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1、Chapter9:VirtualMemoryBackgroundDemandPagingCopy-on-WritePageReplacementAllocationofFramesThrashingMemory-MappedFilesAllocatingKernelMemoryBackgroundVirtualmemorySeparationofuserlogicalmemoryfromphysicalmemoryOnlypartoftheprograminmemoryLogicaladdressspace>=physicaladdressspaceInmany
2、cases,theentireprogramnotneeded:ErrorcodeArrays,lists&tablesallocatedmorethanneedCertainoptions&featuresofaprogramrarelyusedBackgroundBenefits:ProgramscanbelargerthanphysicalmemoryProgrammersneedn’tcareaboutmemorylimitationsMoreprogramscouldberunsimultaneouslyLessI/Oneeded,eachuserpr
3、ogramwouldrunfasterEasy2sharefilesamongseveralprocessesMoreefficientprocesscreationDrawbacks:VirtualMemoryThat isLargerThanPhysicalMemory?Virtual-addressSpaceSparseaddressspaces(holes)SharedLibraryUsingVirtualMemoryLibrarycanbesharedCommunicationFork()systemcallVirtualMemoryVirtualme
4、morycanbeimplementedvia:DemandpagingPagingondemandDemandsegmentationSegmentingondemandDemandPagingPagingondemandBringapageintomemoryonlywhenitisneededLessI/OneededLessmemoryneededFasterresponseMoreusersLazyswapperneverswapsapageintomemoryunlesspagewillbeneededSwapperthatdealswithpage
5、sisapagerTransferofaPagedMemory toContiguousDiskSpaceValid-InvalidBitAvalid–invalidbitforeachpagetableentryAllaresettoiinitiallyv?in-memory&legali?not-in-memory?pagefaultwhenreferencedinvalidreference?abortnot-in-memory?bringtomemoryvvvvii….Frame#valid-invalidbitpagetablePageTableWhe
6、nSomePages AreNotinMainMemoryPageFaultpagefaultFirstreferencetoapagewilltraptooperatingsystem:Invalidreference?abortValidbutnotinmemoryFindafreeframe(fromthefreeframelist)SwappageintoframeResetpagetablesSetvalidationbit=vRestarttheinstructionthatcausedthepagefaultStepsinHandlingaPage
7、FaultPageFault(Cont.)RestartinstructionC=A+B1.fetchtheinstruction2.fetchA3.fetchB4.AddAandB5.StorethesuminC(Cnotinmemory)ProblemscanbeignoredRepetition1instructioncauseNpagefaultsPageFault(Cont.)Restartinstruction1instructionmodifyNlocationsE.g.MVC(movecharacter),canmove256bytesBlock
8、smaystraddle
