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.FortheIntel8086/87and无锡科技职业学院中英文翻译MotorolaMC6800microprocessors,awordis16bitslong,buteachmemorylocationhasonly8bitsandthustwo8-bitlocationsmustbeaccessedtoobtaineachdataword.Inordertousethecontentsofmemory,theprocessormustfetchthecontentsoftherightlocation.Tocarryoutafetch,theprocessorplaces(enables)thebinary-codedaddressofthedesiredlocationontotheaddresslinesoftheexternalprocessorbus.Thememorythenallowsthecontentsoftheaddressedmemorylocationtobereadbytheprocessor.Theprocessoffetchingthecontentsofamemorylocationdoesnotalterthecontentsofthatlocation.InstructionsinmemoryInstructionsstoredinmemoryarefetchedbytheCPUandunlessprogrambranchesoccur,theyareexecutedinthesequencetheyappearinmemory.Aninstructionwrittenasabinarypatterniscalledamachine-languageinstruction.OnewaytoachievemeaningfulpatternsistodivideupthebitsintofieldsasindicatedinFig.3-1A-2,witheachfieldcontainingacodeforadifferenttypeofinformation.Fig.3-1A-2ArrangementofprogramanddatainmemoryEachinstructioninoursimplecomputercanbedividedupintofourfieldsof4bitseach.Eachinstructioncancontainoperationcode(oropcode,eachinstructionhasauniqueopcode),operandaddress,immediateoperands,branchaddress.Inarealinstructionsettherearemanymoreinstructions.Thereisalsoamuchlarge2无锡科技职业学院中英文翻译numberofmemorylocationsinwhichtostoreinstructionsanddata.Inordertoincreasethenumberofmemorylocations,theaddressfieldsandhencetheinstructionsmustbelongerthan16bitsifweusethesameapproach.Thereareanumberofwaystoincreasetheaddressingrangeofthemicroprocessorwithoutincreasingtheinstructionlength:variableinstructionfield,multiwordinstructions,multipleaddressingmodes,variableinstructionlength.Wewillnotdiscussthemindetail.Datainmemorydataisinformationthatisrepresentedinmemoryasacode.Forefficientuseofthememoryspaceandprocessingtime,mostcomputersprovidethecapabilityofmanipulatingdataofdifferentlengthsandrepresentationsinmemory.Thevariousdifferentrepresentationsrecognizedbytheprocessorarecalleditsdatatypes.Thedatatypesnormallyusedare:bit,binary-codeddecimald...
