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Computercontroltechnology1ComputerstructureandfunctionThissectionintroducestheinternalarchitectureofacomputeranddescribeshowinstructionsarestoredandinterpretedandexplainshowtheinstructionexecutioncycleisbrokendownintoitsvariouscomponents.Atthemostbasiclevel,acomputersimplyexecutesbinary-codedresults.Forageneral-purposeprogrammablecomputer,fournecessaryelementsarethememory,centralprocessingunit(CPU,orsimplyprocessor),anexternalprocessorbus,andaninput/outputsystemasindicatedinFig.3-1A-1.Fig.3-1A-1BasicelementsofacomputerThememorystoresinstructionsanddata.TheCPUreadsandinterpretstheinstructions,readsthedatarequiredbyeachinstruction,executestheactionrequiredbytheinstruction,andstorestheresultsbackinmemory.OneoftheactionsthatisrequiredoftheCPUistoreaddatafromorwritedatatoanexternaldevice.Thisiscarriedoutusingtheinput/outputsystem.Theexternalprocessorbusisasetofelectricconductorsthatcarriesdata,addressandcontrolinformationbetweentheothercomputerelements.1-1ThememoryThememoryofacomputerconsistsofasetofsequentiallynumberedlocations.Eachlocationisaregisterinwhichbinaryinformationcanbestored.The“number”ofalocationiscalleditsaddress.Thelowestaddressis0.Themanufacturerdefinesawordlengthfortheprocessorthatisanintegralnumberoflocationslong.Ineachwordthebitscanrepresenteitherdataorinstructions.FortheIntel8086/87andMotorolaMC6800microprocessors,awordis16bitslong,buteachmemorylocation无锡科技职业学院中英文翻译-2-hasonly8bitsandthustwo8-bitlocationsmustbeaccessedtoobtaineachdataword.Inordertousethecontentsofmemory,theprocessormustfetchthecontentsoftherightlocation.Tocarryoutafetch,theprocessorplaces(enables)thebinary-codedaddressofthedesiredlocationontotheaddresslinesoftheexternalprocessorbus.Thememorythenallowsthecontentsoftheaddressedmemorylocationtobereadbytheprocessor.Theprocessoffetchingthecontentsofamemorylocationdoesnotalterthecontentsofthatlocation.InstructionsinmemoryInstructionsstoredinmemoryarefetchedbytheCPUandunlessprogrambranchesoccur,theyareexecutedinthesequencetheyappearinmemory.Aninstructionwrittenasabinarypatterniscalledamachine-languageinstruction.OnewaytoachievemeaningfulpatternsistodivideupthebitsintofieldsasindicatedinFig.3-1A-2,witheachfieldcontainingacodeforadifferenttypeofinformation.000101011000XXXX010000011000XXXX0011XXXXXXXX0100FieldsOpcodeImmediatecodeOperanddataBranchaddressSet‘5’inlocation8Subtract‘1’fromlocation8Ifzero,branchtolocation416-bitinstructionwords......XXXX:notused(or“don’tcare”)Fig.3-1A-2ArrangementofprogramanddatainmemoryEachinstructioninoursimplecomputercanbedividedupintofourfieldsof4bitseach.Eachinstructioncancontainoperationcode(oropcode,eachinstructionhasauniqueopcode),operandaddress,immediateoperands,branchaddress.Inarealinstructionsettherearemanymoreinstructions.Thereisalsoamuchlargenumberofmemorylocationsinwhichtostoreinstructionsanddata.Inordertoincreasethenumberofmemorylocations,theaddressfieldsandhencethe无锡科技职业学院中英文翻译-3-instructionsmustbelongerthan16bitsifweusethesameapproach.Thereareanumberofwaystoincreasetheaddressingrangeofthemicroprocessorwithoutincreasingtheinstructionlength:variableinstructionfield,multiwordinstructions,multipleaddressingmodes,variableinstructionlength.Wewillnotdiscussthemindetail.Datainmemorydataisinformationthatisrepresentedinmemoryasacode.Forefficientuseofthememoryspaceandprocessingtime,mostcomputersprovidethecapabilityofmanipulatingdataofdifferentlengthsandrepresentationsinmemory.Thevariousdifferentrepresentationsrecognizedbytheprocessorarecalleditsdatatypes.Thedatatypesnormallyusedare:bit,binary-codeddecimaldigit(4-bitnibble,BCD),byte(8bits),word(2bytes),doubleword(4bytes).Someprocessorsprovideinstructionsthatmanipulateotherdatatypessuchassingle-precisionfloating-pointdatatypes(32bits)anddouble-precisionfloating-pointdatatypes(64bits).Thereisanothertypeofdata—characterdata.Itisalsousuallyrepresentedin8bits.Eachcomputerterminalkeyandkeycombination(suchasshiftandcontrolfunctions)onastandardterminalkeyboardhasa7-bitscodedefinedbytheAmericanStandardCodeforInformationInterchange(ASCII).TypeofmemoryIntheapplicationsofdigitalcontrolsystem,wealsoconcernedwiththecharacteristicsofdifferentmemorytechniques.Forprimarymemory,weneedittobestoredinformationtemporarilyandtobewrittenandgotinformationfromsuccessiveorfromwidelydifferentlocations.Thistypememoryiscalledrandom-accessmemory(RAM).Insomecasewedonotwanttheinformationinmemorytobelost.Sowearewillingtousespecialtechniquestowriteintomemory.Ifwritingisaccomplishedonlyoncebyphysicallychangingconnections,thememoryiscalledaread-onlymemory(ROM).Iftheinterconnectionpatterncanbeprogrammedtobeset,thememoryiscalledaprogrammableread-onlymemory(PROM).Ifrewritingcanbeaccomplishedwhenitisnecessary,wehaveanerasableprogrammableread-onlymemory(EPROM).AnelectronicallyerasablePROMisabbreviatedEEPROM.无锡科技职业学院中英文翻译-4-1-2TheCPUTheCPU’sjobistofetchinstructionsfrommemoryandexecutetheseinstructions.ThestructureoftheCPUisshowninFig.3-1A-3.Ithasfourmaincomponents:anarithmeticandlogicalunit(ALU),asetofregisters,aninternalprocessorbusandcontroller