The Memory System-II.pdf

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1、SFTW230:SystemsandNetworksIComputerOrganizationTopicscovered:Chapter5.TheMemorySystem(II)Dr.WUWen,FST,UMOutlineßInterleavingmemoryaccessacrossmemorymodulesßVirtualmemoriesßSecondarystoragedevices15.6.1InterleavingßThemainmemoryisstructuredasacollectionofphy

2、sicallyseparatemodules.Eachmodulehasitsownaddressanddatabufferregister(ABR&DBR).……MainmemoryßWehopetheaddressoffetchingdatacanbeevenlydistributed-->severalmodulesarekeptbusyduringtheexecutionprocess.Thisiscalled“interleavingmemoryaccesses”acrossseveralmemor

3、ymodules.2InterleavingßHowtolettheaddressesdistributeoverthemodules?©Twopossibleaddresslayouts.©Whichschemebelowcanbeinterleaved?©Interleavingmemoryaccessescanhideaccesslatency•FasteraccesstoablockofdataIninterleaved•Higheraverageuseofthememorysystemstructu

4、re,theremustbe2kmodules.3Example:theeffectofinterleavingissubstantialßSupposewehaveacachewith8words/blockOnareadmiss,weneedtoloadablockofdatafrommainmemory.What’sthetimeneeded?©Ittakes1clockcycletosendanaddresstothemainmemoryandonetosendbacktocache.DRAMallo

5、wsthefirstwordtobeaccessedin8cycles,andsubsequentwordsoftheblockareaccessedin4cyclesperwordA.Forsinglememorymodule:1+8+(7x4)+1=38cyclesprocessorcachememory4Example:theeffectofinterleavingissubstantialB.Forinterleavedscheme,thememoryisconstructedas4interleav

6、edmodulesßSendaddresses(1),all4modulesaccesstherequireddata(8cycles)ßTransferback(4cycles),atthesametime,thenextwordineachmoduleisaccessedß4cyclestotransferthemtocache1+8+4+4=17cycles.181181181Interleavingreduces181theblocktransfertime141bymorethanafactor14

7、1141of2.14155.7VirtualmemoriesßVirtualmemorytechnique©Themainmemorycanactasa“cache”forthesecondarystorage(magneticdisks).ßThephysicalmainmemoryinacomputerislimited©Alargeprogram•Itcannotcompletelyloadintothemainmemorytorunatonce.•Partsarestoredonsecondaryst

8、orage.•Whenanewpieceofaprogramistobetransferredintothemainmemory–Mainmemoryisfull->Someotherpiecemustbereplaced•Thenalleventuallyarebroughtintothemainmemory(usingDMA)asneeded©Operatingsystemautomatical