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APreliminaryArchitectureforaBasicDataFlowProcessorJackB.DennisandDavidP.MisunasProjectMACMassachusettsInstituteofTechnologyAbstractAprocessorisdescribedwhichcanachievehighlyparallelexecutionofprogramsrepresentedindataflowform.Thelanguageimplementedincorporatesconditionalanditerationmechanisms,andtheprocessorisasteptowardapracticaldataflowprocessorforaFortranleveldataflowlanguage.Theprocessorhasauniquearchitecturewhichavoidstheproblemsofprocessorswitchingandmemory/processorinterconnecionthatusuallylimitthedegreeofrealizableconcurrentprocessing.Thearchitectureoffersanunusualsolutiontotheproblemofstructuringandmanagingatwolevelmemorysystem.IntroductionStudiesofconcurrentoperationwithinacomputersystemandoftherepresentationofparallelisminaprograrmminglanguagehaveyieldedanewformofprogramrepresentation,knownasdataflow.Executionofadataflowprogramisdatadriventhatis,eachinstructionisenabledforexecutionjustwheneachrequiredoperandhasbeensuppliedbytheexecutionofapredecessorinstruction.DataflowrepresentationsforprogramshavebeendescribedbyKarpandMiller8,RodriguezIll,Adamsi,DennisandFosseen5,Bhrs21,Kosinski9,101,andDennis4.Wehavedevelopedanattractivearchitectureforaprocessorthatexecuteselementarydataflowprograms6,7.TheclassofprogramsimplementedbythisprocessorcorrespondstothemodelofKarpandMiller81.Thesedataflowprogramsarewellsuitedtorepresentingsignalprocessingcomputationssuchaswaveformgeneration,modulationandfiltering,inwhichagroupofoperationsistobeperformedonceforeachsampleintimeofthesignalsbeingprocessed.Thiselementarydataflowprocessoravoidstheproblemsofprocessorswitchingandprocessor/memoryinterconnectionpresentinattemptstoadaptconventionalYonNeumantypemachinesforparallelcomputation.Sectionsofthemachinecommunicatebythetransmissionoffixedsizeinformationpackets,andthemachineisorganizedsothatthesectionscantoleratedelaysinpackettransmissionwithoutcompromisingeffectiveutilizationofthehardware.WewishtoexpandthecapabilitiesofthedataflowarchitecturewiththeultimategoalofdevelopingageneralpurposeprocessorusingageneralizeddataflowlanguagesuchasdescribedbyDennis4,Kosinski9,101andBhrs21.Asanintermediatestep,wehavedevelopedapreliminarydesignforabasicdataflowprocessorthatexecutesprogramsexpressedinamorepowerfullanguagethantheelementarymachine,butstillnotachievingageneralizedcapability.ThelanguageofthebasicmachineisthatdescribedbyDennisandFosseen51,andincludesconstructsforexpressingconditionalanditerativeexecutionofprogramparts.Inthispaperwepresentsolutionstothemajorproblemsfacedinthedevelopmentofthebasicmachine.Astraightforwardsolutiontotheincorporationofdecisioncapabilitiesinthemachineisdescribed.Inaddition,thegrowthinprogramsizeandcomplexitywiththeadditionofthedecisioncapabilityrequiresutilizationofatwolevelmemorysystem.Adesignispresentedinwhichonlyactiveinstructionsareintheoperationalmemoryoftheprocessor,andeachinstructionisbroughttothatmemoryonlywhennecessaryforprogramexecution,andremainsthereonlyaslongasitisbeingutilized.TheworkreportedherewassupportedbytheNationalScienceFoundationunderresearchgrantGJ34671.126TheElementaryProcessorTheElementaryProcessorisdesignedtoutilizetheelementarydataflowlanguageasitsbaselanguage.Aprogramintheelementarydataflowlanguageisadirectedgraphinwhichthenodesareoperatorsorlinks.Thesenodesareconnectedbyarcsalongwhichvaluesconveyedbytokensmaytravel.Anoperatoroftheschemaisenabledwhentokensarepresentonallinputarcs.Theenabledoperatormayfir.___atanytime,removingthetokensonitsinputarcs,computingavaluefromtheoperandsassociatedwiththeinputtokens,andassociatingthatvaluewitharesulttokenplacedonitsoutputarc.Aresultmaybesenttomorethanonedestinationbymeansofalinkwhichremovesatokenonitsinputarcandplacestokensonitsoutputarcsbearingcopiesoftheinputvalue.Anoperatororalinkcannotfireunlessthereisnotokenpresentonanyoutputarcofthatoperatororlink.AnexampleofaprogramintheelementarydataflowlanguageisshowninFigureIandrepresentsthefollowingsimplecomputationinputa,byab/xxaabboutputy,xLIL2AllL3MA21/IA4yFigureI.Anelementarydataflowprogram.datapacketsOperationUnit0OperationUnltmIIInstruction__Cell0iMemoryInstructionJ__CellnIi4JoperationpocketsFigure2.Organizationoftheelementarydataflowprocessor.TherectangularboxesinFigure1areoperators,andeacharithmeticoperatorintheabovecomputationisreflectedinacorrespondingoperatorintheprogram.Thesmalldotsarelinks.Thelargedotsrepresenttokensholdingvaluesfortheinitialconfigurationoftheprogram.IntheprogramofFigureI,linksLIandL2areinitiallyenabled.ThefiringofLImakescopiesofthevalueaavailabletooperatorsA1andA3firingL2presentsthevaluebtooperatorsAIandA4.OnceLiandL2havefiredinanyorder,operatorA1isenabledsinceitwillhaveatokenoneachofitsinputarcs.AfterAIhasfiredcompletingthecomputationofab,linkL3willbecomeenabled.ThefiringofL3willenabletheconcurrentfiringofoperatorsA2andA3,andsoon.Thecomputationsrepresentedbyanelementaryprogramareperformedinadatadrivenmannertheenablingofanoperatorisdeterminedonlybythearrivalofvaluesonallinputlinks,andnoseparatecontrolsignalsareutilized.SuchaschemapromptedthedesignofaprocessororganizedasinFigure2.AdataflowschematobeexecutedisstoredintheMe_...moryoftheprocessor.TheMemoryisorganizedintoInstructionCells,eachCellcorrespondingtoanoperatorofthedataflowprogram.EachInstructionCellFigure3iscomposedofthreeregisters.ThefirstregisterholdsaninstructionFigure4whichspecifiestheoperationtobeperformedandtheaddressesofthereglsterstowhichtheresultoftheoperationistobedirected.Thesecondandthirdregistersholdtheoperandsforuseinexecutionoftheinstruction.WhenaCellcontainsaninstructionandthenecessaryoperands,itisenabledandsignalstheArbitrationNetworkthatitisreadytotransmititscontentsasanoperationacetoanOperationUnitwhichcanperformthedesiredfunction.TheoperationpacketflowsthroughtheArbidatapocketdatapacketrlstructionCellregistertinstructi°nregister{,operond,registerIIoperandR.operationpacketfromInstructionCelIstoOperationUnitsFigure5.StructureoftheArbitrationNetwork.tratlonNetworkwhichdirectsittoanappropriateOperationUnitbydecodingtheinstructionportionofthepacket.TheresultofanoperationleavesanOpertionUnitasoneormoredat__.packets,consistingofthecomputedvalueandtheaddressofaregisterintheMemorytowhichthevalueistobedelivered.TheDist£ibutionNetworkacceptsdatapacketsfromtheOperatlonUnitsandutilizestheaddressofeachtodirectthedataitemthroughthenetworktothecorrectregisterintheMemory.TheInstructionCellcontainingthatregistermaythenbeenabledifaninstructionandalloperandsarepresentintheCell.ManyInstructionCellsmaybeenabledsimultaneouslysanditisthetaskoftheArbitrationNetworktoefficientlydeliveroperationpacketstoOperationUnitsandtoqueueoperationpacketswaitingforeachOperationUnit.AstructurefortheArbitrationNetworkprovidingapathforoperationpacketsfromeachInstructionCelltoeachOperationUnitispresentedinFigure5.EachArbitrationUnitpassespacketsarrivingatitsinputportsoneatatimetoitsoutputporttusingaroundrobindisciplinetoresolveanyambiguityaboutwhichpacketsshouldbesentnext.ASwitchUnitassignsapacketatitsinputtooneofitsoutputportssaccordingtosomepropertyofthepacketsinthiscasetheoperationcode.TheDistributionNetworkissimilarlyorganizedusingSwitchUnitstoroutedatapacketsfromtheOperationUnitstotheMemoryRegistersspecifiedbythedestinationaddresses.AfewArbitratlonUnitsarerequiredsodatapacketsfromdifferentOperationUnitscanenterthenetworksimultaneously.SincetheArbitrationNetworkhasmanyinputportsandonlyafewoutputports,therateofpacketflowwillbemuchgreaterattheoutputports.ThussaserialrepresentationofpacketsisappropriateattheinputportstominimizethenumberofconnectionstotheMemory,butamoreparallelrepresentationisrequiredattheoutputportssoahighthroughputmaybeachieved.HencesserlaltoparallelconversionisperformedinstageswithintheArbitrationNetwork.SimilarlyparalleltoserialconversionofthevalueportionofeachresultpacketoccurswithintheDistributionNetwork.TheOperationUnitsoftheprocessorarepipelinedinioperationcodedestinationI2J,..AspecializedfunctionoperationunitdestinationFigure3.OperationofanInstructionCell.127Figure4.Instructionformat.odatalinkbcontrollinkFigure6.Linksofthebasicdataflowlanguage.ordertoallowmaximumthroughput.ThedestinationaddressesofaninstructionareenteredintoidentitypipelinesoftheOratlonUnitsandareutilizedtoformdatapacketswiththeresultwhenitappears.Amoredetailedexplanationoftheelementaryprocessoranditsoperationisgivenin6.Wehavecompleteddesignsforallunitsoftheelementaryprocessorintheformofspeedlndependentinterconnectionsofasmallsetofbasicasynchronousmoduletypes.Thesedesignsarepresentedin7.TheBasicDataFlowLanguaReOursuccessinthearchitectureoftheelementarydataflowprocessorledustoconsiderapplyingtheconceptstothearchitectureofmachinesformorecompletedataflowlanguages.Forthefirststepingeneralization,wehavechosen,a.classofdataflpwprogramsthatcorrespondtoaformaldataflowmodelstudiedbyDennisandFosseen5.Therepresentationofconditionalsanditerationindataflowformrequiresadditionaltypesoflinksandactors.ThetypesoflinksandactorsforthebasicdataflowlanguageareshowninFigures6and7.Datavaluespassthroughdatalinksinthemannerpresentedpreviously.Thetokenstransmittedbycontrollinksareknownascontroltokens,andeachconveysavalueofeithert.rueorfalse.Acontroltokenisgeneratedatadeciderwhich,uponreceivingvaluesfromitsinputarcs,appliesitsassociatedpredicate,andproduceseithera.rueorfalsecontroltokenatitsoutputarc.ThecontroltokenproducedatadecidercanbecombinedwithothercontroltokensbymeansofaBooleanoperatorFigure7f,allowingadecisiontobebuiltupfromsimplerdecisions.COntroltokensdirecttheflowofdatatokensbymeansofTgates,Fgates,ormergeactorsFigure7c,d,e.ATgatepassesthedatatokenonitsinputarctoitsoutputarcwhenitreceivesacontroltokenconveyingnFigure8.Dotaflowrepresentationofthebasicprogram.ooperotorbdecidercTgatedFgateoI,v,,lemergefbooleanoperatorFigure7.Actorsofthebasicdataflowlanguage.thevaluetrueatitscontrolinput.Itwillabsorbthedatatokenonitsinputarcandplacenothingonitsoutputarcifafalsevaluedcontroltokenisreceived,Similarly,theFgatewillpassitsinputdatatokentoitsoutputarconlyonreceiptofafalsevaluedtokenonthecontrolinput.Uponreceiptofatruevaluedtoken,itwillabsorbthedatatoken.Amergeactorhasatrueinput,afalseinput,andaOontro.linput.Stpassestoitsoutputarcadatatokenfromtheinputarccorrespondingtothevalueofthecontroltokenreceived.Anytokensontheotherinputarenotaffected.Aswiththeelementaryschemas,allnkoractorisnotenabledtofireunlessthereisnotokenonanyofitsoutputarcs.Usingtheactorsandlinksofthebasicdataflowlanguage,conditionalsanditerationcanbeeasilyrepresented.Inillustration,Figure8givesabasicdataflowprogramforthefollowingcomputationinputy,xn0whiley}{yalue,\true1}Agatetypecontrolpacketperformsagatingfunctionattheaddressedoperandregister.AvaluetypecontrolpacketprovidesaBooleanoperandvaluetoanInstructionCellthatrepresentsaBooleanoperator.ThesixformatsforthecontentsofInstructionCellsinthebasicprocessoraregiveninFigureI0.TheuseofeachRegisterisspecifiedinitsleftmostfieldIinstructionregisterDoperandregisterfordatavaluesBoperandregisterforBooleanvaluesOnlyRegistersspecifiedtobeoperandregistersofconsistenttypemaybeaddressedbyinstructionsofavalidprogram.TheremainingfieldsintheInstructionCellformatsareaninstructioncode,op,prorbo,thatidentifiestheclassandvariationoftheinstructionintheCellfromonetothreedestinationaddressesdl,d2,d3thatspecifytargetoperandregistersforthepacketsgeneratedbyinstructionexecutioninthecaseofdecidersandBooleanoperators,aresulttadtl,t2,t3foreachdestinationthatspecifieswhetherthecontrolpacketisofgatetypetagmateorofvaluetypetagvalueand,foreachoperandregister,aatingcodegl,g2andeitheradatareceivervl,v2oracontrolreceivercl,c2.129ReceiverfIILvaluedataOrBooleancvalueflagoffnovaluereceivedLonvaluereceivedfof....._f_nogatetypecontrolpacketreceivedgateflagtru._._.etruegatetypecontrolpacketreceivedfalsefalsegatetypecontrolpacketreceivedFigureI1.Structureandstatesofreceivers.InstructionCellOperationThefunctionofeachInstructionCellistoreceivedataandcontrolpackets,and,whentheCellbecomesenabled,totransmitanoperationordecisionpacketthroughtheArbitrationNetworkandresettheInstructionCelltoitsinitialstatus.AnInstructionCellbecomesenabledjustwhenallthreeofitsregistersareenabled.Aregisterspecifiedtoactasaninstructionregisterisalwaysenabled.Registersspecifiedtoactasoperandregisterschangestatewiththearrivalofpacketsdirectedtothem.ThestatetransitionsandenablingrulesfordataoperandregistersaredefinedinFig.12.InFig.12thecontentsofanoperandregisterarerepresentedasfollowsD,off,of_atttttDgatingcodenolJemptyttruetrue,o__.nfilledandenabledgateflogJregisteruseindicatorTheasteriskindicatesthattheRegisterisenabled.EventsdenotingarrivalofdataandcontrolpacketsarelabelledthusddatapacketttruegatetypecontrolpacketffalsegatetypecontrolpacketWiththisexplanationofnotation,thestatechangesandenablingrulesgiveninFig.12shouldbeclear.SimilarrulesapplytothestatechangesandenablingofBooleanoperandregisters.NotethatarrivalofagatetypecontrolpacketthatdoesnotmatchthegatingcodeoftheRegistercausestheassociateddatapackettobediscarD,n£of__f,offlld,off,o_.fD.tru.._eoff,off{.e,.true,offj\dhtrue,on.dfflse°ff/\J/td.off,¢77.LffdD,falseoff,of\o,false,offr..bfalse,onzTdLtue°\JD,con__._of._f,onFigure12.Statetransitionandenablingrulesfordataoperandregisters.30ded,andresetstheRegistertoitsstartingcondition.TheoperationpacketssenttoOperationUnitsanddecisionpacketssenttoDecisionUnitsconsistoftheentirecontentsoftheInstructionCellexceptforthegatingcodesandreceiverstatusfields.ThusthepacketssentthroughtheArbitrationNetworkhavethefollowingformatsTotheOperationUnitsop,vl,v2,dlop,vl,dl,d2TotheDecisionUnitspr,vl,v2,tl,dlpr,vl,tl,dl,t2,d2bo,cl,c2,tl,dl,t2,d2,t3,d3bo,cl,tl,dl,t2,d2,t3,d3AninitialconfigurationofInstructionCellscorrespondingtothebasicdataflowprogramofFig.8isgiveninFig.13.Forsimplicity,Cellscontainingcontroldistributionanddataforwardinginstructionsarenotshown.Instead,wehavetakenthelibertyofwritinganynumberofaddressesinthedestinationfieldsofinstructions.TheinitialvaluesofxandyareplacedinRegisters2and5.Cellsiand2,containingthesevalues,arethenenabledandpresenttotheArbitrationNetworktheoperationpackets{ident,ii,14}andThesepacketsaredirectedtoanidentityOperationUnitwhichmerelycreatesthedesireddatapacketswiththevaluesofxandyanddeliversthepacketstotheDistributionNetwork.UponreceiptbytheMemoryofthedatapacketsdirectedtoRegisters7and8,cell3willbeenabledandwilltransmititsdecisionpackettoaDecisionUnittoperformthelessthanfunction.TheresultofthedecisionwillbereturnedthroughtheControlNetworkasfivecontrolpackets.Iftheresultistrue,Cells4,5and6willbeenabledandwillsendtheircontentsthroughthecellIO0I\ident8,11,14\02Dn__oxcell512lplus7,15,2013Dtrue14Onocell2031\ident7,13,20\04I05Dn_.ycelI615\plus16,2516Dtrue017DconsIcell306Ilessgate11,13,16,20,2307Dno08iDn._£Icell718Iprint19Dconsi20IDifalsecell409IIl\ident{8,11,14\I0IIIDtru._._e212223cell8iprintDconsDJfalseFigure13.InstructionCellinitializationforthebasicdataflowprograminFigure8.ArbitrationNetworktoOperationUnitscapableofperformingtheidentityandadditionoperations.Iftheresultofthedecisionisfalse,outputcells7and8willbeenabled,andcells4,5,and6willhavetheirgatedoperandsdeleted.TwoLevelMemoryHierarchyThehighlevelofparallelactivityachievableindataflowprocessorsmakesauniqueformofmemoryhierarchyfeasibletheInstructionCellsarearrangedtoactasacacheforthemostactiveinstructionsofthedataflowprogram.IndividualinstructionsareretrievedfromauxiliarymemorytheInstructionMemoryastheybecomerequiredbytheprogressofcomputation,andinstructionsarereturnedtotheInstructionMemorywhentheInstructionCellsholdingthemarerequiredformoreactivepartsoftheprogram.TheorganizationofabasicdataflowprocessorwithInstructionMemoryisgiveninFig.14.InstructionMemoryTheInstructionMemoryhasastoragelocationforeachpossibleregisteraddressofthebasicprocessor.Thesestoragelocationsareorganizedintogroupsofthreelocationsidentifiedbytheaddressofthefirstlocationofthegroup.EachgroupcanholdthecontentsofoneInstructionCellintheformatsalreadygiveninFig.I0.Amemorycommandpacketa,retr}presentedtothecommannportoftheInstructionMemory,requestsretrievalofaninstructionpacket\a,x}inwhichxistheCellcontentsstoredinthegroupoflocationsspecifiedbyaddressa.TheinstructionpacketisdeliveredattheretrieveportoftheInstructionMemory.Aninstructionpacket{a,x\presentedatthestoreportoftheInstructionMemoryrequestsstorageofCellcontentsxinthethreelocationgroupspecifiedbyaddressa.However,thestorageisnoteffectiveuntilamemorycommandpacket{a,store\i8receivedbytheInstructiondatapacketsOperationUnitsDecisioncontrolUnitspocketsControlNetworkoperationpacketsdecisionpacketsCeilsCellBlock0MemoryCellBlockkIinstructionpacketsMemoryCommandNetworkinstructionpacketsmemorycommandpacketscommandIretrievestore,InstructionMemoryFigure14.Organizationofthebasicdataflowprocessorwithauxiliarymemory.131Memoryatitscommandport,andanypriorretrievalre.questhasbeenhonored.Similarly,retrievalrequestsarenothonoreduntilpriorstoragerequestsforthegrouphavetakeneffect.WeenvisionthattheInstructionMemorywouldbedesignedtohandlelargenumbersofstorageandretrievalequestsconcurrently,muchastheinput/outputfacilitiesofcontemporarycomputersystemsoperateundersoftwarecontrol.CellBlockOperationForapplicationofthecacheprincipletothebasicdataflowprocessor,anInstructionMemoryaddressisdividedintoamajoraddressandaminoraddress,eachcontaininganumberofbitsoftheaddress.OneCellBlockoftheprocessorisassociatedwitheachpossiblemajoraddress.AllinstructionshavingthesamemajoraddressareprocessedbytheInstructionCellsofthecorrespondingCellBlock.ThustheDistributionandControlNetworksusethemajoraddresstodirectdatapackets,controlpackets,andinstructionpacketstotheappropriateCellBlock.ThepacketsdeliveredtotheCellBlockincludetheminoraddress,whichissufficienttodeterminehowthepacketshouldbetreatedbytheCellBlock.OperationanddecisionpacketsleavingaCellBlockhaveexactlythesameformatasbefore.InstructionpacketsleavingaCellBlockhavetheform\m,x}wheremisaminoraddressandxisthecontentsofanInstructionCell.ThemajoraddressoftheCellBlockisappendedtoeachinstructionpacketasittravelsthroughtheArbitrationNetwork.Inthesameway,memorycommandpacketsleavetheCellBlockwithjustaminoraddress,whichisaugmentedbythemajoraddressoftheCellBlockduringitstripthroughtheMemoryCommandNetwork.Fig.15showsthestructureofaCellBlock.EachInstructionCellisabletoholdanyinstructionwhosemajoraddressisthatoftheCellBlock.SincemanymoreinstructionsshareamajoraddressthanthereareCellsinaCellBlock,theCellBlockincludesanAssociationTablewhichhasanentry{m,iforeachInstructionCelmistheminoraddressoftheinstructiontowhichtheCellisassigned,andiisaCellstatusindicatorwhosevalueshavesignificanceasfollowsstatusvaluemeanlnfreetheCellisnotassignedtoanyinstructionenaedtheCellhasbeenengagedfortheinstructionhavingminoraddressm,byarrivalofadataorcontrolpacketoccupiedtheCellisoccupiedbyaninstructionwithminoraddressmTheStackelementofaCellBlockholdsanorderingoftheInstructionCellsascandidatesfordisplacementoftheircontentsbynewlyactivatedinstructions.OnlyCellsinoccupiedstatusarecandiatesfordisplacement.OperationofaCellBlockcanbespecifiedbygivingtwoproceduresoneinitiatedbyarrivalofadataorcontrolpacketattheCellBlock,andtheotheractivatedbyarrivalofaninstructionpacketfromtheInstructionMemory.ProcedureiArrivalofadataorcontrolpacketIn,ywherenisaminoraddressandyisthepacketcontent.stepI.DoestheAssociationTablehaveanentrywithminoraddressnIfso,letpbetheCellcorrespondingtotheentry,andgotostep5.Otherwisecontinuewithstep2.step2.IftheAssociationTableshowsthatnoInstructionCellhasstatusfree,gotostep3.OtherwiseletpbeaCellwithstatusfree.LettheAssociaLControlNetTkIeontrolpocketstAssociotiOIo0odataTablepockets,pocketsdecisionpacketsDistributionArbitrationNetworkNetworkinstructioninstructionpacketsc2°JApocketsINetworkFilure15.StructureofeCellBlock.tlonTa61eentryforpbem,free}gotostep4.step3.UsetheStacktochooseaCellpinoccupiedstatusforpreemptionlettheAssociationTableentryforpbem,occupie.d}transmitthecontentszofCellpasaninstructionpacket{m,z}totheInstructionMemoryviatheArbitrationNetworktransmitthememorycormmandpacket{m,storetotheInstructionMemorythroughtheMemoryCommandNetwork.step4.Makeanentry{n,enKaged}forCellpintheAssociationTabletransmitthememorycozmandpacketn,re.tr}totheInstructionMemoryviatheMemoryCommandNetwork.step5.UpdatetheoperandregisterofCellphavingminoraddressnaccordingtothecontentyofthedataorcontrolpackettherulesforupdatingarethosegiveninFig.12.IfCellpisoccupiedthestatechangeoftheregistermustbeconsistentwiththeinstructioncodeortheprogramisinvalid.IfCellpisengaged,thechangesmostbeconsistentwiththeregisterstatusleftbyprecedingpacketarrivals.step6.IfCellpisoccupiedandallthreeregisersareenabledaccordingtotherulesofFig,12,theCellpisenabledtransmitanoperationordecisionpackettotheOperationUnitsorDecisionUnitsthroughtheArbitrationNetworkleaveCellpinoccupiedstatusholdingthesameinstructionwithitsoperandregistersresetreceiversemptywiththegateandvalueflagssettoof_l.ChangetheorderofCellsintheStacktomakeCellpthelastcandidatefordisplacement.Procedure2ArrivalofaninstructionpacketIn,x}withminoraddressnandcontentx.steI.LetpbetheInstructionCellwithentry{n,engaged}intheAssociationTable,st2.ThestatusoftheoperandregistersofCellpmustbeconsistentwiththecontentxoftheinstructionpacketsortheprogramisinvalid.UpdatethecontentsofCellptoincorporatetheinstructionandoperandstatusinformationintheinstructionpacket.step3.ChangetheAssociationTableentryforCellpfrom{n,engaged}to{n,occupied}.step4.IfallregistersofCellpareenabled,then132CellpisenabledtransmitanoperationordecisionpackettotheOperationUnitsorDecisionUnitsthroughtheArbitrationNetworkleaveCellpinoccupiedstatusholdingthesameinstructionwithitsoperandregistersreset.ChangetheorderofCellsintheStacktomakeCellpthelastcandidatefordisplacement.ConclusionTheorganizatibnofacomputerwhichallowstheexecutionofprogramsrepresentedindataflowformoffersaverypromisingsolutiontotheproblemofachievinghighlyparallelcomputation.Thusfar,thedesignoftwoprocessors,theelementaryandthebasicdataflowprocessors,hasbeeninvestigated.Theelementaryprocessorisattractiveforstreamorientedsignalprocessingapplications.ThebasicprocessordescribedhereisafirststeptowardahighlyparallelprocessorfornumericalalgorithmsexpressedinaFortranlikedataflowlanguage.However,thisgoalrequiresfurtherelaborationofthedataflowarchitecturetoencompassarrays,concurrentactivationofprocedures,andsomemeansofexploitingthesortofparallelismpresentinvectoroperations.Weareoptimisticthatextensionsofthearchitecturetoprovidethesefeaturescanbedevised,andwearehopefulthattheseconceptscanbefurtherextendedtothedesignofcomputersforgeneralpurposecomputationbasedonmorecompletedataflowmodelssuchaspresentedbyDemiis4.ReferencesI.Adams,D.A.ACompgationModelWithDataFlowSequencing.TechnicalReportcs117sComputerScienceDepartment,SchoolOfHumanitiesandSciences,StanfordUniversity,Stanford,Calif.,December1968.2.Bhrs,A.OperationpatternsAnextensiblemodelofanextensiblelanguage.SymposiumonneoreticalProRrafmning,Novosibirsk,USSR,August1972preprint.3.Dennis,J.B.Programminggenerality,parallelismandcomputerarchitecture.InformationProcessing68,NorthHollandPublishingCo.,Amsterdam1969,484492.4.Dennis,J.B.Firstversionofadataflowprocedurelanguage.Symposiumo_nProrammlng,InstitutdeProgranmmtion,UniversityofParis,Paris,Prance,April1974,241271.5.Dennis,J.B.,andJ.B.Fosseen.IntroductiontoDataFlowSchemas.November1973submittedforpublication.6.Dennis,J.B.,andD.P.Misunas.Acomputerarchitectureforhighlyparallelsignalprocessing.ProceedingsoftheACM1974NationalConference,ACM,NewYork,November1974.7.Dennis,J.B.,andD.P.Misunas.Th_eDesIKnofaHighlyParallelComputerfo_.2.SignalPrqcessinAplicati.onsCdmputationStructuresGroupMemoI01,ProjectMAC,M.I.T.,Cambridge,Mass.,July1974.8.Karp,R.M.,andR.E.Miller.Propertiesofamodelforparallelcomputationsdeterminacy,termination,queuelng..SIAMJ.Appi.Math.14November1966,13901411.9.Kosinski,P.R.ADataFlowProKTammlngLanKuage.ReportRC4264,IBMT.J.WatsonResearchCenter,YorktownHeights,N.Y.,March1973.Kosinskl,P.R.Adataflowlanguageforoperatingsystemsprogramming.ProceedingsofACMSIGPLANSCOPSInterfaceMeeting,SIGPLANNotices8,9September1973,8994.II.Rodriguez,J.E.AGraphMode____fo__Paralle____Computation.ReportTK64,ProjectMAC,M.I.T.,Cambridge,Mass.,September1969.I0.
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