ATYPICALCOMPUTERSYSTEM

　　Atypicaldigitalcomputerconsistsof:

　　a)Acentralprocessorunit(CPU)

　　b)Amemory

　　c)Input/output(1/0)ports

　　ThememoryservesasaplacetostoreInstructions,thecodedpiecesofinformationthatdirecttheactivitiesoftheCPU,andData,thecodedpiecesofinformationthatareprocessedbytheCPU.AgroupoflogicallyrelatedinstructionsstoredinmemoryisreferredtoasaProgram.TheCPU"reads"eachinstructionfrommemoryinalogicallydeterminedsequence,andusesittoinitiateprocessingactions.Iftheprogramsequenceiscoherentandlogical,processingtheprogramwillproduceintelligibleandusefulresults.

　　Thememoryisalsousedtostorethedatatobemanipulated,aswellastheinstructionsthatdirectthatmanipulationTheprogrammustbeorganizedsuchthattheCPUdoesnotreadanon-instructionwordwhenitexpectstoseeaninstruction.TheCPUcanrapidlyaccessanydatastoredinmemory;butoftenthememoryisnotlargeenoughtostoretheentiredatabankrequiredforaparticularapplication.TheproblemcanberesolvedbyprovidingthecomputerwithoneormoreInputPorts.TheCPUcanaddresstheseportsandinputthedatacontainedthere.Theadditionofinputportsenablesthecomputertoreceiveinformationfromexternalequipment(suchasapapertapereaderorfloppydisk)athighratesofspeedandinlargevolumes.

　　AcomputeralsorequiresoneormoreOutputPortsthatpermittheCPUtocommunicatetheresultofitsprocessingtotheoutsideworld.Theoutputmaygotoadisplay,forusebyahumanoperator,toaperipheraldevicethatproduces"hardcopy,"suchasaline-printer,toaperipheralstoragedevice,suchasafloppydiskunit,ortheoutputmayconstituteprocesscontrolsignalsthatdirecttheoperationsofanothersystem,suchasanautomatedassemblyline.Likeinputports,outputportsareaddressable.Theinputandoutputportstogetherpermittheprocessortocommunicatewiththeoutsideworld.

　　TheCPUunifiesthesystem.Itcontrolsthefunctionsperformedbytheothercomponents.TheCPUmustbeabletofetchinstructionsfrommemory,decodetheirbinarycontentsandexecutethem.Itmustalsobeabletoreferencememoryand1/0portsasnecessaryintheexecutionofinstructions.Inaddition,theCPUshouldbeabletorecognizeandrespondtocertainexternalcontrolsignals,suchasINTERRUPTandWAITrequests.ThefunctionalunitswithinaCPUthatenableittoperformthesefunctionsaredescribedbelow.