Lesson 3Notes Pipelining PDF

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Pipelining Pipeliningiscrucialtoimprovingperformanceinprocessors;itincreasesthroughputand reducescycletime.Thedownsideofpipelinesaretheincreaseinhazards,bothcontroland data.  PipelininginaProcessor Fivestagesinabasicpipeline: Fetch,Read/Decode,ALU,MemoryAccess,WritetheRegisters  Pipeliningtheinstructionstakesthesameamountoftime,butthroughputisimproved.  PipeliningCPI Ifthereaninstructionhastowaitatapipelinestage,alltheinstructionsaheadofitproceed throughthepipeline,alltheinstructionsbehinditarealsostalled.Thisiscalledadelayinthe pipeline.Thepipelineaheadofthedelaywillnothaveinstructionstoexecuteasthepipeline emptiesandtheinstructionsbehindthedelaywillbestalled.  Asthenumberofdelaysincreasethroughthepipeline,theCPIwillincrease.  PipelineStallsandFlushes PipelineFlush:Branchescancausebubbleswhentheincorrectbranchistaken.Whenthis happensalltheincorrectinstructionsthatwerefetchedmustbeflushedfromthepipelineand replacedwithNOPs.Thenthecorrectinstructionsmustbefetched.  ControlDependencies Controldependencies:Whenaninstructionisdependentontheoutcomeofabranchdecision, theseinstructionsaresaidtohaveacontroldependence. 20%ofinstructionsarebranchesandjumps 50%ofbranchandjumpinstructionsaretaken  OverallCPI=CPIofprogram+%ofinstructionsmispredicted*penaltyformisprediction  DataDependencies Datadependence:Whenaninstructionneedsdatafromanotherinstructionthatiscalledadata dependence. Typeofdatadependencies: 1.RAWreadafterwrite.AlsocalledFlow,TrueDependence. ThefollowingdependenciesareFalseorNamedependencies. 2.WAWwriteafterwrite.AlsocalledOutputDependence. 3.WARwriteafterread.AlsocalledAntiDependence RARreadafterreadisnotadependence. 

DependenciesandHazards Dependenciesarecausedbytheprogram,notthepipeline. Somedependenceswillnotcauseproblems,butsome,likeRAW,cancauseproblemsin pipelines. Hazardsaretruedependenciesthatresultinincorrectexecutionoftheprogram,butnotalltrue dependencieswillresultinahazard.Hazardsarecausedbyboththeprogramandthepipeline.  HandlingofHazards Firststeptohandlinghazards:Detectonlythosedependenciesthatwillcausehazards. SecondSteptohandlinghazards:Removethehazard. Optionsforremovinghazardsare: 1.Flushdependentinstructionsoutofthepipeline. Thismethodisusedwithcontroldependencies.Sincethe wrongvalueisinthepipelineitneedstobeflushed. 2.Stalldependentinstructionsinthepipeline. Thismethodisusedfordatadependencies.Thiswillgivethe datatimetogetwrittentoregistersbeforeitisneededbyalater instruction. 3.Fixthevaluesreadbydependentinstructions. Thismethodisalsousedfordatadependencies.Insteadof stallingtheinstructionuntilthecorrectvaluesareintheregisters, thepipelinecanforwardtherequiredvalues.Thevaluesfroman ALUforexamplecanbeusedassoonastheycomputed,rather thanwaiting. Thismethoddoesnotalwayswork.  HowManyStages EverypipelineshouldbeachievingtherequiredCPI,itisdifferentforeverypipeline. Whenmorestagesareaddedtoapipeline: 1.Therearemorehazardsintroducedintothepipeline. 2.Thepenaltyforhazardsincreases. 3.Thereislessworkforeachstage,sothecycletimecanbesmaller.  TheIronLawofPerformancesays: ThenumberofstagesmustbebalancedbetweentheCPIandthecycletime.  Ifonlyperformanceisconsidered,3040stagesisideal.Butifpowerconsumptionisalso considered,theidealpipelineis1015stages.    ...