrokernelisverylimited:handlethecommunicationbetweentheprocesses,andgivetheprimitives Theideabehindtheclient-serverstructureforoperatingsystemsisthefollowing.One:put server8,leserver,memorymanager,etc.theremainingmicrokernelisleftwithonlyonetask: necessarytopreventraceconditions. 7MINIXisaUnix-likeoperatingsystem(BSD)writtenbyA.S.Tanenbaum(Tanenbaum,1987)forPC's.The Two:migrateasmuchaspossiblefunctionalitytostand-aloneclients,suchastheprocess ofminixasanoperatingsystemislimited,however,duetothefactthatitisratherslow thecommunication overhead,whilethreads(seebelow)arenotavailable,neckitwhentoomanyprocessesareactive. fromscratch,partlyforteachingpurposes,andendedupwithaverywell-structuredproduct.thepopularity distributionandthereforestudyoftheiroperatingsystem.tanenbaumthendecidedtorewriteasimilarproduct reasonhewroteitwasthat,inthe1980's,at&tbecameawareofthedistributabilityofunix,andprohibitedthe ComputerSystemenenApplicaties 8Inthiscasetheword`server'hasanotherconnotationthanthesamewordwhenusedfortheclient-server September1994

26 handlethecommunicationbetweenthevariousclients. 26 CHAPTER2.OPERATINGSYSTEMS AlthoughMINIXisnotactuallyusedonmanycomputers,itsclearstructureaidsintheunderstandingofhowoperatingsystemswork.ThestructureoftheMINIXoperatingsystemisbuild upoutoffourlevels,asdepictedingure2.5.atthelowestlevel,thecommunicationbetween Asaveryneatexampleofaclient-serverbasedoperatingsystemwewillconsiderisMINIX. clockdiskcamera::: useruser manager memory user system le::: processmanagement Figure2.5:Theclient-serverstructureoftheMINIXoperatingsystem. andmessagepassing Themicro-kernelonlyperformsprocessmanagementandhandlesthecommunication(passingof theprocessesistakencareof,andtrapsandinterrupts(explainedbelow)arecaught.this messages)betweenthememorymanager,lesystem,etc.i.e.,thekernelservestheother lowestlevelisverysimilartothemicrokerneldiscussedbefore,exceptthatprocessmanagement processors. devicedrivers.theseprocessesareallcompletelystand-aloneandcommunicatewitheachother bymessagepassingthroughthemicrokernel.layer3,containing(amongothers)thememory isalsodoneatthislevelandnotbyaseparateclient.atthesecondlowestlevelarethei/o memorymanagement(e.g.,forkforstartinganewprocessorbrkforchangingtheamountof managerandlesystem,presentstheuserwithsystemcalls.forinstance,allsystemcallsfor manager,whichwillsubsequentlysendmessagestothelowerlevels. inthreedierentways: memorythatisreservedforaprocesstostoreitsdata)arecallstofunctionsinthememory interruptaninterrupt,aswesawbefore,isaconditionthatisnotinuencedbytherunning Atlayer4theuserprocessesarerunning.Ingeneral,userprocesseshaveaccesstothekernel hascompletedthetransferofablockofdata.thediskcontrollerchipwillgeneratean Asanotherinterruptexample,assumeaprogramArunningwhilethediskhardware areused,amongotherthings,fortheschedulertochoosewhentorunanextprocess. programbutratherbysomethingexternal,e.g.,adisk,ortheclock.clockinterrupts backtoa.so,aninterruptcanbeseenasatransparentprocedurecall,instantiatedby thediskinterrupthandler.whenthisprogramisnished,controlwillbetransferred interrupt.theprocessorwillthenstopexecutingprogramaandtransferexecutionto trapatrapissimilartoaninterrupt,butisdirectlygeneratedbytherunningprogramitself. externalinuences. Therearetwokindoftraps:thoseresultingfromanerrorofsomekind,andthoseresulting themachineinkernelmode.theformer,error-generated,trapsresultforinstancewhen servestotransfercontrolfromauser-programtotheoperatingsystem'skernel,andput aspecialinstructioncalledthetrapinstruction,alsoknownasakernelcall.thistrap fromasystemcall,e.g.,readingfromale.thissystemcallmakestheprocessorexecute model;admitted,veryunfortunate::: adivisionby0occurs,oraoatingpointoverow,anillegalinstruction,orareferenceto apartofmemorywhereaprogramisnotallowedtoreadorwrite. September1994 ComputerSystemenenApplicaties