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21-Instruction Issue Logic for High-Performance, Interruptable Pipelined Processors.pdf

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21-Instruction Issue Logic for High-Performance, Interruptable Pipelined Processors.pdf

INSTRUCTIONISSUELOGICFORHIGHPERFORMANCE,INTERRUPTABLEP1PELINEDPROCESSORSGURINDARSSOHIANDSRIRAMVAJAPEYAMCOMPUTERSCIENCESDEPARTMENTUNIVERSITYOFWISCONSINMADISON1210WESTDAYTONSTREETMADISON,WISCONSIN53706ABSTRACTTHEPERFORMANCEOFPIPELINEDPROCESSORSISSEVERELYLIMITEDBYDATADEPENDENCIESINORDERTOACHIEVEHIGHPERFORMANCE,AMECHANISMTOALLEVIATETHEEFFECTSOFDATADEPENDENCIESMUSTEXISTIFAPIPELINEDCPUWITHMULTIPLEFUNCTIONALUNITSISTOBEUSEDINTHEPRESENCEOFAVIRTUALMEMORYHIERARCHY,AMECHANISMMUSTALSOEXISTFORDETERMININGTHESTATEOFTHEMACHINEPRECISELYINTHISPAPER,WECOMBINETHEISSUESOFDEPENDENCYRESOLUTIONANDPRECISENESSOFSTATEWEPRESENTADESIGNFORINSTRUCTIONISSUELOGICTHATRESOLVESDEPENDENCIESDYNAMICALLYAND,ATTHESAMETIME,GUARANTEESAPRECISESTATEOFTHEMACHINE,WITHOUTASIGNIFICANTHARDWAREOVERHEADDETAILEDSIMULATIONSTUDIESFORTHEPROPOSEDMECHANISM,USINGTHELAWRENCELIVERMORELOOPSASABENCHMARK,AREPRESENTED1INTRODUCTIONASTHEDEMANDFORPROCESSINGPOWERINCREASES,COMPUTERSYSTEMDESIGNERSAREFORCEDTOUSETECHNIQUESTHATRESULTINHIGHPERFORMANCEPROCESSINGUNITSAWIDELYUSEDTECHNIQUEISPIPELINING\1\,INWHICHTHEOVERALLLOGICOFTHESYSTEMISSPLITINTOSEVERALSTAGESWITHEACHSTAGEPERFORMINGASUBTASKOFACOMPLETETASKCONSIDERABLEOVERLAPCANBEACHIEVEDBECAUSEEACHSTAGECANPERFORMASUBTASKFORADIFFERENTTASKPIPELINEDCPUSHAVETWOMAJORIMPEDIMENTSTOTHEIRPERFORMANCEIDATADEPENDENCIESANDIIBRANCHINSTRUCTIONSANINSTRUCTIONCANNOTBEGINEXECUTIONUNTILITSOPERANDSAREAVAILABLEIFANOPERANDISTHERESULTOFAPREVIOUSINSTRUCTION,THEINSTRUCTIONMUSTWAITTILLTHEPREVIOUSINSTRUCTIONHASCOMPLETEDEXECUTION,THEREBYDEGRADINGPERFORMANCETHEPERFORMANCEDEGRADATIONDUETOBRANCHINSTRUCTIONSISEVENMORESEVERENOTONLYMUSTACONDITIONALBRANCHINSTRUCTIONWALTFORITSCONDITIONTOBEKNOWNRESULTINGINBUBBLESINTHEPIPELINE,ANADDITIONALPENALTYISINCURREDINFETCHINGANINSTRUCTIONFROMTHETAKENBRANCHPATHTOTHEINSTRUCTIONDECODEANDISSUESTAGEPERMISSIONTOCOPYWITHOUTFEEALLORPARTOFTHISMATERIALISGRANTEDPROVIDEDTHATTHECOPIESARENOTMADEORDISTRIBUTEDFORDIRECTCOMMERCIALADVANTAGE,THEACMCOPYRIGHTNOTICEANDTHETITLEOFTHEPUBLICATIONANDITSDATEAPPEAR,ANDNOTICEISGIVENTHATCOPYINGISBYPERMISSIONOFTHEASSOCIATIONFORCOMPUTINGMACHINERYTOCOPYOTHERWISE,ORTOREPUBLISH,REQUIRESAFEEAND/ORSPECIFICPERMISSIONAMAJORPROBLEMTHATARISESINPIPELINEDCOMPUTERDESIGNISTHATANINTERRUPTCANBEIMPRECISE\2,3\THISPROBLEMISESPECIALLYSEVEREINMULTIPLEFUNCTIONALUNITCOMPUTERSINWHICHINSTRUCTIONSCANCOMPLETEEXECUTIONOUTOFPROGRAMORDER\2,4\FORAHIGHPERFORMANCE,PIPELINEDCPU,ANADEQUATESOLUTIONMUSTBEFOUNDFORTHEIMPRECISEINTERRUPTPROBLEMANDMEANSMUSTBEPROVIDEDFOROVERCOMINGTHEPERFORMANCEDEGRADINGFACTORS11BACKGROUNDANDPREVIOUSWORKTHEDETRIMENTALEFFECTSOFBRANCHINSTRUCTIONSCANBEALLEVIATEDBYUSINGDELAYEDBRANCHINSTRUCTIONSHOWEVER,THEUTILITYOFDELAYEDBRANCHINSTRUCTIONSISLIMITEDFORLONGPIPELINESINSUCHCASES,OTHERMEANSMUSTEXISTTOALLEVIATETHEDETRIMENTALEFFECTSACOMMONAPPROACHISTOUSEBRANCHPREDICTION\5,6\USINGPREDICTIONTECHNIQUES,THEPROBABLEEXECUTIONPATHOFABRANCHINSTRUCTIONISDETERMINEDINSTRUCTIONSFROMTHEPREDICTEDPATHCANTHENBEFETCHEDINTOINSTRUCTIONBUFFERSOREVENEXECUTEDINACONDITIONALMODEL2,7,8\WHILETHECONDITIONALMODEOFEXECUTIONWILLRESULTINAHIGHERPIPELINETHROUGHPUT,ESPECIALLYIFTHEOUTCOMEOFTHEBRANCHESISPREDICTEDCORRECTLY,AHARDWAREMECHANISMMUSTEXISTWHICHWILLALLOWTHEMACHINETORECOVERFROMANINCORRECTSEQUENCEOFCONDITIONALINSTRUCTIONSBOTHHARDWAREANDSOFTWARESOLUTIONSEXISTTOTHEDATADEPENDENCYPROBLEMSOFTWARESOLUTIONSUSECODESCHEDULINGTECHNIQUESCOMBINEDWITHALARGESETOFREGISTERSTOINCREASETHEDEPENDENCYDISTANCEANDTOPROVIDEINTERLOCKS\9\HARDWARESOLUTIONSEMPLOYWAITINGSTATIONSORRESERVATIONSTATIONSWHEREANINSTRUCTIONCANWAITFORITSOPERANDSANDALLOWSUBSEQUENTINSTRUCTIONSTOPROCEED\10\INAPIPELINEDMACHINE,IMPRECISEINTERRUPTSCANBECAUSEDBYINSTRUCTIONGENERATEDTRAPSSUCHASARITHMETICEXCEPTIONSANDPAGEFAULTSANIMPRECISEINTERRUPTCANLEAVETHEMACHINEINANIRRECOVERABLESTATEWHILETHEOCCURRENCEOFARITHMETICEXCEPTIONSISRARE,THEOCCURRENCEOFPAGEFAULTSINAMACHINETHATSUPPORTSVUALMEMORYISNOTTHEREFORE,IFVIRTUALMEMORYISTOBEUSEDWITHAPIPELINEDCPU,ITISCRUCIALTHATINTERRUPTSBEPRECISESEVERALHARDWARESOLUTIONSTOTHEPROBLEMAREDESCRIBEDIN\3\WEAREUNAWAREOFANYSOFTWARESOLUTIONSTOTHEIMPRECISEINTERRUPTPROBLEMFORMULTIPLEFUNCTIONALUNITCOMPUTERSASOFTWARESOLUTIONWILLBEEXTREMELYDIFFICULT,IFNOTIMPOSSIBLENOTONLYMUSTTHESOFTWAREALLOWFORTHEWORSTCASEEXECUTIONTIMEFORANYINSTRUCTION,ITMUSTALSOKEEPTRACKOFINSTRUCTIONSTHATHAVECOMPLETEDOUTOFPRE1987ACM00847495/87/06000027500,7527GRAMORDERANDGENERATETHEAPPROPRIATECODESEQUENCETOUNDOTHEEFFECTSOFTHOSEINSTRUCTIONSINEITHERCASE,SOMEHARDWARESUPPORTMUSTBEPROVIDEDTOMAINTAINRUNTIMEINFORMATION12OUTLINEOFTHEPAPERINTHISPAPER,WETREATTHEPROBLEMSOFDEPENDENCYRESOLUTIONANDIMPRECISEINTERRUPTSSIMULTANEOUSLYSINCEAHARDWAREMECHANISMMUSTEXISTFORIMPLEMENTINGPRECISEINTERRUPTS,WHYNOTEXTENDTHISMECHANISMTORESOLVEDEPENDENCIESANDALLOWOUTOFORDERINSTRUCTIONEXECUTIONINSECTION2,WEDISCUSSTOMASULOSDEPENDENCYRESOLUTIONALGORITHMANDEXTENDIT,GIVINGSEVERALVARIATIONS,SOTHATTHECOSTOFIMPLEMENTINGITISNOTPROHIBITIVEEVENFORALARGENUMBEROFREGISTERSINSECTION3,WEDISCUSSTHEPROBLEMOFIMPRECISEINTERRUPTSANDPRESENTSOLUTIONSSECTION4DESCRIBESAUNITTHATRESOLVESDEPENDENCIESASWELLASIMPLEMENTSPRECISEINTERRUPTSTHEPRECISEINTERRUPTANDDEPENDENCYRESOLUTIONMECHANISMSMUTUALLYAIDANDSIMPLIFYEACHOTHERASIMULATIONANALYSISOFTHEPROPOSEDMECHANISMUSINGSEVERALLIVERMORELOOPSASBENCHMARKSISCARRIEDOUTINSECTION5FINALLY,WEDISCUSSHOWOURMECHANISMMIGHTBEUSEDTOALLEVIATETHEDEGRADATIONDUETOBRANCHINSTRUCTIONSTHROUGHOUTTHEPAPER,WEDISCUSSINCREMENTALMODIFICATIONSTOTHEBASICPRINCIPLESDATASUPPORTINGOURCLAIMSFORSUCHMODIFICATIONSHAVEBEENOMITTEDFORREASONSOFCONCISENESSHOWEVER,WEDOPRESENTDETAILEDSIMULATIONDATAFOROURFINALDESIGN13MODELARCHITECTURETHEMODELARCHITECTURETHATWEUSEFOROURSTUDIESISPRESENTEDINFIGURE1ITHASTHESAMECAPABILITIESANDEXECUTESTHESAMEINSTRUCTIONSETASTHESCALARUNITOFTHECRAY1\4,11\HOWEVER,THEREISAMAJORDIFFERENCEINOURARCHITECTURE,ALLINSTRUCTIONS,WHETHERTHEYARECOMPOSEDOFIPARCEL16BITSOR2PARCELS32BITSCANISSUEINASINGLECYCLEIFISSUECONDITIONSAREFAVORABLETHEREFORE,THEBESTCASEEXECUTIONTIMEOFACONDITIONALBRANCHINSTRUCTIONIS4CLOCKCYCLESAFTERTHECONDITIONISKNOWNASOPPOSEDTO5CLOCKCYCLESFORTHECRAY1\11\THECRAY1WASCHOSENBECAUSEITREPRESENTSASTATEOFTHEARTSCALARUNITANDITSEXECUTIONCANBEMODELEDPRECISELYTHEAUTHORSALSOHADEASYACCESSTOTOOLSTHATCOULDBEUSEDTOGENERATEINSTRUCTIONTRACESFORTHECRAY1SCALARUNIT\12\THEMODELMACHINE,THEREFORE,CONSISTSOFSEVERALFUNCTIONALUNITSCONNECTEDTOACOMMONRESULTBUSONLYONEFUNCTIONCANOUTPUTDATAONTOTHERESULTBUSINANYCLOCKCYCLEINSTRUCTIONSAREFETCHEDBYTHEINSTRUCTIONFETCHUNITANDDECODEDANDISSUEDBYTHEDECODEANDISSUEUNITONCEDEPENDENCIESHAVEBEENRESOLVEDINTHEDECODEANDISSUEUNIT,INSTRUCTIONSAREFORWARDEDTOTHEFUNCTIONALUNITSFOREXECUTIONTHERESULTSOFTHEFUNCTIONALUNITSAREWRITTENDIRECT2YINTOTHEREGISTERFILETHEREGISTERFILECONSISTSOF8A,8S,64BAND64TREGISTERS2DEPENDENCYRESOLUTIONOUTOFORDERINSTRUCTIONEXECUTIONWHENANINSTRUCTIONREACHESTHEDECODEANDISSUESTAGEINTHEPIPELINE,CHECKSMUSTBEMADETODETERMINEIFTHEOPERANDSFORTHEINSTRUCTIONAREAVAILABLE,IE,IFALLDEPENDENCIESFORTHISINSTRUCTIONHAVEBEENRESOLVEDIFANOPERANDISNOTAVAILABLE,THEINSTRUCTIONMUSTWAITCONSEQUENTLY,SUBSEQUENTINSTRUCTIONSCANNOTPROCEEDEVENTHOUGHTHEYMAYBEREADYTOEXEFUNCTIONALUNITSIFROMMEMORYIREGISTERINSTRUCTIONFETCHUNIT,IRFILELI\,IZRESTBUSFIGURE1THEBASICARCHITECTURECUTESUBSEQUENTINSTRUCTIONSCANPROCEEDIFTHEWAITINGINSTRUCTIONSTEPSASIDE,ANDALLOWSOTHERINSTRUCTIONSTOBYPASSITWHILEITWAITSFORITSOPERANFSRESERVATIONSTATIONSPERMITANINSTRUCTIONTODOTHIS\10\21TOMASULOSALGORITHMTOMASULOSDEPENDENCYRESOLUTIONALGORITHMWASFIRSTPRESENTEDFORTHEFLOATINGPOINTUNITOFTHEIBM360/91\10\ANEXTENSIONOFTHISALGORITHMFORTHESCALARUNITOFTHECRAY1ISPRESENTEDIN\13\THEALGORITHMOPERATESASFOLLOWSANINSTRUCTIONWHOSEOPERANDSARENOTAVAILABLEWHENITENTERSTHEDECODEANDISSUESTAGEISFORWARDEDTOARESERVATIONSTATIONRSASSOCIATEDWITHTHEFUNCTIONALUNITTHATITWILLBEUSINGITWAITSINTHERSUNTILITSDATADEPENDENCIESHAVEBEENRESOLVED,IE,ITSOPERANDSAREAVAILABLEONCEATARESERVATIONSTATION,ANINSTRUCTIONCANRESOLVEITSDEPENDENCIESBYMONITORINGTHECOMMONDATABUSTHERESULTBUSINOURMODELARCHITECTUREWHENALLTHEOPERANDSFORANINSTRUCTIONAREAVAILABLE,ITISDISPATCHEDTOTHEAPPROPRIATEFUNCTIONALUNITFOREXECUTIONTHERESULTBUSCANBERESERVEDEITHERWHENTHEINSTRUCTIONISDISPATCHEDTOTHEFUNCTIONALUNIT\13\ORSOONBEFOREITISABOUTTHELEAVETHEFUNCTIONALUNIT\10\EACHSOURCEREGISTERISASSIGNEDABITTHATDETERMINESIFRTHEREGISTERISBUSYAREGISTERISBUSYIFITISTHEDESTINATIONOFANINSTRUCTIONTHATISSTILLINEXECUTIONADESTINATIONREGISTERISALSOCALLEDASINKREGISTER\10\EACHSINKREGISTERISASSIGNEDATAGWHICHIDENTIFIESTHERESULTTHATMUSTBEWRITTENINTOTHEREGISTERSINCEANYREGISTERINTHEREGISTERFILECANBEASINK,EACHREGISTERMUSTBEASSIGNEDATAGEACHRESERVATIONSTATIONHASTHEFOLLOWINGFIELDSSOURCEOPERAND1SOURCEOPERAND2DESTINATION28IFASOURCEREGISTERISBUSYWHENTHEINSTRUCTIONREACHESTHEISSUESTAGE,THETAGFORTHESOURCEREGISTERISOBTAINEDANDTHEINSTRUCTIONISFORWARDEDTOARESERVATIONSTATIONIFTHESINKREGISTERISBUSY,THEINSTRUCTIONFETCHESANEWTAG,UPDATESTHETAGOFTHESINKREGISTERANDPROCEEDSTOARESERVATIONSTATIONTHEREGISTERSASWELLASTHERESERVATIONSTATIONSMONITORTHERESULTBUSANDUPDATETHEIRCONTENTSWHENAMATCHINGTAGISFOUNDMEMORYISTREATEDASASPECIALFUNCTIONALUNITDETAILSOFTHEALGORITHMCANBEFOUNDIN\10\AND\13\WHILETHISALGORITHMISSTRAIGHTFORWARDANDEFFECTIVE,ITISEXPENSIVETOIMPLEMENTBECAUSEEACHREGISTERNEEDSTOBETAGGEDANDEACHTAGNEEDSASSOCIATIVECOMPARISONHARDWARETOCARRYOUTTHETAGMATCHINGPROCESSTHISMAYNOTBEPRACTICALIFTHENUMBEROFPOSSIBLESINKFIELDS,IE,THENUMBEROFREGISTERSISLARGEFOROURMODELARCHITECTUREWHICHHAS8A,8S,64BAND64TREGISTERS,CLEARLYTHEUSEOF144TAGMATCHINGHARDWAREUNITSISIMPRACTICAL22EXTENSIONSTOTOMASULOSALGORITHM221ASEPARATETAGUNITONCLOSERINSPECTIONWESEETHATVERYFEWOFALLPOSSIBLESINKREGISTERSMAYACTUALLYBEACTIVE,IE,BEWAITINGFORARESULTATANYGIVENTIMETHEREFORE,IFWEASSOCIATEATAGWITHEACHPOSSIBLESINKREGISTER,ALOTOFASSOCIATIVETAGMATCHINGHARDWAREWILLBEIDLEATANYGIVENTIMEWHYNOTHAVEACOMMONTAGPOOLANDASSIGNATAGONLYTOACURRENTLYACTIVESINKREGISTERRATHERTHANASSOCIATINGATAGWITHEACHPOSSIBLESINKFIELDINTOMASULOSALGORITHM,ACURRENTLYACTIVEREGISTERISONEWHOSEBUSYBITISONWECONSOLIDATETHETAGSFROMALLCURRENTLYACTIVEREGISTERSINTOATAGUNITTUEACHREGISTERNOWHASONLYASINGLEBUSYBITATINSTRUCTIONISSUETIME,IFASOURCEREGISTERISBUSY,THETUISQUE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