+第三版++答桉.docx

+第三版++答桉.docx

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+第三版++答桉

SOLUTIONSTOCHAPTER1PROBLEMS

1.Anoperatingsystemmustprovidetheuserswithanextended（i.e.,virtual）machine,anditmustmanagetheI/Odevicesandothersystemresources.

2.MultiprogrammingistherapidswitchingoftheCPUbetweenmultipleprocessesinmemory.ItiscommonlyusedtokeeptheCPUbusywhileoneormoreprocessesaredoingI/O.

3.Inputspoolingisthetechniqueofreadinginjobs,forexample,fromcards,ontothedisk,sothatwhenthecurrentlyexecutingprocessesarefinished,therewillbeworkwaitingfortheCPU.Outputspoolingconsistsoffirstcopyingprintablefilestodiskbeforeprintingthem,ratherthanprintingdirectlyastheoutputisgenerated.

Acronymforsimultaneousperipheraloperationson-line,spoolingreferstoputtingjobsinabuffer,aspecialareainmemoryoronadiskwhereadevicecanaccessthemwhenitisready.

Inputspoolingonapersonalcomputerisnotverylikely,butoutputspoolingis.

4.TheprimereasonformultiprogrammingistogivetheCPUsomethingtodowhilewaitingforI/Otocomplete.IfthereisnoDMA,theCPUisfullyoccupieddoingI/O,sothereisnothingtobegained（atleastintermsofCPUutilization）bymultiprogramming.NomatterhowmuchI/Oaprogramdoes,theCPUwillbe100percentbusy.Thisofcourseassumesthemajordelayisthewaitwhiledataarecopied.ACPUcoulddootherworkiftheI/Owereslowforotherreasons（arrivingonaserialline,forinstance）.

5.Secondgenerationcomputersdidnothavethenecessaryhardwaretoprotecttheoperatingsystemfrommalicioususerprograms.

6.Itisstillalive.Forexample,IntelmakesPentiumI,II,andIII,and4CPUswithavarietyofdifferentpropertiesincludingspeedandpowerconsumption.Allofthesemachinesarearchitecturallycompatible.Theydifferonlyinpriceandperformance,whichistheessenceofthefamilyidea.

Producememorybank

7.A25×80charactermonochrometextscreenrequiresa2000-bytebuffer.The1024×768pixel24-bitcolorbitmaprequires2,359,296bytes.In1980thesetwooptionswouldhavecost$10and$11,520,respectively.Forcurrentprices,checkonhowmuchRAMcurrentlycosts,probablylessthan$1/MB.

8.Choices（a）,（c）,and（d）shouldberestrictedtokernelmode.

9.Personalcomputersystemsarealwaysinteractive,oftenwithonlyasingleuser.Mainframesystemsnearlyalwaysemphasizebatchortimesharingwithmanyusers.Protectionismuchmoreofanissueonmainframesystems,asisefficientuseofallresources.

10.Everynanosecondoneinstructionemergesfromthepipeline.Thismeansthemachineisexecuting1billioninstructionspersecond.Itdoesnotmatteratallhowmanystagesthepipelinehas.A10-stagepipelinewith1nsecperstagewouldalsoexecute1billioninstructionspersecond.Allthatmattersishowoftenafinishedinstructionspopsouttheendofthepipeline.

11.Themanuscriptcontains80×50×700=2.8millioncharacters.Thisis,ofcourse,impossibletofitintotheregistersofanycurrentlyavailableCPUandistoobigfora1-MBcache,butifsuchhardwarewereavailable,themanuscriptcouldbescannedin2.8msecfromtheregistersor5.8msecfromthecache.Thereareapproximately27001024-byteblocksofdata,soscanningfromthediskwouldrequireabout27seconds,andfromtape2minutes7seconds.Ofcourse,thesetimesarejusttoreadthedata.Processingandrewritingthedatawouldincreasethetime.

12.Logically,itdoesnotmatterifthelimitregisterusesavirtualaddressoraphysicaladdress.However,theperformanceoftheformerisbetter.Ifvirtualaddressesareused,theadditionofthevirtualaddressandthebaseregistercanstartsimultaneouslywiththecomparisonandthencanruninparallel.Ifphysicaladdressesareused,thecomparisoncannotstartuntiltheadditioniscomplete,increasingtheaccesstime.

13.Maybe.Ifthecallergetscontrolbackandimmediatelyoverwritesthedata,whenthewritefinallyoccurs,thewrongdatawillbewritten.However,ifthedriverfirstcopiesthedatatoaprivatebufferbeforereturning,thenthecallercanbeallowedtocontinueimmediately.Anotherpossibilityistoallowthecallertocontinueandgiveitasignalwhenthebuffermaybereused,butthisistrickyanderrorprone.

14.Atrapiscausedbytheprogramandissynchronouswithit.Iftheprogramisrunagainandagain,thetrapwillalwaysoccuratexactlythesamepositionintheinstructionstream.Aninterruptiscausedbyanexternaleventanditstimingisnotreproducible.

15.Base=40,000andlimit=10,000.Anansweroflimit=50,000isincorrectforthewaythesystemwasdescribedinthisbook.Itcouldhavebeenimplementedthatway,butdoingsowouldhaverequiredwaitinguntiltheaddress+basecalculationwascompletedbeforestartingthelimitcheck,thusslowingdownthecomputer.

16.Theprocesstableisneededtostorethestateofaprocessthatiscurrentlysuspended,eitherreadyorblocked.Itisnotneededinasingleprocesssystembecausethesingleprocessisneversuspended.

17.Mountingafilesystemmakesanyfilesalreadyinthemountpointdirectoryinaccessible,somountpointsarenormallyempty.However,asystemadministratormightwanttocopysomeofthemostimportantfilesnormallylocatedinthemounteddirectorytothemountpointsotheycouldbefoundintheirnormalpathinanemergencywhenthemounteddevicewasbeingcheckedorrepaired

18.Forkcanfailiftherearenofreeslotsleftintheprocesstable（andpossiblyifthereisnomemoryorswapspaceleft）.Execcanfailifthefilenamegivendoesnotexistorisnotavalidexecutablefile.Unlinkcanfailifthefiletobeunlinkeddoesnotexistorthecallingprocessdoesnothavetheauthoritytounlinkit.

19.Ifthecallfails,forexamplebecausefdisincorrect,itcanreturn−1.Itcanalsofailbecausethediskisfullanditisnotpossibletowritethenumberofbytesrequested.Onacorrecttermination,italwaysreturnsnbytes.

20.Itcontainsthebytes:

1,5,9,2.

21.Blockspecialfilesconsistofnumberedblocks,eachofwhichcanbereadorwrittenindependentlyofalltheotherones.Itispossibletoseektoanyblockandstartreadingorwriting.Thisisnotpossiblewithcharacterspecialfiles.

22.Systemcallsdonotreallyhavenames,otherthaninadocumentationsense.Whenthelibraryprocedurereadtrapstothekernel,itputsthenumberofthesystemcallinaregisteroronthestack.Thisnumberisusedtoindexintoatable.Thereisreallynonameusedanywhere.Ontheotherhand,thenameofthelibraryprocedureisveryimportant,sincethatiswhatappearsintheprogram.

23.Yesitcan,especiallyifthekernelisamessage-passingsystem.

24.Asfarasprogramlogicisconcerneditdoesnotmatterwhetheracalltoalibraryprocedureresultsinasystemcall.Butifperformanceisanissue,ifataskcanbeaccomplishedwithoutasystemcalltheprogramwillrunfaster.Everysystemcallinvolvesoverheadtimeinswitchingfromtheusercontexttothekernelcontext.Furthermore,onamultiusersystemtheoperatingsystemmayscheduleanotherprocesstorunwhenasystemcallcompletes,furtherslowingtheprogressinrealtimeofacallingprocess.

25.SeveralUNIXcallshavenocounterpartintheWin32API:

Link:

aWin32programcannotrefertoafilebyanalternatenameorseeitinmorethanonedirectory.Also,attemptingtocreatealinkisaconvenientwaytotestforandcreatealockonafile.Mountandumount:

aWindowsprogramcannotmakeassumptionsaboutstandardpathnamesbecauseonsystemswithmultiplediskdrivesthedrivenamepartofthepathmaybedifferent.Chmod:

Windowsprogrammershavetoassumethateveryusercanaccesseveryfile.Kill:

Windowsprogrammerscannotkillamisbehavingprogramthatisnotcooperating.

26.Theconversionsarestraightforward:

（a）Amicroyearis10-6×365×24×3600=31.536sec.

（b）1000metersor1km.

（c）Thereare240bytes,whichis1,099,511,627,776bytes.

（d）Itis6×1024kg.

SOLUTIONSTOCHAPTER2PROBLEMS

1.Thetransitionfromblockedtorunningisconceivable.SupposethataprocessisblockedonI/OandtheI/Ofinishes.IftheCPUisotherwiseidle,theprocesscouldgodirectlyfromblockedtorunning.Theothermissingtransition,fromreadytoblocked,isimpossible.AreadyprocesscannotdoI/Ooranythingelsethatmightblockit.Onlyarunningprocesscanblock.

2.Youcouldhavearegistercontainingapointertothecurrentprocesstableentry.WhenI/Ocompleted,theCPUwouldstorethecurrentmachinestateinthecurrentprocesstableentry.Thenitwouldgototheinterruptvectorfor

theinterruptingdeviceandfetchapointertoanotherprocesstableentry（theserviceprocedure）.Thisprocesswouldthenbestartedup.

3.Generally,high-levellanguagesdonotallowonethekindofaccesstoCPUhardwarethatisrequired.Forinstance,aninterrupthandlermayberequiredtoenableanddisabletheinterruptservicingaparticulardevice,ortomanipulatedatawithinaprocess’stackarea.Also,interruptserviceroutinesmustexecuteasrapidlyaspossible.

4.Thereareseveralreasonsforusingaseparatestackforthekernel.Twoofthemareasfollows.First,youdonotwanttheoperatingsystemtocrashbecauseapoorlywrittenuserprogramdoesnotallowforenoughstackspace.Second,ifthekernelleavesstackdatainauserprogram’smemoryspaceuponreturnfromasystemcall,amalicioususermightbeabletousethisdatatofindoutinformationaboutotherprocesses.

5.Itwouldbedifficult,ifnotimpossible,tokeepthefilesystemconsistent.Supposethataclientprocesssendsarequesttoserverprocess1toupdateafile.Thisprocess