本文格式為Word版，下載可任意編輯——操作系統第九版部分課后作業習題答案CHAPTER9VirtualMemory

PracticeExercises

9.1Underwhatcircumstancesdopagefaultsoccur?Describetheactionstakenbytheoperatingsystemwhenapagefaultoccurs.Answer:

Apagefaultoccurswhenanaccesstoapagethathasnotbeenbroughtintomainmemorytakesplace.Theoperatingsystemveri?esthememoryaccess,abortingtheprogramifitisinvalid.Ifitisvalid,afreeframeislocatedandI/Oisrequestedtoreadtheneededpageintothefreeframe.UponcompletionofI/O,theprocesstableandpagetableareupdatedandtheinstructionisrestarted.

9.2Assumethatyouhaveareferencestringforaprocesswithmframes(initiallyallempty).Thereferencestringhaslengthp;ndistinctpagenumbersoccurinit.Answerthesequestionsforanyreplacementalgorithms:

a.Whatisalowerboundonthenumberofpagefaults?b.Whatisanupperboundonthenumberofpagefaults?Answer:a.nb.p

9.3ConsiderthepagetableshowninFigure9.30forasystemwith12-bitvirtualandphysicaladdressesandwith256-bytepages.Thelistoffree

pageframesisD,E,F(thatis,Disattheheadofthelist,Eissecond,andFislast).

Convertthefollowingvirtualaddressestotheirequivalentphysicaladdressesinhexadecimal.Allnumbersaregiveninhexadecimal.(Adashforapageframeindicatesthatthepageisnotinmemory.)?9EF?111

2930Chapter9VirtualMemory?700?0FFAnswer:?9EF-0EF?111-211?700-D00?0FF-EFF

9.4Considerthefollowingreplacementalgorithms.Rankthesealgorithmsona?ve-pointscalefrom“bad〞to“perfect〞accordingtotheirfaultrate.SeparatethosealgorithmsthatsufferfromBelady’sanomalyfromthosethatdonot.a.LRUreplacementb.FIFOreplacementc.Optimalreplacement

d.Second-chancereplacementAnswer:

RankAlgorithmSufferfromBelady’sanomaly1Optimalno2LRUno

3Second-chanceyes4FIFOyes

9.5Discussthehardwaresupportrequiredtosupportdemandpaging.Answer:

Foreverymemory-accessoperation,thepagetableneedstobeconsultedtocheckwhetherthecorrespondingpageisresidentornotandwhethertheprogramhasreadorwriteprivilegesforaccessingthepage.Thesecheckshavetobeperformedinhardware.ATLBcouldserveasacacheandimprovetheperformanceofthelookupoperation.

9.6Anoperatingsystemsupportsapagedvirtualmemory,usingacentralprocessorwithacycletimeof1microsecond.Itcostsanadditional1microsecondtoaccessapageotherthanthecurrentone.Pageshave1000words,andthepagingdeviceisadrumthatrotatesat3000revolutionsperminuteandtransfers1millionwordspersecond.Thefollowingstatisticalmeasurementswereobtainedfromthesystem:

?1percentofallinstructionsexecutedaccessedapageotherthanthecurrentpage.

?

Oftheinstructionsthataccessedanotherpage,80percentaccessedapagealreadyinmemory.PracticeExercises31?

Whenanewpagewasrequired,thereplacedpagewasmodi?ed50percentofthetime.

Calculatetheeffectiveinstructiontimeonthissystem,assumingthatthesystemisrunningoneprocessonlyandthattheprocessorisidleduringdrumtransfers.Answer:

effectiveaccesstime=0.99×(1??sec+0.008×(2??sec)+0.002×(10,000??sec+1,000??sec)+0.001×(10,000??sec+1,000??sec)=(0.99+0.016+22.0+11.0)??sec=34.0??sec

9.7Considerthetwo-dimensionalarrayA:intA[][]=newint[100][100];

whereA[0][0]isatlocation200inapagedmemorysystemwithpagesofsize200.Asmallprocessthatmanipulatesthematrixresidesinpage0(locations0to199).Thus,everyinstructionfetchwillbefrompage0.Forthreepageframes,howmanypagefaultsaregeneratedbythefollowingarray-initializationloops,usingLRUreplacementand

assumingthatpageframe1containstheprocessandtheothertwoareinitiallyempty?a.for(intj=0;j<100;j++)for(inti=0;i<100;i++)A[i][j]=0;

b.for(inti=0;i<100;i++)for(intj=0;j<100;j++)A[i][j]=0;Answer:a.5,000b.50

9.8Considerthefollowingpagereferencestring:1,2,3,4,2,1,5,6,2,1,2,3,7,6,3,2,1,2,3,6.

Howmanypagefaultswouldoccurforthefollowingreplacementalgorithms,assumingone,two,three,f