操作系统1new

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1、Chapter10VirtualMemoryMaincontent:1.Background2.DemandPaging3.ProcessCreation4.PageReplacement5.AllocationofFrames6.Thrashing1.Background(1)introduceSomeinstances:Programsoftenhavecodetohandleunusualerrorconditions.Arrays,lists,andtablesareoftenallocatedmorememorythantheyactuallyneed.Cer

2、tainoptionsandfeaturesofaprogrammaybeusedrarely.Benefitsofpartiallyinmemory:Aprogramwouldnolongerbeconstrainedbytheamountofphysicalmemoryavailable.Moreprogramscouldberunatthesametime.Useprogramwouldrunfaster.(2)VirtualmemorySpecification:separationofuserlogicalmemoryfromphysicalmemory..C

3、haracters:Onlypartoftheprogramneedstobeinmemoryforexecution.Logicaladdressspacecanthereforebemuchlargerthanphysicaladdressspace.Allowsaddressspacestobesharedbyseveralprocesses.Allowsformoreefficientprocesscreation.VirtualMemoryThatisLargerThanPhysicalMemory:(3)implementedDemandpagingDema

4、ndsegmentation2.DemandPaging(1)AboutDemandPagingspecification:Ademandpagingsystemissimilartoapagingsystemwithswapping.Processesresideonsecondarymemory.Bringapageintomemoryonlywhenitisneeded.Ratherthanswappingtheentireprocessintomemory,weusealazyswapper.Lazyswapper:Alazyswapperneverswapsa

5、pageintomemoryunlessthatpagewillbeneeded.example:(2)BasicmethodValid-InvalidBit:Witheachpagetableentryavalid–invalidbitisassociated(1⇒in-memory,0⇒not-in-memory).Initiallyvalid–invalidbutissetto0onallentries.example:PageFault:Iftheprocesstriestoaccessapagethatwasnotbroughtintomemory,acces

6、stoapagemarkedinvalidcauseatraptoOS⇒pagefault.HowtohandlePageFault:OSlooksatanothertabletodecide:Invalidreference⇒abort.Justnotinmemory⇒pageitin.Getemptyframe.Swappageintoframe.Resettables,validationbit=1.RestartinstructionStepsinHandlingaPageFault:Whathappensifthereisnofreeframe?Pagerep

7、lacement–findsomepageinmemory,butnotreallyinuse,swapitout.Problem:Samepagemaybebroughtintomemoryseveraltimes.Howtorestartanyinstructionafterapagefault:Apagefaultcouldoccuratanymemoryreference.Ifthepagefaultoccursontheinstructionfetch,wecanrestartbyfetchingtheinstructionag