操作系统概念第七版答案含编程代码.docx

操作系统概念第七版答案含编程代码.docx

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操作系统概念第七版答案含编程代码

solutionstotheexercises

Chapter1

1.1Inamultiprogrammingandtime-sharingenvironment,severaluserssharethesystemsimultaneously.Thissituationcanresultinvarioussecurityproblems.

a.Whataretwosuchproblems?

b.Canweensurethesamedegreeofsecurityinatime-sharedmachineasinadedicatedmachine?

Explainyouranswer.

Answer:

a.Stealingorcopyingone’sprogramsordata;usingsystemresources（CPU,memory,diskspace,peripherals）withoutproperaccounting.

b.Probablynot,sinceanyprotectionschemedevisedbyhumanscaninevitablybebrokenbyahuman,andthemorecomplexthescheme,themoredifficultitistofeelconfidentofitscorrectimplementation.

1.2Theissueofresourceutilizationshowsupindifferentformsindifferenttypesofoperatingsystems.Listwhatresourcesmustbemanaged

carefullyinthefollowingsettings:

a.Mainframeorminicomputersystemsb.Workstationsconnectedtoserversc.Handheldcomputers

Answer:

a.Mainframes:

memoryandCPUresources,storage,networkbandwidth.

b.Workstations:

memoryandCPUresouces

c.Handheldcomputers:

powerconsumption,memoryresources.

1.3UnderwhatcircumstanceswouldauserbebetteroffusingatimesharingsystemratherthanaPCorsingle-userworkstation?

Answer:

Whentherearefewotherusers,thetaskislarge,andthehardwareisfast,time-sharingmakessense.Thefullpowerofthesystemcanbebroughttobearontheuser’sproblem.Theproblemcanbesolvedfasterthanonapersonalcomputer.Anothercaseoccurswhenlotsofotherusersneedresourcesatthesametime.

Apersonalcomputerisbestwhenthejobissmallenoughtobeexecutedreasonablyonitandwhenperformanceissufficienttoexecutetheprogramtotheuser’ssatisfaction.

1.4Whichofthefunctionalitieslistedbelowneedtobesupportedbytheoperatingsystemforthefollowingtwosettings:

（a）handhelddevicesand（b）real-timesystems.a.Batchprogrammingb.Virtualmemoryc.Timesharing

Answer:

Forreal-timesystems,theoperatingsystemneedstosupportvirtualmemoryandtimesharinginafairmanner.Forhandheldsystems,theoperatingsystemneedstoprovidevirtualmemory,butdoesnotneedtoprovidetime-sharing.Batchprogrammingisnotnecessaryinbothsettings.

1.5Describethedifferencesbetweensymmetricandasymmetricmultiprocessing.Whatarethreeadvantagesandonedisadvantageofmultiprocessorsystems?

Answer:

Symmetricmultiprocessingtreatsallprocessorsasequals,andI/OcanbeprocessedonanyCPU.AsymmetricmultiprocessinghasonemasterCPUandtheremainderCPUsareslaves.Themasterdistributestasksamongtheslaves,andI/Oisusuallydonebythemasteronly.

Multiprocessorscansavemoneybynotduplicatingpowersupplies,housings,andperipherals.Theycanexecuteprogramsmorequicklyandcanhaveincreasedreliability.Theyarealsomorecomplexinbothhardwareandsoftwarethanuniprocessorsystems.

1.6Howdoclusteredsystemsdifferfrommultiprocessorsystems?

Whatisrequiredfortwomachinesbelongingtoaclustertocooperatetoprovideahighlyavailableservice?

Answer:

Clusteredsystemsaretypicallyconstructedbycombiningmultiplecomputersintoasinglesystemtoperformacomputationaltaskdistributedacrossthecluster.MultiprocessorsystemsontheotherhandcouldbeasinglephysicalentitycomprisingofmultipleCPUs.Aclusteredsystemislesstightlycoupledthanamultiprocessorsystem.Clusteredsystemscommunicateusingmessages,whileprocessorsinamultiprocessorsystemcouldcommunicateusingsharedmemory.

Inorderfortwomachinestoprovideahighlyavailableservice,thestateonthetwomachinesshouldbereplicatedandshouldbeconsistentlyupdated.Whenoneofthemachinesfail,theothercouldthentake-overthefunctionalityofthefailedmachine.

1.7Distinguishbetweentheclient-serverandpeer-to-peermodelsofdistributedsystems.

Answer:

Theclient-servermodelfirmlydistinguishestherolesoftheclientandserver.Underthismodel,theclientrequestsservicesthatareprovidedbytheserver.Thepeer-to-peermodeldoesn’thavesuchstrictroles.Infact,allnodesinthesystemareconsideredpeersandthusmayactaseitherclientsorservers-orboth.Anodemayrequestaservicefromanotherpeer,orthenodemayinfactprovidesuchaservicetootherpeersinthesystem.

Forexample,let’sconsiderasystemofnodesthatsharecookingrecipes.Undertheclient-servermodel,allrecipesarestoredwiththeserver.Ifaclientwishestoaccessarecipe,itmustrequesttherecipefromthespecifiedserver.Usingthepeer-to-peermodel,apeernodecouldaskotherpeernodesforthespecifiedrecipe.Thenode（orperhapsnodes）withtherequestedrecipecouldprovideittotherequestingnode.Noticehoweachpeermayactasbothaclient（i.e.itmayrequestrecipes）andasaserver（itmayproviderecipes.）

1.8Consideracomputingclusterconsistingoftwonodesrunningadatabase.Describetwowaysinwhichtheclustersoftwarecanmanageaccesstothedataonthedisk.Discussthebenefitsanddisadvantagesofeach.

Answer:

Considerthefollowingtwoalternatives:

asymmetricclusteringandparallelclustering.Withasymmetricclustering,onehostrunsthedatabaseapplicationwiththeotherhostsimplymonitoringit.Iftheserverfails,themonitoringhostbecomestheactiveserver.Thisisappropriateforprovidingredundancy.However,itdoesnotutilizethepotentialprocessingpowerofbothhosts.Withparallelclustering,thedatabaseapplicationcanruninparallelonbothhosts.Thedifficultyimplementingparallelclustersisprovidingsomeformofdistributedlockingmechanismforfilesontheshareddisk.

1.9Howarenetworkcomputersdifferentfromtraditionalpersonalcomputers?

Describesomeusagescenariosinwhichitisadvantageoustousenetworkcomputers.

Answer:

Anetworkcomputerreliesonacentralizedcomputerformostofitsservices.Itcanthereforehaveaminimaloperatingsystemtomanageitsresources.Apersonalcomputerontheotherhandhastobecapableofprovidingalloftherequiredfunctionalityinastandalonemannerwithoutrelyingonacentralizedmanner.Scenarioswhereadministrativecostsarehighandwheresharingleadstomoreefficientuseofresourcesarepreciselythosesettingswherenetworkcomputersarepreferred.

1.10Whatisthepurposeofinterrupts?

Whatarethedifferencesbetweenatrapandaninterrupt?

Cantrapsbegeneratedintentionallybyauserprogram?

Ifso,forwhatpurpose?

Answer:

Aninterruptisahardware-generatedchange-of-flowwithinthesystem.Aninterrupthandlerissummonedtodealwiththecauseoftheinterrupt;controlisthenreturnedtotheinterruptedcontextandinstruction.Atrapisasoftware-generatedinterrupt.AninterruptcanbeusedtosignalthecompletionofanI/Otoobviatetheneedfordevicepolling.Atrapcanbeusedtocalloperatingsystemroutinesortocatcharithmeticerrors.

1.11Directmemoryaccessisusedforhigh-speedI/OdevicesinordertoavoidincreasingtheCPU′sexecutionload.

a.HowdoestheCPUinterfacewiththedevicetocoordinatethetransfer?

b.HowdoestheCPUknowwhenthememoryoperationsarecomplete?

c.TheCPUisallowedtoexecuteotherprogramswhiletheDMAcontrolleristransferringdata.Doesthisprocessinterferewiththeexecutionoftheuserprograms?

Ifso,describewhatformsofinterferencearecaused.

Answer:

TheCPUcaninitiateaDMAoperationbywritingvaluesintospecialregistersthatcanbeindependentlyaccessedbythedevice.ThedeviceinitiatesthecorrespondingoperationonceitreceivesacommandfromtheCPU.Whenthedeviceisfinishedwithitsoperation,itinterruptstheCPUtoindicatethecompletionoftheoperation.

BoththedeviceandtheCPUcanbeaccessingmemorysimultaneously.Thememorycontrollerprovidesaccesstothememorybusinafairmannertothesetwoentities.ACPUmightthereforebeunabletoissuememoryoperationsatpeakspeedssinceithastocompetewiththedeviceinordertoobtainaccesstothememorybus.

1.12Somecomputersystemsdonotprovideaprivilegedmodeofoperationinhardware.Isitpossibletoconstructasecureoperatingsystemforthesecomputersystems?

Giveargumentsboththatitisandthatitisnotpossible.

Answer:

Anoperatingsystemforamachineofthistypewouldneedtoremainincontrol（ormonitormode）atalltimes.Thiscouldbeaccomplishedbytwomethods:

a.Softwareinterpretationofalluserprograms（likesomeBASIC,Java,andLISPsystems,forexample）.Thesoftwareinterpreterwouldprovide,insoftware,whatthehardwaredoesnotprovide.

b.Requiremeantthatallprogramsbewritteninhigh-levellanguagessothatallobjectcodeiscompiler-produced.Thecompilerwouldgenerate（eitherin-lineorbyfunctioncalls）theprotectionchecksthatthehardwareismissing.

1.13Givetworeasonswhycachesareuseful.Whatproblemsdotheysolve?

Whatproblemsdotheycause?

Ifacachecanbemadeaslargeasthedeviceforwhichitiscaching（forinstance,acacheaslargeasadisk）,whynotmakeitthatlargeandeliminatethedevice?

Answer:

Cachesareusefulwhentwoormorecomponentsneedtoexchangedata,andthecomponentsperformtransfersatdifferingspeeds.Cachessolvethetransferproblembyprovidingabufferofintermediatespeedbetweenthecomponents.Ifthefastdevicefindsthedataitneedsinthecache,itneednotwaitfortheslowerdevice.Thedatainthecachemustbekeptconsistentwiththedatainthecomponents.Ifaomponenthasadatavaluechange,andthedatumisalsointhecache,thecachemustalsobeupdated.Thisisespeciallyaproblemonmultiprocessorsystemswheremorethanoneprocessmaybeaccessingadatum.Acomponen