计算机操作系统原理9

[e-mail]20109.1OperatingSystemConceptsChapter9:MemoryManagementBackgroundSwappingContiguousAllocationPagingSegmentationSegmentationwithPaging*[e-mail]20109.2OperatingSystemConcepts9.1BackgroundProgrammustbebroughtintomemoryandplacedwithinaprocessforittoberun.Inputqueue–thecollectionofprocessesonthediskthatarewaitingtobebroughtintomemorytoruntheprogram.Thenormalprocedureistoselectoneoftheprocessesintheinputqueueandtoloadthatprocessintomemory.Mostsy[e-mail]20109.3OperatingSystemConcepts9.1.1AddressBindingUsually,userprogramsgothroughseveralstepsbeforebeingrun.Addressesmayberepresentedindifferentwaysduringthesesteps.Addressesinsourceprogramaregenerallysymbolic(suchascount,i,jetc.)Compilertypicallybindsthesesymbolicaddressestorelocatableaddresses(suchas“14bytesfromthebeginningofthismodule”)Linkageeditororloaderwillbindtheserelocatableaddressestoabsoluteaddressesinturn.[e-mail]20109.4OperatingSystemConcepts9.1.1AddressBinding(1)Bindingofinstructionsanddatatomemoryaddressescanhappenatthreedifferentstages.Compiletime--Ifknownstartinglocationinmemory,absolutecodecanbegenerated;butmustrecompilecodeifstartinglocationchanges.(e.g..COMfilesinMS-DOS)Loadtime--Mustgeneraterelocatablecodeifmemorylocationisnotknownatcompiletime.Thefinalbindingisdelayeduntilloadtime.Executiontime–Bindingisdelayeduntilruntime,iftheprocesscanbemovedduringitsexecutionfromonememorysegmenttoanother.Needspecialhardwaresupportforaddressmaps(e.g.,baseandlimitregisters).[e-mail]20109.5Ope[e-mail]20109.6OperatingSystemConcepts9.1.2LogicalvsPhysicalAddressSpaceTheconceptofalogicaladdressspacethatisboundtoaseparatephysicaladdressspaceiscentraltopropermemorymanagement.Logicaladdress–generatedbyCPU;alsoreferredtoasvirtualaddress.Thesetofalllogicaladdressesgeneratedbyaprogramisalogical-addressspace.Physicaladdress–addressseenbythememoryunit.Thesetofallphysicaladdressescorrespondingtotheselogicaladdressesisaphysical-addressspace.Compile-timeandload-timeaddress-bindingschemesgenerateidenticallogicalandphysicaladdresses;[e-mail]20109.7OperatingSystemConcepts9.1.2LogicalvsPhysicalAddressSpace(1)Memory-ManagementUnit(MMU)--Hardwaredevicethatmapsvirtualaddresstophysicaladdressinrun-time.InMMUscheme,valueinrelocationregisterisaddedtoeveryaddressgeneratedbyauserprocesswhenitissenttomemory.RangesfordifferenttypesofaddressesLogicaladdresses—range0tomaxPhysicaladdresses-–rangeR+0toR+maxforabasevalueRTheuserprogramdealswithlogicaladdressesandneverseestherealphysicaladd[e-mail]20109.8OperatingSystemConcepts9.1.2LogicalvsPhysicalAddressSpace(2)[e-mail]20109.9OperatingSystemConcepts9.1.3DynamicLoadingDynamicloading–Routineisnotloadeduntilitiscalled,whicharekeptondiskinarelocatableloadformat.Usedtoobtainbettermemory-spaceutilization.Unusedroutineisneverloaded.Itisusefulwhenlargeamountsofcodeareneededtohandlecasesoccurringinfrequently.Nospecialsupportisrequiredfromtheoperatingsystemwhenimplementedthroughprogramdesign.Operatingsystemcanpr[e-mail]20109.10OperatingSystemConcepts9.1.4DynamicLinkingLinkingispostponeduntilexecutiontime.Smallpieceofcode,stub,usedtolocatetheappropriatememory-residentlibraryroutine.Stubreplacesitselfwiththeaddressoftheroutine,andexecutestheroutine.Operatingsystemneededtocheckiftheroutineisinprocesses’(itselforanother)memoryaddress.Dynamiclinkingisparticularlyusefulforsharedlibrariesupdating(suchasbugfixes),whichcalleddynamicallylinkedlibraries(DLL)[e-mail]20109.11OperatingSystemConcepts9.1.5OverlaysOverlays–tokeepinmemoryonlythoseinstructionsanddatathatareneededatanygiventime.Whenotherinstructionsareneeded,theyareloadedintospaceoccupiedpreviouslybyinstructionsthatarenolongerneeded.Neededwhenprocessislargerthantheamountofmemoryallocatedtoit.Implementedbyuser,nospecialsupportneededfromoperatingsystem,programmingdesignofoverlaystructureiscomplex.Overlaysiscurrentlylimitedtomicrocomputerandothersystemsthathavelimitedamountsofphysica[e-mail]2[e-mail]20109.13OperatingSystemConcepts9.2SwappingAprocesscanbeswappedtemporarilyoutofmemorytoabackingstore,andthenbroughtbackintomemoryforcontinuedexecution.Backingstore–afastdiskthatmustbelargeenoughtoaccommodatecopiesofallmemoryimagesforallusers;mustprovidedirectaccesstothesememoryimages.Rollout,rollin–avariantofswappingpolicyisusedforpriority-basedschedulingalgorithms;lower-priorityprocessisswappedout,sohigher-priorityp