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12-The Case for the Reduced Instruction Set Computer.pdf

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12-The Case for the Reduced Instruction Set Computer.pdf

THECASEFORTHEREDUCEDINSTRUCTIONSETCOMPUTERDAVIDAPATTERSONCOMPUTERSCIENCEDIVISIONUNIVERSITYOFCALIFORNIABERKELEY,CALIFORNIA94720DAVIDRDITZELBELLLABORATORIESCOMPUTINGSCIENCERESEARCHCENTERMURRAYHILL,NEWJERSEY07974INTRODUCTIONONEOFTHEPRIMARYGOALSOFCOMPUTERARCHITECTSISTODESIGNCOMPUTERSTHATAREMORECOSTEFFECTIVETHANTHEIRPREDECESSORSCOSTEFFECTIVENESSINCLUDESTHECOSTOFHARDWARETOMANUFACTURETHEMACHINE,THECOSTOFPROGRAMMING,ANDCOSTSINCURREDRELATEDTOTHEARCHITECTUREINDEBUGGINGBOTHTHEINITIALHARDWAREANDSUBSEQUENTPROGRAMSIFWEREVIEWTHEHISTORYOFCOMPUTERFAMILIESWEFINDTHATTHEMOSTCOMMONARCHITECTURALCHANGEISTHETRENDTOWARDEVERMORECOMPLEXMACHINESPRESUMABLYTHISADDITIONALCOMPLEXITYHASAPOSITIVETRADEOFFWITHREGARDTOTHECOSTEFFECTIVENESSOFNEWERMODELSINTHISPAPERWEPROPOSETHATTHISTRENDISNOTALWAYSCOSTEFFECTIVE,ANDINFACT,MAYEVENDOMOREHARMTHANGOODWESHALLEXAMINETHECASEFORAREDUCEDINSTRUCTIONSETCOMPUTERRISCBEINGASCOSTEFFECTIVEASACOMPLEXINSTRUCTIONSETCOMPUTERCISCTHISPAPERWILLARGUETHATTHENEXTGENERATIONOFVLSICOMPUTERSMAYBEMOREEFFECTIVELYIMPLEMENTEDASRISCSTHANCISCSASEXAMPLESOFTHISINCREASEINCOMPLEXITY,CONSIDERTHETRANSITIONSFROMIBMSYSTEM/3TOTHESYSTEM/38UTLEY78ANDFROMTHEDECPDP11TOTHEVAXLLTHECOMPLEXITYISINDICATEDQUANTITATIVELYBYTHESIZEOFTHECONTROLSTORE;FORDECTHESIZEHASGROWNFROM256X56INTHEPDP11/40TO5120X96INTHEVAX11/780REASONSFORINCREASEDCOMPLEXITYWHYHAVECOMPUTERSBECOMEMORECOMPLEXWECANTHINKOFSEVERALREASONSSPEEDOFMEMORYVSSPEEDOFCPUJOHNCOCKESAYSTHATTHECOMPLEXITYBEGANWITHTHETRANSITIONFROMTHE701TOTHE709COCKE80THE701CPUWASABOUTTENTIMESASFASTASTHECOREMAINMEMORY;THISMADEANYPRIMITIVESTHATWEREIMPLEMENTEDASSUBROUTINESMUCHSLOWERTHANPRIMITIVESTHATWEREINSTRUCTIONSTHUSTHEFLOATINGPOINTSUBROUTINESBECAMEPARTOFTHE709ARCHITECTUREWITHDRAMATICGAINSMAKINGTHE709MORECOMPLEXRESULTEDINANADVANCETHATMADEITMORECOSTEFFECTIVETHANTHE701SINCETHEN,MANYHIGHERLEVELINSTRUCTIONSHAVEBEENADDEDTOMACHINESINANATTEMPTTOIMPROVEPERFORMANCENOTETHATTHISTRENDBEGANBECAUSEOFTHEIMBALANCEINSPEEDS;ITISNOTCLEARTHATARCHITECTSHAVEASKEDTHEMSELVESWHETHERTHISIMBALANCESTILLHOLDSFORTHEIRDESIGNS25MICROCODEANDLSITECHNOLOGYMICROPROGRAMMEDCONTROLALLOWSTHEIMPLEMENTATIONOFCOMPLEXARCHITECTURESMORECOSTEFFECTIVELYTHANHARDWIREDCONTROL\HUSSON70\ADVANCESININTEGRATEDCIRCUITMEMORIESMADEINTHELATE60SANDEARLY70SHAVECAUSEDMICROPROGRAMMEDCONTROLTOBETHEMORECOSTEFFECTIVEAPPROACHINALMOSTEVERYCASEONCETHEDECISIONISMADETOUSEMICROPROGRAMMEDCONTROL,THECOSTTOEXPANDANINSTRUCTIONSETISVERYSMALL;ONLYAFEWMOREWORDSOFCONTROLSTORESINCETHESIZESOFCONTROLMEMORIESAREOFTENPOWERSOF2,SOMETIMESTHEINSTRUCTIONSETCANBEMADEMORECOMPLEXATNOEXTRAHARDWARECOSTBYEXPANDINGTHEMICROPROGRAMTOCOMPLETELYFILLTHECONTROLMEMORYTHUSTHEADVANCESINIMPLEMENTATIONTECHNOLOGYRESULTEDINCOSTEFFECTIVEIMPLEMENTATIONOFARCHITECTURESTHATESSENTIALLYMOVEDTRADITIONALSUBROUTINESINTOTHEARCHITECTUREEXAMPLESOFSUCHINSTRUCTIONSARESTRINGEDITING,INTEGERTOFLOATINGCONVERSION,ANDMATHEMATICALOPERATIONSSUCHASPOLYNOMIALEVALUATIONCODEDENSITYWITHEARLYCOMPUTERS,MEMORYWASVERYEXPENSIVEITWASTHEREFORECOSTEFFECTIVETOHAVEVERYCOMPACTPROGRAMSCOMPLEXINSTRUCTIONSETSAREOFTENHERALDEDFORTHEIRSUPPOSEDCODECOMPACTIONATTEMPTINGTOOBTAINCODEDENSITYBYINCREASINGTHECOMPLEXITYOFTHEINSTRUCTIONSETISOFTENADOUBLEEDGEDSWORDHOWEVER,ASMOREINSTRUCTIONSANDADDRESSINGMODESREQUIREMOREBITSTOREPRESENTTHEMEVIDENCESUGGESTSTHATCODECOMPACTIONCANBEASEASILYACHIEVEDMERELYBYCLEANINGUPTHEORIGINALINSTRUCTIONSETWHILECODECOMPACTIONISIMPORTANT,THECOSTOF10MOREMEMORYISOFTENFARCHEAPERTHANTHECOSTOFSQUEEZING10OUTOFTHECPUBYARCHITECTURALINNOVATIONSCOSTFORALARGESCALECPUISINADDITIONALCIRCUITPACKAGESNEEDEDWHILECOSTFORASINGLECHIPCPUISMORELIKELYTOBEINSLOWINGDOWNPERFORMANCEDUETOLARGERHENCESLOWERCONTROLPLASMARKETINGSTRATEGYUNFORTUNATELY,THEPRIMARYGOALOFACOMPUTERCOMPANYISNOTTODESIGNTHEMOSTCOSTEFFECTIVECOMPUTER;THEPRIMARYGOALOFACOMPUTERCOMPANYISTOMAKETHEMOSTMONEYBYSELLINGCOMPUTERSINORDERTOSELLCOMPUTERSMANUFACTURERSMUSTCONVINCECUSTOMERSTHATTHEIRDESIGNISSUPERIORTOTHEIRCOMPETITORSCOMPLEXINSTRUCTIONSETSARECERTAINLYPRIMARYMARKETINGEVIDENCEOFABETTERCOMPUTERINORDERTOKEEPTHEIRJOBS,ARCHITECTSMUSTKEEPSELLINGNEWANDBETTERDESIGNSTOTHEIRINTERNALMANAGEMENTTHENUMBEROFINSTRUCTIONSANDTHEIRPOWERISOFTENUSEDTOPROMOTEANARCHITECTURE,REGARDLESSOFTHEACTUALUSEORCOSTEFFECTIVENESSOFTHECOMPLEXINSTRUCTIONSETINSOMESENSETHEMANUFACTURERSANDDESIGNERSCANNOTBEBLAMEDFORTHISASLONGASBUYERSOFCOMPUTERSDONOTQUESTIONTHEISSUEOFCOMPLEXITYVSCOSTEFFECTIVENESSFORTHECASEOFSILICONHOUSES,AFANCYMICROPROCESSORISOFTENUSEDASADRAWCARD,ASTHEREALPROFITCOMESFROMLURINGCUSTOMERSINTOBUYINGLARGEAMOUNTSOFMEMORYTOGOWITHTHEIRRELATIVELYINEXPENSIVECPUUPWARDCOMPATIBILITYCOINCIDENTWITHMARKETINGSTRATEGYISTHEPERCEIVEDNEEDFORUPWARDCOMPATIBILITYUPWARDCOMPATIBILITYMEANSTHATTHEPRIMARYWAYTOIMPROVEADESIGNISTOADDNEW,ANDUSUALLYMORECOMPLEX,FEATURESSELDOMAREINSTRUCTIONSORADDRESSINGMODESREMOVEDFROMANARCHITECTURE,RESULTINGINAGRADUALINCREASEINBOTHTHENUMBERANDCOMPLEXITYOFINSTRUCTIONSOVERASERIESOFCOMPUTERSNEWARCHITECTURESTENDTOHAVEAHABITOFINCLUDINGALLINSTRUCTIONSFOUNDINTHEMACHINESOFSUCCESSFULCOMPETITORS,PERHAPSBECAUSEARCHITECTSANDCUSTOMERSHAVENOREALGRASPOVERWHATDEFINESAGOODINSTRUCTIONSETSUPPORTFORHIGHLEVELLANGUAGESASTHEUSEOFHIGHLEVELLANGUAGESBECOMESINCREASINGLYPOPULAR,MANUFACTURERSHAVEBECOMEEAGERTOPROVIDEMOREPOWERFULINSTRUCTIONSTOSUPPORTTHEMUNFORTUNATELYTHEREISLITTLEEVIDENCETOSUGGESTTHATANYOFTHEMORECOMPLICATEDINSTRUCTIONSETSHAVEACTUALLYPROVIDEDSUCHSUPPORTONTHECONTRARY,WESHALLARGUETHATINMANYCASESTHECOMPLEXINSTRUCTIONSETSAREMOREDETRIMENTALTHANUSEFULTHEEFFORTTOSUPPORTHIGHLEVELLANGUAGESISLAUDABLE,BUTWEFEELTHATOFTENTHEFOCUSHASBEENONTHEWRONGISSUES26USEOFMULTIPROGRAMMINGTHERISEOFTIMESHARINGREQUIREDTHATCOMPUTERSBEABLETORESPONDTOINTERRUPTSWITHTHEABILITYTOHALTANEXECUTINGPROCESSANDRESTARTITATALATERTIMEMEMORYMANAGEMENTANDPAGINGADDITIONALLYREQUIREDTHATINSTRUCTIONSCOULDBEHALTEDBEFORECOMPLETIONANDLATERRESTARTEDTHOUGHNEITHEROFTHESEHADALARGEEFFECTONTHEDESIGNOFINSTRUCTIONSETSTHEMSELVES,THEYHADADIRECTEFFECTONTHEIMPLEMENTATIONCOMPLEXINSTRUCTIONSANDADDRESSINGMODESINCREASETHESTATETHATHASTOBESAVEDONANYINTERRUPTSAVINGTHISSTATEOFTENINVOLVESTHEUSEOFSHADOWREGISTERSANDALARGEINCREASEINTHECOMPLEXITYOFTHEMICROCODETHISCOMPLEXITYLARGELYDISAPPEARSONAMACHINEWITHOUTCOMPLICATEDINSTRUCTIONSORADDRESSINGMODESWITHSIDEEFFECTSHOWHAVECISCSBEENUSEDONEOFTHEINTERESTINGRESULTSOFRISINGSOFTWARECOSTSISTHEINCREASINGRELIANCEONHIGHLEVELLANGUAGESONECONSEQUENCEISTHATTHECOMPILERWRITERISREPLACINGTHEASSEMBLYLANGUAGEPROGRAMMERINDECIDINGWHICHINSTRUCTIONSTHEMACHINEWILLEXECUTECOMPILERSAREOFTENUNABLETOUTILIZECOMPLEXINSTRUCTIONS,NORDOTHEYUSETHEINSIDIOUSTRICKSINWHICHASSEMBLYLANGUAGEPROGRAMMERSDELIGHTCOMPILERSANDASSEMBLYLANGUAGEPROGRAMMERSALSORIGHTFULLYIGNOREPARTSOFTHEINSTRUCTIONSETWHICHARENOTUSEFULUNDERTHEGIVENTIMESPACETRADEOFFSTHERESULTISTHATOFTENONLYAFAIRLYSMALLPARTOFTHEARCHITECTUREISBEINGUSEDFOREXAMPLE,MEASUREMENTSOFAPARTICULARIBM360COMPILERFOUNDTHAT10INSTRUCTIONSACCOUNTEDFOR80OFALLINSTRUCTIONSEXECUTED,16FOR90,21FOR95,AND30FOR99ALEXANDER75ANOTHERSTUDYOFVARIOUSCOMPILERSANDASSEMBLYLANGUAGEPROGRAMSCONCLUDEDTHATLITTLEFLEXIBILITYWOULDBELOSTIFTHESETOFINSTRUCTIONSONTHECDC3600WEREREDUCEDTOOROFTHEINSTRUCTIONSNOWAVAILABLEFOSTER71SHUSTEKPOINTSOUTFORTHEIBM370THATASHASBEENOBSERVEDMANYTIMES,VERYFEWOPEODESACCOUNTFORMOSTOFAPROGRAMSEXECUTIONTHECOBOLPROGRAM,FOREXAMPLE,EXECUTES84OFTHEAVAILABLE183INSTRUCTIONS,BUT48REPRESENT9908OFALLINSTRUCTIONSEXECUTED,AND26REPRESENT9028SHUSTEK78SIMILARSTATISTICSAREFOUNDWHENEXAMININGTHEUSEOFADDRESSINGMODESCONSEQUENCESOFCISCIMPLEMENTATIONSRAPIDCHANGESINTECHNOLOGYANDTHEDIFFICULTIESINIMPLEMENTINGCISCSHAVERESULTEDINSEVERALINTERESTINGEFFECTSFASTERMEMORYTHEADVANCESINSEMICONDUCTORMEMORYHAVEMADESEVERALCHANGESTOTHEASSUMPTIONSABOUTTHERELATIVEDIFFERENCEINSPEEDBETWEENTHECPUANDMAINMEMORYSEMICONDUCTORMEMORIESAREBOTHFASTANDRELATIVELYINEXPENSIVETHERECENTUSEOFCACHEMEMORIESINMANYSYSTEMSFURTHERREDUCESTHEDIFFERENCEBETWEENCPUANDMEMORYSPEEDSIRRATIONALIMPLEMENTATIONSPERHAPSTHEMOSTUNUSUALASPECTOFTHEIMPLEMENTATIONOFACOMPLEXARCHITECTUREISTHATITISDIFFICULTTOHAVERATIONALIMPLEMENTATIONSBYTHISWEMEANTHATSPECIALPURPOSEINSTRUCTIONSARENOTALWAYSFASTERTHANASEQUENCEOFSIMPLEINSTRUCTIONSONEEXAMPLEWASDISCOVEREDBYPEUTOANDSHUSTEKFORTHEIBM370PEUTO,SHUSTEK77;THEYFOUNDTHATASEQUENCEOFLOADINSTRUCTIONSISFASTERTHANALOADMULTIPLEINSTRUCTIONFORFEWERTHAN4REGISTERSTHISCASECOVERS40OFTHELOADMULTIPLEINSTRUCTIONSINTYPICALPROGRAMSANOTHERCOMESFROMTHEVAX11/780THEINDEXINSTRUCTIONISUSEDTOCALCULATETHEADDRESSOFANARRAYELEMENTWHILEATTHESAMETIMECHECKINGTOSEETHATTHEINDEXFITSINTHEARRAYBOUNDSTHISISCLEARLYANIMPORTANTFUNCTIONTOACCURATELYDETECTERRORSINHIGHLEVELLANGUAGESSTATEMENTSWEFOUNDTHATFORTHEVAX11/780,REPLACINGTHISSINGLEHIGHLEVELINSTRUCTIONBYSEVERALSIMPLEINSTRUCTIONSCOMPARE,JUMPLESSUNSIGNED,ADD,MULTIPLYTHATWECOULDPERFORMTHESAMEFUNCTION4527FASTERFURTHERMORE,IFTHECOMPILERTOOKADVANTAGEOFTHECASEWHERETHELOWERBOUNDWASZERO,THESIMPLEINSTRUCTIONSEQUENCEWAS60FASTERCLEARLYSMALLERCODEDOESNOTALWAYSIMPLYFASTERCODE,NORDOHIGHERLEVELINSTRUCTIONSIMPLYFASTERCODELENGTHENEDDESIGNTIMEONEOFTHECOSTSTHATISSOMETIMESIGNOREDISTHETIMETODEVELOPANEWARCHITECTUREEVENTHOUGHTHEREPLICATIONCOSTSOFACISCMAYBELOW,THEDESIGNTIMEISGREATLYEXPANDEDITTOOKDECONLY6MONTHSTODESIGNANDBEGINDELIVERYOFTHEPDP1,BUTITNOWTAKESATLEASTTHREEYEARSTOGOTHROUGHTHESAMECYCLEFORAMACHINELIKETHEVAX1THISLONGDESIGNTIMECANHAVEAMAJOREFFECTONTHEQUALITYOFTHERESULTINGIMPLEMENTATION;THEMACHINEISEITHERANNOUNCEDWITHATHREEYEAROLDTECHNOLOGYORTHEDESIGNERSMUSTTRYTOFORECASTAGOODIMPLEMENTATIONTECHNOLOGYANDATTEMPTTOPIONEERTHATTECHNOLOGYWHILEBUILDINGTHEMACHINEITISCLEARTHATREDUCEDDESIGNTIMEWOULDHAVEVERYPOSITIVEBENEFITSONTHERESULTINGMACHINEINCREASEDDESIGNERRORSONEOFTHEMAJORPROBLEMSOFCOMPLEXINSTRUCTIONSETSISDEBUGGINGTHEDESIGN;THISUSUALLYMEANSREMOVINGERRORSFROMTHEMICROPROGRAMCONTROLALTHOUGHDIFFICULTTODOCUMENT,ITISLIKELYTHATTHESECORRECTIONSWEREAMAJORPROBLEMWITHTHEIBM360FAMILY,ASALMOSTEVERYMEMBEROFTHEFAMILYUSEDREADONLYCONTROLSTORETHE370LINEUSESALTERABLECONTROLSTOREEXCLUSIVELY,DUEPERHAPSTODECREASEDHARDWARECOSTS,BUTMORELIKELYFROMTHEBADEXPERIENCEWITHERRORSONTHE360THECONTROLSTOREISLOADEDFROMAFLOPPYDISKALLOWINGMICROCODETOBEMAINTAINEDSIMILARLYTOOPERATINGSYSTEMS;BUGSAREREPAIREDANDNEWFL

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