操作系统习地的题目.docx

操作系统习地的题目.docx

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操作系统习地的题目

1.1Whatarethethreemainpurposesofanoperatingsystem?

Answer:

•Toprovideanenvironmentforacomputerusertoexecuteprogramsoncomputerhardwareinaconvenientandefficientmanner.

•Toallocatetheseparateresourcesofthecomputerasneededtosolvetheproblemgiven.Theallocationprocessshouldbeasfairandefficientaspossible.

•Asacontrolprogramitservestwomajorfunctions:

（1）supervisionoftheexecutionofuserprogramstopreventerrorsandimproperuseofthecomputer,and

（2）managementoftheoperationandcontrolofI/Odevices.

1.2Whatarethemaindifferencesbetweenoperatingsystemsformainframecomputersandpersonalcomputers?

Answer:

Generally,operatingsystemsforbatchsystemshavesimplerrequirementsthanforpersonalcomputers.Batchsystemsdonothavetobeconcernedwithinteractingwithauserasmuchasapersonalcomputer.Asaresult,anoperatingsystemforaPCmustbeconcernedwithresponsetimeforaninteractiveuser.Batchsystemsdonothavesuchrequirements.Apurebatchsystemalsomayhavenottohandle

timesharing,whereasanoperatingsystemmustswitchrapidlybetweendifferentjobs.

1.3Listthefourstepsthatarenecessarytorunaprogramonacompletelydedicatedmachine.

Answer:

a.Reservemachinetime.

b.Manuallyloadprogramintomemory.

c.Loadstartingaddressandbeginexecution.

d.Monitorandcontrolexecutionofprogramfromconsole.

1.4Wehavestressedtheneedforanoperatingsystemtomakeefficientuseofthecomputinghardware.Whenisitappropriatefortheoperatingsystemtoforsakethisprincipleandto“waste”resources?

Whyissuchasystemnotreallywasteful?

Answer:

Single-usersystemsshouldmaximizeuseofthesystemfortheuser.AGUImight“waste”CPUcycles,butitoptimizestheuser’sinteractionwiththesystem.

1.5Whatisthemaindifficultythataprogrammermustovercomeinwritinganoperatingsystemforareal-timeenvironment?

Answer:

Themaindifficultyiskeepingtheoperatingsystemwithinthefixedtimeconstraintsofareal-timesystem.Ifthesystemdoesnotcompleteataskinacertaintimeframe,itmaycauseabreakdownoftheentiresystemitisrunning.Thereforewhenwritinganoperatingsystemforareal-timesystem,thewritermustbesurethathisschedulingschemesdon’tallowresponsetimetoexceedthetimeconstraint.

1.6Considerthevariousdefinitionsofoperatingsystem.ConsiderwhethertheoperatingsystemshouldincludeapplicationssuchasWebbrowsersAndmailprograms.Argueboththatitshouldandthatitshouldnot,andsupportyouranswer.

Answer:

Point.Applicationssuchaswebbrowsersandemailtoolsareperforminganincreasinglyimportantroleinmoderndesktopcomputersystems.Tofulfillthisrole,theyshouldbeincorporatedaspartoftheoperatingsystem.Bydoingso,theycanprovidebetterperformanceandbetterintegrationwiththerestofthesystem.Inaddition,these

importantapplicationscanhavethesamelook-and-feelastheoperatingsystemsoftware.

Counterpoint.ThefundamentalroleoftheoperatingsystemistomanagesystemresourcessuchastheCPU,memory,I/Odevices,etc.Inaddition,it’sroleistorunsoftwareapplicationssuchaswebbrowsersandemailapplications.Byincorporatingsuchapplicationsintotheoperatingsystem,weburdentheoperatingsystemwithadditionalfunctionality.Suchaburdenmayresultintheoperatingsystemperformingaless-thansatisfactoryjobatmanagingsystemresources.Inaddition,weincreasethesizeoftheoperatingsystemtherebyincreasingthelikelihoodofsystemcrashesandsecurityviolations.

1.7Howdoesthedistinctionbetweenkernelmodeandusermodefunctionasarudimentaryformofprotection（security）system?

Answer:

Thedistinctionbetweenkernelmodeandusermodeprovidesarudimentaryformofprotectioninthefollowingmanner.CertaininstructionscouldbeexecutedonlywhentheCPUisinkernelmode.Similarly,hardwaredevicescouldbeaccessedonlywhentheprogram

isexecutinginkernelmode.ControloverwheninterruptscouldbeenabledordisabledisalsopossibleonlywhentheCPUisinkernelmode.Consequently,theCPUhasverylimitedcapabilitywhenexecutinginusermode,therebyenforcingprotectionofcriticalresources.

1.8Whichofthefollowinginstructionsshouldbeprivileged?

a.Setvalueoftimer.

b.Readtheclock.

c.Clearmemory.

d.Issueatrapinstruction.

e.Turnoffinterrupts.

f.Modifyentriesindevice-statustable.

g.Switchfromusertokernelmode.

h.AccessI/Odevice.

Answer:

Thefollowingoperationsneedtobeprivileged:

Setvalueoftimer,clearmemory,turnoffinterrupts,modifyentriesindevice-statustable,accessI/Odevice.Therestcanbeperformedinusermode.

1.9Someearlycomputersprotectedtheoperatingsystembyplacingitinamemorypartitionthatcouldnotbemodifiedbyeithertheuserjobortheoperatingsystemitself.Describetwodifficultiesthatyouthinkcouldarisewithsuchascheme.

Answer:

Thedatarequiredbytheoperatingsystem（passwords,accesscontrols,accountinginformation,andsoon）wouldhavetobestoredinorpassedthroughunprotectedmemoryandthusbeaccessibletounauthorizedusers.

1.10SomeCPUprovideformorethantwomodesofoperation.Whataretwopossibleusesofthesemultiplemodes?

Answer:

Althoughmostsystemsonlydistinguishbetweenuserandkernelmodes,someCPUshavesupportedmultiplemodes.Multiplemodescouldbeusedtoprovideafiner-grainedsecuritypolicy.Forexample,ratherthandistinguishingbetweenjustuserandkernel-mode,

youcoulddistinguishbetweendifferenttypesofusermode.Perhapsusersbelongingtothesamegroupcouldexecuteeachother’scode.Themachinewouldgointoaspecifiedmodewhenoneoftheseuserswasrunningcode.Whenthemachinewasinthismode,amemberofthe

groupcouldruncodebelongingtoanyoneelseinthegroup.Anotherpossibilitywouldbetoprovidedifferentdistinctionswithinkernelcode.Forexample,aspecificmodecouldallowUSBdevicedriverstorun.ThiswouldmeanthatUSBdevicescouldbeservicedwithout

havingtoswitchtokernelmode,therebyessentiallyallowingUSBdevicedriverstoruninaquasi-user/kernelmode.

1.11Timerscouldbeusedtocomputethecurrenttime.Provideashortdescription

ofhowthiscouldbeaccomplished.

Answer:

Aprogramcouldusethefollowingapproachtocomputethecurrenttimeusingtimerinterrupts.Theprogramcouldsetatimerforsometimeinthefutureandgotosleep.Whenitisawakenedbytheinterrupt,itcouldupdateitslocalstate,whichitisusingtokeeptrackofthenumberofinterruptsithasreceivedthusfar.Itcouldthenrepeatthisprocessofcontinuallysettingtimerinterruptsandupdatingitslocalstatewhentheinterruptsareactuallyraised.

1.12IstheInternetaLANoraWAN?

Answer:

TheInternetisaWANasthevariouscomputersarelocatedatgeographicallydifferentplacesandareconnectedbylong-distancenetworklinks.

2.1Whatisthepurposeofsystemcalls?

Answer:

Systemcallsallowuser-levelprocessestorequestservicesoftheoperatingsystem.

2.2Whatarethefivemajoractivitiesofanoperatingsysteminregardtoprocessmanagement?

Answer:

a.Thecreationanddeletionofbothuserandsystemprocesses

b.Thesuspensionandresumptionofprocesses

c.Theprovisionofmechanismsforprocesssynchronization

d.Theprovisionofmechanismsforprocesscommunication

e.Theprovisionofmechanismsfordeadlockhandling

2.3Whatarethethreemajoractivitiesofanoperatingsysteminregardtomemorymanagement?

Answer:

a.Keeptrackofwhichpartsofmemoryarecurrentlybeingused

andbywhom.

b.Decidewhichprocessesaretobeloadedintomemorywhenmemory

spacebecomesavailable.

c.Allocateanddeallocatememoryspaceasneeded.

2.4Whatarethethreemajoractivitiesofanoperatingsysteminregardtosecondary-storagemanagement?

Answer:

•Free-spacemanagement.

•Storageallocation.

•Diskscheduling.

2.5Whatisthepurposeofthecommandinterpreter?

Whyisitusuallyseparatefromthekernel?

Answer:

Itreadscommandsfromtheuserorfromafileofcommandsandexecutesthem,usuallybyturningthemintooneormoresystemcalls.Itisusuallynotpartofthekernelsincethecommandinterpreterissubjecttochanges.

2.6Whatsystemcallshavetobeexecutedbyacommandinterpreterorshell

inordertostartanewprocess?

Answer:

InUnixsystems,aforksystemcallfollowedbyanexecsystem

callneedtobeperformedtostartanewprocess.Theforkcallclonesthe

currentlyexecutingprocess,whiletheexeccalloverlaysanewprocess

basedonadifferentexecutableoverthecallingprocess.

2.7Whatisthepurposeofsystemprograms?

Answer:

Systemprogramscanbethoughtofasbundlesofuseful

systemcalls.Theyprovidebasicfunctionalitytouserssothatusersdo

notneedtowritetheirownprogramstosolvecommonproblems.

2.8Whatisthemainadvantageofthelayeredapproachtosystemdesign?

Whatarethedisadvantagesofusingthelayeredapproach?

Answer:

Asinallcasesofmodulardesign,designinganoperating

systeminamodularwayhasseveraladvantages.Thesystemiseasier

todebugandmodifybecausechangesaffectonlylimitedsectionsof

thesystemratherthantouchingallsectionsoftheoperatingsystem.

Informationiskeptonlywhereitisneededandisaccessibleonlywithin

adefinedandrestrictedarea,soanybugsaffectingthatdatamustbe

limitedtoaspecificmoduleorlayer.

2.9Listfiveservicesprovidedbyanoperatingsystem.Explainhoweach

providesconveniencetotheusers.Explainalsoinwhichcasesitwould

beimpossibleforuser-levelprogramstoprovidetheseservi