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1、Chapter6Memoryfenlei.wdsxx.com分类信息news.wdsxx.com新闻2Chapter6ObjectivesMastertheconceptsofhierarchicalmemoryorganization.Understandhoweachlevelofmemorycontributestosystemperformance,andhowtheperformanceismeasured.Mastertheconceptsbehindcachememory,virtualmem
2、ory,memorysegmentation,pagingandaddresstranslation.fenlei.wdsxx.com分类信息news.wdsxx.com新闻36.1IntroductionMemoryliesattheheartofthestored-programcomputer.Inpreviouschapters,westudiedthecomponentsfromwhichmemoryisbuiltandthewaysinwhichmemoryisaccessedbyvarious
3、ISAs.Inthischapter,wefocusonmemoryorganization.Aclearunderstandingoftheseideasisessentialfortheanalysisofsystemperformance.fenlei.wdsxx.com分类信息news.wdsxx.com新闻46.2TypesofMemoryTherearetwokindsofmainmemory:randomaccessmemory,RAM,andread-only-memory,ROM.Ther
4、earetwotypesofRAM,dynamicRAM(DRAM)andstaticRAM(SRAM).DynamicRAMconsistsofcapacitorsthatslowlyleaktheirchargeovertime.Thustheymustberefreshedeveryfewmillisecondstopreventdataloss.DRAMis“cheap”memoryowingtoitssimpledesign.fenlei.wdsxx.com分类信息news.wdsxx.com新闻
5、56.2TypesofMemorySRAMconsistsofcircuitssimilartotheDflip-flopthatwestudiedinChapter3.SRAMisveryfastmemoryanditdoesn’tneedtoberefreshedlikeDRAMdoes.Itisusedtobuildcachememory,whichwewilldiscussindetaillater.ROMalsodoesnotneedtoberefreshed,either.Infact,itne
6、edsverylittlechargetoretainitsmemory.ROMisusedtostorepermanent,orsemi-permanentdatathatpersistsevenwhilethesystemisturnedoff.fenlei.wdsxx.com分类信息news.wdsxx.com新闻66.3TheMemoryHierarchyGenerallyspeaking,fastermemoryismoreexpensivethanslowermemory.Toprovideth
7、ebestperformanceatthelowestcost,memoryisorganizedinahierarchicalfashion.Small,faststorageelementsarekeptintheCPU,larger,slowermainmemoryisaccessedthroughthedatabus.Larger,(almost)permanentstorageintheformofdiskandtapedrivesisstillfurtherfromtheCPU.fenlei.w
8、dsxx.com分类信息news.wdsxx.com新闻6.3TheMemoryHierarchyThisstorageorganizationcanbethoughtofasapyramid:786.3TheMemoryHierarchyToaccessaparticularpieceofdata,theCPUfirstsendsarequesttoitsnearestmemory,usuallycache.I
