Memory Systems

1.3MemorySystems,Chapter1ComputerHardware,1.3MemorySystem,MainpointMemorysystemspecifications存储系统规范Hierarchicalstructureofmemory分级结构存储DevicesandoperationofmemoryRAM(RANDOMACCESSMEMORY)ROM(READONLYMEMORY)CD-ROMCacheMagneticrecordingdevicesMagneticdiskMagnetictape,Keywords,Hierarchicalmemory存储器层次结构Cache高速缓冲存储器,高速缓存Chip芯片On-chipcache单片高速缓存Silicondie硅片Magneticdisk磁盘Mainmemory主存储器Pagedvirtualmemory页式虚拟存储器,Keywords,RANDOMACCESSMEMORY(RAM)随机（访问）存储器ReadOnlyMemory(ROM)只读存储器Boot引导，启动，自举CompactDiskROM(CD-ROM)只读光盘Diskdrive磁盘驱动器Floppydrive(diskette)软磁盘Writeonce,readmany(WORM)一次写多次读Magnetictape磁带,Keywords,Registerfile寄存器组Latency潜伏时间、等待时间Pageframe页帧Realmemory(storage)实存储器DynamicRAM(DRAM)动态随机存储器Benchmark基准测试程序，基准程序，测试标准，基准Volatile易失性Laserstorage激光存储器,1.MemorySystemDesiderata,MemorySystemDesiderataSize:infinitelylarge,noconstraintsonprogramordatasetsizeSpeed:infinitelyfast,latencyequaltothefastestmemorytechnologyavailableCost:theperbitcostshouldapproachthelowest-costtechnologyavailableClearlythesespecificationscannotallbeachievedastheyaremutuallyexclusive.,MemorySystem,However,withthesemiconductor（半导体）andmagneticmemorytechnologyoftoday,thesespecifications（规格、说明书）arecloselyapproximated.（接近的、估计的）,2.MemoryHierarchy,CPU,SecondaryCache(L2)256K4MB(10ns),PrimaryCache(L1)64KB4MB(1ns),MainMemory64MB-1024MB(50ns),TapeBackup10GB-10TB(seconds),InternetUnlimited(minutes),Latency（潜伏期）1ns=0.0000000001seconds(nanosecond)Storage1byte=8bits1KB(kilobyte)=1024bytes=(103)bytes1MB(megabyte)=1024KB=(106)bytes1GB(gigabyte)=1024MB=(109)bytes1TB(terabyte)=1024MB=(1012)bytes1PB(petabyte)=1024TB=(1015)bytes1EB(exabyte)=1024PB=(1018)bytes1ZB(zettabyte)=1024EB=(1021)bytes1YB(yottabyte)=1024ZB=(1024)bytes,Faster,Larger,HardDrive50GB-250GB(10ms),Instructionoperands(HW),Blocks(HW),Pages(OS),Files(OS),MemoryHierarchy,BigdataWhatisthesizeoftheInternet8.2billionpagesEverydataabout2.51018bytesofdataarecreatedor2.5EB(exabytes)二的十六次方位元组,MemoryHierarchy,Wecanseethememorygrowsexponentially（以指数形式）insizeanddecreasesexponentiallyinaccesstime.（访问时间）ClosestmemorytotheCPUisthesmallest,typicallyintherangeof1Kbyte10008-bitcharacters(froma0-255charactertable)to1Mbyteofdata(1000000characters).Theseon-chipcachememoriescanhaveaccesstimesasshortasto1nanosecond十亿分之一秒orone-billionth十亿分之一ofasecond.,MemoryHierarchy,Mainmemoryismuchslowerthanon-chipcachememory(L1,L2).Buttheygiveyoumuchmorememoryandarecheaperandupgradable.（能提到更高一级的）,MemoryHierarchy,Asabenchmark,（标准检查程序）lightcantravelonefootthroughtheairinonenanosecond.（十亿分之一秒）WecallthiscachetheLevel1orL1cache.Level2orL2cacheisusuallylocatedontheprocessorchipitself.Usuallytakesthebiggestpercentageofthechiparea.Secondarymemorysitsinyourmainmemory.Thesearememorysticksyoucanbuyatacomputershop.,3.PagedVirtualMemory,Pagedvirtualmemoryprovidesthesolutiontothefirstdesideratum（迫切需要的；所愿望之物）ofaverylargememorysbeingavailabletotheprocessor.Withvirtualmemory,theprocessoraddressesthedisk,thelarge,slowendofthehierarchy.ThememoriesbetweentheprocessorandthediskaretheretoincreaseeffectiveperformanceReducelatencyandincreasedbandwidth)oftheveryslowdisk.,3.PagedVirtualMemory,Ifeveryinstructionanddatareferencedweremadetothedisk,theprocessorperformancewouldbeslowindeed.WhyisVirtualMemorysoimportantEarlycomputerswithsmallrealmemoryrequiredthatthetransferofdatabetweenrealmemoryanddiskbemanagedexplicitlybytheoperatingsystemortheuser.,3.PagedVirtualMemoryPageAllocationandAddresstranslation,-Virtualmemoryisdividedintoequal-Sizedgroupscalledpages.Apageinamoderncomputeris1K,2K,4K,8KbytesRealmemoryisalsodividedintothesameequal-sizedgroups,calledPageframes.-WheninformationismovedbetweenVirtualmemoryspaceandrealmemorySpace,acompletepageismoved.ProvidesProtectionTranslationSharing,ContAddresstranslation,Caches,Wenowknowhowvirtualmemoryextendstheaddressspaceofaprocessor.However,thelatencyofrealmemoryistoolongtosupporthigh-performanceprocessor.Evenwiththehigh-speedDRAMsusedforrealmemory,somethingmustbedonetoovercomethislatencyproblem.Addanotherleveltothememoryhierarchy,calledthecache.Allocationofmemoryin3levels(diagram),4.CachesRealmemoryandcacheallocation,RealMemory,VirtualMemory,4.Caches,Blocksof16-32bytesaretakenfrompageframesinrealmemoryandplacedintoblockslotsforaccessbytheprocessor.ForModernprocessors,thecacheusuallyhasalatencyofoneprocessorclock,sothattheinstructionsanddatamaybefetchedwithoutdelayexceptwhenthereferenceditemisnotinthecache.,5.MemoryDevices,RANDOMACCESSMEMORY(RAM)Needspower,lossesinformationwhenthereisnopowerthusvolatileREAD-ONLYMEMORY(ROM)MAGNETICDISKSCD-ROMSMAGNETICTAPE,5.MemoryDevices,RANDOMACCESSMEMORY(RAM)Needspower,lossesinformationwhenthereisnopowerthusvolatileDataisnotpermanentREAD-ONLYMEMORY(ROM)Holdshardwareconfigurations硬件配置(BIOSBasicInput/OutputSystem)HardwiredcannotbechangedCriticalprogramsforstartingupacomputer(booting)SlowerthanRAM,usuallytransferredtoRAM,5.MemoryDevices,MagneticDiskYoutypicalharddriveinyourcomputerCircularplatterwithasmoothsurfacewithacoatingthatcanbemagnetizedSpinsandhasaheadwhichreadsandwritestothedisks.,5.MemoryDevices,CD-ROMs(CompactDiskReadOnlyMemory)Opticaldisksneedthinbeamsofconcentratedlighttostoreandreaddata.Aformoflaserstorage.TwotypesWORMwriteoncereadmanyErasableCanbeerasedandre-written