操作系统教学课件.ppt

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1、Chapter9VirtualMemoryOperatingSystemConceptsChapter9:VirtualMemoryBackgroundDemandPagingProcessCreation(Copy-on-Write)PageReplacementAllocationofFramesThrashingMemory-MappedFilesOtherConsiderations2ObjectivesTodescribethebenefitsofavirtualmemorysystemToexplaintheconceptsofd

2、emandpaging,page-replacementalgorithms,andallocationofpageframesTodiscusstheprincipleoftheworking-setmodel39.1BackgroundAllmemorymanagementschemesattempttohavemanyprocessesinmemoryatthesametimetosupportahighdegreeofmultiprogramming.Virtualmemory–separationofuserlogicalmemor

3、yfromphysicalmemory.Virtualmemorycanbeimplementedvia:DemandpagingDemandsegmentation49.1BackgroundVirtualmemory–isatechniquethatdoesnotrequiretheentirelogicaladdressspacetobeinmemoryforexecution.Onlypartoftheprogramneedstobeinmemoryforexecution.Additionalpartsoftheaddressspa

4、cearebroughtinasneeded.Logicaladdressspacecanthereforebemuchlargerthanphysicaladdressspace.Thisalsoallowssomeorganizationsofaprocess’saddressspacetobesharedbyotherprocesses.Thiscertainlyallowsformoreefficientprocesscreation,astheentireaddressspacedoesnotneedtobebroughtintop

5、rimarymemoryinthebeginningofexecution.5VirtualMemoryThatisLargerThanPhysicalMemorySomebasicrequirementsstillhold:Instructionstobeexecutedmustbeinphysicalmemory.Unfortunately,thislimitsthesizeofaprogramtothesizeofmemory.Intruth,oftentimesmajorpartsofprogramsareneverexecutedf

6、oragiven‘run.’6VirtualMemoryThatisLargerThanPhysicalMemorySo,theabilitytoexecuteaprogramonlypartiallyinmemoryisagreatstepforward.Programmersmaynowwriteprogramsnotconstrainedbymemorysize.Theymaythusconcentrateontheproblemthey’resolvingratherthanbylimitationsofthecomputingsys

7、tem.Moreso,manyprogramsmaybemorereadily(容易地)runatthesametimebecauseentireprogramsneednotbephysicallyinmemoryatthesametime.7VirtualMemoryThatisLargerThanPhysicalMemory(P316)Considerthisfigure:Wecanseethatonlypartsofaprogramresideinphysicalmemoryatanygivensnapshotintime.Othe

8、r‘parts’ofaprogrammayresideondiskuntil(andif)needed.Clearly,thepagesarenotcontiguo