Memory Devices and Microprocessors.docx
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Memory Devices and Microprocessors.docx
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MemoryDevicesandMicroprocessors
Lesson5MemoryDevicesandMicroprocessors
5.1MemoryDevices
Memoriescanbemadeinmechanical,magnetic,optical,biologicalandelectronictechnologies.Examplesofmagneticmemoriesaretapes,floppydisks,harddrivesandferroelectricRAMs.ExamplesofopticalmemoriesareCD-ROMs,rewritableCDs.Electronicmemoryisusedextensivelyincomputerequipmentsinceitisthefastestavailable.Forapplicationswherespeedislessimportant,magneticandopticaltechnologiesareoftenused.
Theflip-flopAflip-flopisbasicallyabi-statecircuitinwhicheithera0or1statecanresides.Becauseofitssimplicity,theflip-flopisextremelyfast.Asabasicelement,theflip-flopisusedindigitalcircuitsandICs.Aflip-flopwillloseitsstatewhenthesupplyvoltageisremoved.Therefore,itisvolatile.
TheregisterAregisterisasetofflip-flopsinparallel.Typicallyaregisteris8,16,32or64bitswide.Oftenaregisterisusedtoholddata,addresspointers,etc.Aregisterisvolatileandveryfastjustliketheflip-flop.
SRAM(StaticRandomAccessMemory)AnSRAMisanarrayofaddressableflip-flops.Thearraycanbeconfiguredassuchthatthedatacomesoutinsinglebit,4-bit,8-bit,andetcformat.SRAMissimple,fastandvolatilejustliketheflip-flop,itsbasicmemorycell.SRAMcanbefoundonmicrocontrollerboards(eitheronorofftheCPUchip),wheretheamountofmemoryrequiredissmallanditwillnotpayofftobuildtheextrainterfacecircuitryforDRAMs.Inaddition,SRAMisoftenusedascachebecauseofitshighspeed.
SRAMcomesinmanyspeedclasses,rangingfromseveralnsforcacheapplicationsto200nsforlowpowerapplications.SRAMexistsinbothbipolarandMOStechnology.CMOStechnologyboaststhehighestdensityandthelowestpowerconsumption.FastcachememorycanbeconstructedinBiCMOStechnology,ahybridtechnologythatusesbipolartransistorsforextradrive.ThefastestSRAMmemoriesareavailableinECL(EmitterCoupledLogic)bipolartechnology.Becauseofthehighpowerconsumption,thememorysizeislimitedinthetechnology.
AspecialcaseofSRAMmemoryisContentAddressableMemory(CAM).Inthistechnology,thememoryconsistsofanarrayofflip-flops,inwhicheachrowisconnectedtoadatacomparator.Thememoryisaddressedbypresentingdatatoit(notanaddress!
).AllcomparatorswillthenchecksimultaneouslyiftheircorrespondingRAMregisterholdsthesamedata.TheCAMwillrespondwiththeaddressoftherow(register)correspondingtotheoriginaldata.Themainapplicationforthistechnologyisfastlookuptables.Theseareoftenusedinnetworkrouters.
DRAM(DynamicRandomAccessMemory)Theword“dynamic”indicatesthatthedataisnotheldinaflip-flopbutratherinastoragecell.Thedatainastoragecellmustberefreshed(readoutandre-written)regularlybecauseofleakage.Therefreshtimeintervalisusually4to64ms.Thestoragecellonlyrequiresonecapacitorandonetransistor,whereasaflip-flopconnectedinanarrayrequires6transistors.Intrenchcapacitormemorytechnology,whichisusedinallmodernDRAMs,thetransistorisconstructedabovethecapacitorsothatthespaceonchipisultimatelyminimized.Forthisreason,DRAMtechnologyhasalowercostperbitthanSRAMtechnology.Thedisadvantageoftheextracircuitryrequiredforrefreshingiseasilyoffsetbythelowerpriceperbitwhenusinglargememorysizes.
DRAMmemoryis,justlikeSRAMmemoryconstructedasanarrayofmemorycell.AmajordifferencebetweenSRAMandDRAM,however,liesintheaddressingtechnique.WithanSRAM,anaddressneedstobepresentedandthechipwillrespondwithpresentingthedataofthememorycellattheoutput,oracceptingthedataattheinputandwriteitintotheaddressedcell.WithDRAMtechnology,thissimpleapproachisimpossiblesinceaddressingarowofdatawithoutrewritingiswilldestructalldataintherowbecauseofthedynamicnature.
ROM(ReadOnlyMemory)ROMsarealsocalledmask-ROMsormaskprogrammedROMs.ThisisbecauseaROMneedstobeprogrammedbysettingitscellstoeither0or1atthetimeofmanufacture.Usuallythe0or1isformedbythepresenceorabsenceofanaluminumline.Thisaluminumpatternisdefinedbyalithographicmaskusedinoneofthelaststepsofmanufacture.Thereforethesedevicesareoftencalledmask-ROMs.
TheadvantageofROMisthatitcanbemanufacturedatthelowestpriceinhighvolumes.Anotheradvantageinsomeapplicationsisthatitisimpossibletoalterthedataoncethechipsaremade,andthatnofurtherprogrammingandtestingarerequired.Ontheotherhand,ifthedataorcodemustbechangedthiscanbeasmalldisaster.Therestofthechipswillendinthedustbinandnewchipswillhavetobemade.
EEPROM(ElectricallyErasableProgrammableROM)ThismeansthatthechipcanbeprogrammedlikeanEPROM,butcanbeerasedelectrically.Asaresult,noUVsourceisrequired.EEPROMscanbeerasedonabyte-by-bytebasis.
5.2Microprocessor
Amicroprocessorisacompletecomputationenginethatisfabricatedonasinglechip.ThefirstmicroprocessorwastheIntel4004,introducedin1971.The4004wasnotverypowerful–allitcoulddowastoaddandsubtract,anditcouldonlydothat4bitsatatime.Butitwasamazingthateverythingwasononechip.Priortothe4004,engineersbuiltcomputerseitherfromcollectionsofchipsorfromdiscretecomponents.The4004poweredoneofthefirstportableelectroniccalculators.
ThefirstmicroprocessortomakeitintoahomecomputerwastheIntel8080,acomplete8-bitcomputerononechip,introducedin1974.ThefirstmicroprocessortomakearealsplashinthemarketwastheIntel8088,introducedin1979andincorporatedintotheIBMPC.ThePCmarketmovedfromthe8088tothe80286tothe80386tothe80486tothePentiumtothePentiumIItothePentiumIIItothePentium4.AllofthesemicroprocessorsaremadebyIntelandallofthemareimprovementonthebasicdesignofthe8088.ThePentium4canexecuteanypieceofcodethatranonoriginal8088,butitdoesitabout5000timesfaster!
InsideaMicroprocessoramicroprocessorexecutesacollectionofmachineinstructionsthattelltheprocessorwhattodo.Basedontheinstructions,amicroprocessordoesthreebasicthings:
1.UsingitsALU(Arithmetic/LogicUnit),amicroprocessorcanperformmathematicaloperationslikeaddition,subtraction,multiplicationanddivision.Modernmicroprocessorscontaincompletefloatingpointprocessorsthatcanperformextremelysophisticatedoperationonlargefloatingpointnumbers.
2.Amicroprocessorcanmovedatafromonememorylocationtoanother.
3.Amicroprocessorcanmakedecisionsandjumptoanewsetofinstructionsbasedonthosedecisions.
Theremaybeverysophisticatedthingsthatamicroprocessordoes,butthoseareitsthreebasicactivities.
RAMstandsforrandom-accessmemoryRAMcontainsbytesofinformationandthemicroprocessorandreadorwritestothosebytesdependingonwhethertheRDorWRlineissignaled.Oneproblemwithtoday’sRAMchipsisthattheyforgeteverythingoncethepowergoesoff.ThatiswhythecomputerneedsROM.
Bytheway,nearlyallcomputerscontainsomeamountofROM(itispossibletocreateasimplecomputerthatcontainsnoRAM—manymicrocontrollersdothisbyplacingahandfulofRAMbytesontheprocessorchipitself—butgenerallyimpossibletocreateonethatcontainsnoROM).OnaPC,theROMiscalledtheBIOS(BasicInput/OutputSystem).Whenthemicroprocessorstarts,itbeginsexecutinginstructionsitfindsintheBIOSTheBIOSinstructionsdothingsliketestthehardwareinthemachine,andthenitgoestotheharddisktofetchthebootsector.Thisbootsectorisanothersmallprogram,andtheBIOSstoreitinRAMafterreadingitoffthedisk.Themicroprocessorthenbeginsexecutingthebootsector’sinstructionsfromRAM.ThebootsectorprogramwilltellthemicroprocessortofetchsomethingelsefromtheharddiskintoRAM,whichthemicroprocessorthenexecutes,andsoon.Thisishowthemicroprocessorloadsandexecutestheentireoperatingsystem.
MicroprocessorInstructionsEventheincrediblysimplemicroprocessorshownherewillhaveafairlylargesetofinstructionsthatitcanperform.Thecollectionofinstructionsisimplementedasbitpatterns,eachoneofwhichhasadifferentmeaningwhenloadedintotheinstructionregister.Humansarenotparticularlygoodatrememberingbitpatterns,soasetofshortwordsaredefinedtorepresentthedifferentbitpatterns.Thiscollectionofwordsiscalledtheassemblylanguageoftheprocessor.Anassemblercantranslatethewordsintotheirbitpatternsveryeasily,andthentheoutputoftheassemblerisplacedinmemoryforthemicroprocessortoexecute.IfyouuseClanguageprogramming,aCcompilerwilltranslatetheCcodeintoassemblylanguage.
Sonowthequestionis,“HowdoalloftheseinstructionslookinROM?
”Eachoftheseassemblylanguageinstructionsmustberepresentedbyabinarynumber.Thesenumbersareknownasopcodes.Theinstructiondecoderneedstoturneachoftheopcodesintoasetofsignalsthatdrivethedifferentcomponentsinsidethemicroprocessor.
MicroprocessorPerformanceThenumberoftransistorsavailablehasahugeeffectontheperformanceofaprocessor.Asseenearlier,atypicalinstructioninaprocessorlikean8088took15clockcyclestoexecute.Becauseofthedesignofthemultiplier,ittookapproximately80cyclesjusttodoone16-bitmultiplicationonthe8088.Withmoretransistors,muchmorepowerfulmultiplierscapableofsingle-cyclespeedsbecomepossible.
Moretransistorsalsoallowforatechnologyca
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