力传感器及电子秤设计文献翻译输入输出访问Word格式.docx

力传感器及电子秤设计文献翻译输入输出访问Word格式.docx

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Inthisarticle,wewilllookatthethreebasicmethodsofI/Oaccessing－programmedI/O,interrupt-drivenI/O,anddirectmemoryaccess（DMA）.ThekeyissuethatdistinguishesthesethreemethodsishowdeeplytheprocessorisinvolvedinI/Ooperations.Thediscussionemphasizesinterrupt-drivenI/O,becauseitisbasedontheconceptofinterrupthandling,whichisageneralproblemthatgoesbeyondInput/Outputoperations.Thestudyofinterrupthandlingalsoaidsinunderstandingthegeneralconceptofexceptionprocessing,whichisanimportantissuenotonlyforI/O,butalsoforinterfacingacomputerwithothersystemcontrolfunctions.

AddressingI/ORegisters

Input/OutputdevicescommunicatewithaprocessorthroughInput/Outputports.Throughtheinputports,sprocessorreceivesdatafromtheI/Odevices.Throughtheoutputports,aprocessorsendsdatatotheI/Odevices.EachI/Oportconsistsofasmallsetofregisters,suchasdatabufferregisters（theinputbufferand/ortheoutputbuffer）,thestatusregister,andthecontrolregister.Theprocessormusthavesomemeanstoaddresstheseregisterswhilecommunicatingwiththem.TherearetwocommonmethodsofaddressingI/Oregister－memory-mappedI/OanddirectI/O.

1.Memory-MappedI/O

Memory-mappedI/OmapstheI/Oregistersandmainmemoryintoaunifiedaddressspaceinthecomputersystem.I/Oregisterssharethesameaddressspacewithmainmemory,butaremappedtoaspecificsectionthatisreservedjustforI/O.Thus,theI/Oregistercanbeaddressedinordinarymemoryreferenceinstructionsasiftheyarepartofthemainmemorylocations.TherearenospeciallydesignedI/Oinstructionsintheinstructionsetofthesystem.AnyinstructionthatreferencesalocationinthisareaisanI/Oinstruction.AnyinstructionthatcanspecifyamemoryaddressiscapableofperformingI/Ooperations.TheMotorolaMC68000isanexampleofacomputersystemthatusesthisaddressingmethod.

2.DirectI/O

ThemethodofaddressingI/OregistersdirectlywithoutsharingtheaddressspacewiththemainmemoryiscalleddirectI/OorI/O-mappedI/O.Inotherwords,I/Oregistersarenotmappedtothesameaddressspacewiththemainmemory.EachI/Oregisterhasanindependentaddressspace.Asaresult,instructionsthatreferencethemainmemoryspacecannotbeusedforInput/Output.Intheinstructionsetofthecomputersystem,specialI/OinstructionsmustbedesignedforI/Ooperations.IntheseI/Oinstructions,distinctI.D.numbersmustbeusedtoaddressdifferentI/Ocommunicationchannels（i.e.,I/Oports）.Theyarecalledportnumbers.TheI/OregistersofanI/OportareconnectedtothesystemI/Obus,throughwhichtheprocessorcanreferencetheI/Oregistersdirectlytosend/receivedatato/fromanI/Odevice.AnI/Oportnumberisnotfromthesameaddressspaceasmainmemory.ThePentiumisanexampleofacomputersystemthatusesthedirectI/Oaddressingmethod.Ithasa64GBmemoryaddressspace（32addressbits）and,atthesametime,a64KBI/Oaddressspace（16bitsI/Oaddress/portnumber）.

Wecancomparememory-mappedI/OandthedirectI/OandthedirectI/Oasfollows:

●Memory-mappedI/OusesordinarymemoryreferenceinstructionstoaccessI/O,soitprovidesflexibilityforI/OprogrammingandsimplifiesI/Osoftware.DirectI/OdoesnotprovideanyflexibilityinI/Oprogramming,sinceonlyasmallsetofspecialI/OinstructionsareallowedtoreferenceI/Oregisters.

●formemory-mappedI/O,theprocessorusesthesameaddresslinestoaccessalltheaddressableI/Oregistersandthesamedatalinestosend/receivedatato/formtheseregisters.ThissimplifiestheconnectionbetweenI/Oportandtheprocessor,andthusleadstoalow-costhardwaredesignandimplementation.FordirectI/O,theconnectionbetweenI/Oportsandtheprocessormaybemoreexpensive.Thisisbecauseeither

（1）specialhardwareisneededtoimplementseparateI/Oaddresslinesor

（2）whenmemoryaddresslinesareusedforI/O;

aspecialflagisneeded,indicatingthattherequestedaddressisforanI/Ooperation.

●InspiteoftheadvantageofusingordinarymemoryreferenceinstructionstoaccessI/Oregisters,memory-mappedI/OmaycomplicatethecontrolunitdesigninregardstotheimplementationofI/O-relatedinstructions.ThisisbecauseusuallytheI/Obuscyclesneedtobelongerthantheequivalentmemorybuscycles,andthismeansthatthedesignofdifferenttimingcontrollogicisrequired.Thiscanbeusedtoexplainwhymemory-mappedI/Obenefitsprogrammers,butnotelectronicsengineers.

●DirectI/Oaddressinghasanotheradvantageovermemory-mappedI/Ointhatlow-leveldebuggingonadifferentiatedaddressingsystemmaybeeasier,becausebreak-pointsorerrortrapscanbeimposedmoregenerally.

●withmemory-mappedI/O,I/Oregisterssharethesameaddressspacewithmainmemory;

hence,thememoryspaceavailableforprogramsanddataisreduced.FordirectI/Oaddressing,I/Odoesnotsharememoryspacewithmainmemory,andasinglecontiguousmemoryspacecanbemaintainedandusedbyprogrammers.

ProgrammedI/O

ProgrammedI/Orequiresthatalldatatransferoperationsbeputunderthecompletecontroloftheprocessorwhenexecutingprograms.Itissometimescalledpolling,becausetheprogramrepeatedlypolls（checks）thestatusflagofanI/Odevice,sothatitsinput/outputoperationcanbesynchronizedwiththeprocessor.AgeneralflowchartofsuchaprogramisshowninFigure1.TheprogramcontinuouslypollsthestatusofanI/Odevicetofindoutwhether

（1）dataisavailableintheinputbufferor

（2）theoutputdeviceisreadyforreceivingdatafromtheprocessor.Ifthestatusshows“available”theprogramwillexecuteadatatransferinstructiontocompletetheI/Ooperation;

otherwise,thebusystatusoftheI/Odevicewillforcetheprogramtocirculateinabusy-waitingloopuntilthestatusbecomesavailable.Suchabusy-waitingloop,whichcontinuouslychecksthestatusofdataavailability（forinput）ordeviceavailability（forout-put）,formsthetypicalprogramstructureofprogrammedI/O.Itisthistime-consumingbusy-waitingloopthatwastesprocessortimeandmakesprogrammedI/Overyinefficient.TheprocessormustbeinvolvedcontinuouslyintheentireI/Oprocess.Duringthistimeinterval,theprocessorcannotperformanyusefulcomputation,butonlyserveasingleI/Odevice.ForcertainslowI/Odevices,thisbusy-waitingloopintervalmaybelongenoughthattheprocessorcouldexecutemillionsofinstructionsbeforetheI/Oeventoccurs,e.g.,akeystrokeonakeyboard.

TheoperationalmodelfprogrammedI/Ostatedaboveischaracterizedbythebusywaitingloopoftheprogram,duringwhichtheprocessorspendstimepollinganI/Odevice.Becauseofthededicationoftheprocessortoasingletask,thismodeofprogrammedI/Oiscalleddedicatedpollingorspinpolling.Althoughdedicatedpollingishighlyinefficient,sometimesitisnecessaryandevenunavoidable.Inaparticularcase,ifanurgenteventneedsanimmediateresponsewithoutdelay,thendedicatedpollingbyadedicatedprocessormaybethebestwaytohandleit.Oncetheexpectedeventhappens,theprocessorcantracttoitimmediately.Forexample,certainrealtimesystems（e.g.,radarechoprocessingsystems）requireareactiontoincomingdatathatissoquickthatevenaninterruptresponseistooslow.Undersuchacircumstance,onlyafastdedicatedpollingloopmaysuffice.

AnothermodeofoperationofprogrammedI/Oiscalledintermittentpollingortimedpolling.Inthismode,theprocessormaypollthedeviceataregulartimedinterval,whichcanbeexpectedorprescheduled.Suchadevicecanbefoundinmanyembeddedsystemswhereaspecial-purposecomputerisusedforprocesscontrol,dataacquisition,environmentalmonitoring,trafficcounting,etc.thesedevices,whichmeasure,collect,orrecorddata,areusuallypolledperiodicallyinaregularscheduledeterminedbytheneedsoftheapplication.Suchamethodofintermittentpollingcanhelpsavetimelostinspinpollingandavoidthecomplexityofinterruptprocessing.However,itshouldbenotedthatintermittentpollingmaynotbeapplicableinsomespecialcases,inwhichthereisonlyonedevicetobepolledandthecorrectpollingratemustbeachievedwiththeassistanceofaninterrupt-drivenclock.Usingtimedpollinginthiscasewouldresultinsimplyswappingoneinterrupt-drivenclock.Usingtimepollinginthiscasewouldresultinsimplyswappingoneinterruptrequirementforanother.

Interrupt-DrivenI/O

Interrupt-drivenI/Oisameanstoavoidtheinefficientbusy-waitingloops,whichcharacterizeprogrammedI/O.InsteadofwaitingwhiletheI/Odeviceisbusydoingitsjobofinput/output,theprocessorcanrunotherprograms.WhentheI/Odevicecompletesitsjobanditsstatusbecomes“available”,itwillissueaninterruptrequesttotheprocessor,askingforCPUservice.Inresponse,theprocessorsuspendswhateveritiscurrentlydoing,inordertoattendtotheneedsofthatI/Odevice.

Inrespondtoaninterruptrequest,theprocessorwillfirstsavethecontentsofboththeprogramcounterandthestatusregisterfortherunningprogram,andthentransferthecontroltothecorrespondinginterruptserviceroutinetoperformtherequireddatainput/outputoperation.Whentheinterruptserviceroutinehascompleteditsexecutionandifnomoreinterruptrequestsarepending,theprocessorwillresumetheexecutionofthepreviouslyinterruptedprogramandrestorethecontentsofthestatusesandprogramcounter.Theprocessorhardwareshouldchecktheinterruptrequestsignaluponcompletionofexecutionofeveryinstruction.Ifm