外文翻译computerprogram英文doc.docx
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外文翻译computerprogram英文doc
ComputerProgram
1Introduction
ComputerProgram,setofinstructionsthatdirectsacomputertoperformsomeprocessingfunctionorcombinationoffunctions.Fortheinstructionstobecarriedout,acomputermustexecuteaprogram,thatis,thecomputerreadstheprogram,andthenfollowthestepsencodedintheprograminapreciseorderuntilcompletion.Aprogramcanbeexecutedmanydifferenttimes,witheachexecutionyieldingapotentiallydifferentresultdependingupontheoptionsanddatathattheusergivesthecomputer.
Programsfallintotwomajorclasses:
applicationprogramsandoperatingsystems.Anapplicationprogramisonethatcarriesoutsomefunctiondirectlyforauser,suchaswordprocessingorgame-playing.Anoperatingsystemisaprogramthatmanagesthecomputerandthevariousresourcesanddevicesconnectedtoit,suchasRAM,harddrives,monitors,keyboards,printers,andmodems,sothattheymaybeusedbyotherprograms.ExamplesofoperatingsystemsareDOS,Windows95,OS\2,andUNIX.
2ProgramDevelopment
Softwaredesignerscreatenewprogramsbyusingspecialapplicationsprograms,oftencalledutilityprogramsordevelopmentprograms.Aprogrammerusesanothertypeofprogramcalledatexteditortowritethenewprograminaspecialnotationcalledaprogramminglanguage.Withthetexteditor,theprogrammercreatesatextfile,whichisanorderedlistofinstructions,alsocalledtheprogramsourcefile.Theindividualinstructionsthatmakeuptheprogramsourcefilearecalledsourcecode.Atthispoint,aspecialapplicationsprogramtranslatesthesourcecodeintomachinelanguage,orobjectcode—aformatthattheoperatingsystemwillrecognizeasaproperprogramandbeabletoexecute.
Threetypesofapplicationsprogramstranslatefromsourcecodetoobjectcode:
compilers,interpreters,andassemblers.Thethreeoperatedifferentlyandondifferenttypesofprogramminglanguages,buttheyservethesamepurposeoftranslatingfromaprogramminglanguageintomachinelanguage.
Acompilertranslatestextfileswritteninahigh-levelprogramminglanguage--suchasFORTRAN,C,orPascal—fromthesourcecodetotheobjectcodeallatonce.ThisdiffersfromtheapproachtakenbyinterpretedlanguagessuchasBASIC,APLandLISP,inwhichaprogramistranslatedintoobjectcodestatementbystatementaseachinstructionisexecuted.Theadvantagetointerpretedlanguagesisthattheycanbeginexecutingtheprogramimmediatelyinsteadofhavingtowaitforallofthesourcecodetobecompiled.Changescanalsobemadetotheprogramfairlyquicklywithouthavingtowaitforittobecompiledagain.Thedisadvantageofinterpretedlanguagesisthattheyareslowtoexecute,sincetheentireprogrammustbetranslatedoneinstructionatatime,eachtimetheprogramisrun.Ontheotherhand,compiledlanguagesarecompiledonlyonceandthuscanbeexecutedbythecomputermuchmorequicklythaninterpretedlanguages.Forthisreason,compiledlanguagesaremorecommonandarealmostalwaysusedinprofessionalandscientificapplications.
Anothertypeoftranslatoristheassembler,whichisusedforprogramsorpartsofprogramswritteninassemblylanguage.Assemblylanguageisanotherprogramminglanguage,butitismuchmoresimilartomachinelanguagethanothertypesofhigh-levellanguages.Inassemblylanguage,asinglestatementcanusuallybetranslatedintoasingleinstructionofmachinelanguage.Today,assemblylanguageisrarelyusedtowriteanentireprogram,butisinsteadmostoftenusedwhentheprogrammerneedstodirectlycontrolsomeaspectofthecomputer’sfunction.
Programsareoftenwrittenasasetofsmallerpieces,witheachpiecerepresentingsomeaspectoftheoverallapplicationprogram.Aftereachpiecehasbeencompiledseparately,aprogramcalledalinkercombinesallofthetranslatedpiecesintoasingleexecutableprogram.
Programsseldomworkcorrectlythefirsttime,soaprogramcalledadebuggerisoftenusedtohelpfindproblemscalledbugs.Debuggingprogramsusuallydetectaneventintheexecutingprogramandpointtheprogrammerbacktotheoriginoftheeventintheprogramcode.
Recentprogrammingsystems,suchasJava,useacombinationofapproachestocreateandexecuteprograms.AcompilertakesaJavasourceprogramandtranslatesitintoanintermediateform.SuchintermediateprogramsarethentransferredovertheInternetintocomputerswhereaninterpreterprogramthenexecutestheintermediateformasanapplicationprogram.
3ProgramElements
Mostprogramsarebuiltfromjustafewkindsofstepsthatarerepeatedmanytimesindifferentcontextsandindifferentcombinationsthroughouttheprogram.Themostcommonstepperformssomecomputation,andthenproceedstothenextstepintheprogram,intheorderspecifiedbytheprogrammer.
Programsoftenneedtorepeatashortseriesofstepsmanytimes,forinstanceinlookingthroughalistofgamescoresandfindingthehighestscore.Suchrepetitivesequencesofcodearecalledloops.
Oneofthecapabilitiesthatmakescomputersousefulistheirabilitytomakeconditionaldecisionsandperformdifferentinstructionsbasedonthevaluesofdatabeingprocessed.If-then-elsestatementsimplementthisfunctionbytestingsomepieceofdataandthenselectingoneoftwosequencesofinstructionsonthebasisoftheresult.Oneoftheinstructionsinthesealternativesmaybeagotostatementthatdirectsthecomputertoselectitsnextinstructionfromadifferentpartoftheprogram.Forexample,aprogrammightcomparetwonumbersandbranchtoadifferentpartoftheprogramdependingontheresultofthecomparison:
Ifxisgreaterthany
Then
Gotoinstruction#10
Elsecontinue
Programoftenuseaspecificsequenceofstepsmorethanonce.Suchasequenceofstepscanbegroupedtogetherintoasubroutine,whichcanthenbecalled,oraccessed,asneededindifferentpartsofthemainprogram.Eachtimeasubroutineiscalled,thecomputerrememberswhereitwasintheprogramwhenthecallwasmade,sothatitcanreturnthereuponcompletionofthesubroutine,allowingaverygeneralpieceofcodetobewrittenonceandusedinmultipleways.
Mostprogramsuseseveralvarietiesofsubroutines.Themostcommonofthesearefunctions,procedures,libraryroutines,systemroutines,anddevicedrivers.Functionsareshortsubroutinesthatcomputesomevalue,suchascomputationsofangles,whichthecomputercannotcomputewithasinglebasicinstruction.Proceduresperformamorecomplexfunction,suchassortingasetofnames.Libraryroutinesaresubroutinesthatarewrittenforusebymanydifferentprograms.Systemroutinesaresimilartolibraryroutinesbutareactuallyfoundintheoperatingsystem.Theyprovidesomeservicefortheapplicationprograms,suchasprintingalineoftext.Devicedriversaresystemroutinesthatareaddedtoanoperatingsystemtoallowthecomputertocommunicatewithanewdevice,suchasascanner,modem,orprinter.Devicedriversoftenhavefeaturesthatcanbeexecuteddirectlyasapplicationsprograms.Thisallowstheusertodirectlycontrolthedevice,whichisusefulif,forinstance,acolorprinterneedstoberealignedtoattainthebestprintingqualityafterchanginganinkcartridge.
4ProgramFunction
Moderncomputersusuallystoreprogramsonsomeformofmagneticstoragemediathatcanbeaccessedrandomlybythecomputer,suchastheharddrivediskpermanentlylocatedinthecomputer,oraportablefloppydisk.Additionalinformationonsuchdisks,calleddirectories,indicatethenamesofthevariousprogrambeginsonthediskmedia.Whenauserdirectsthecomputertoexecuteaparticularapplicationprogram,theoperatingsystemlooksthroughthesedirectories,locatestheprogram,andreadsacopyintoRAM.TheoperatingsystemthendirectstheCPUtostartexecutingtheinstructionsatthebeginningoftheprogram.InstructionsatthebeginningoftheprogrampreparethecomputertoprocessinformationbylocatingfreememorylocationsinRAMtoholdworkingdata,retrievingcopiesofthestandardoptionsanddefaultstheuserhasindicatedfromadisk,anddrawinginitialdisplaysonthemonitor.
Theapplicationprogramrequestscopyofanyinformationtheuserentersbymakingacalltoasystemroutine.Theoperatingsystemconvertsanydatasoenteredintoastandardinternalform.Theapplicationthenusesthisinformationtodecidewhattodonext---forexample,performsomedesiredprocessingfunctionsuchasreformattingapageoftext,orobtainsomeadditionalinformationfromanotherfileonadisk.Ineithercase,callstoothersystemroutinesareusedtoactuallycarryoutthedisplayoftheresultsortheaccessingofthefilefromthedisk.
Whentheapplicationreachescompletionorispromptedtoquit,itmakesfurthersystemcallstomakesurethatalldatathatneedstobesavedhasbeenwrittenbacktodisk.Itthenmakesafinalsystemcalltotheoperatingsystemindicatingthatitisfinished.TheoperatingsystemthenfreesuptheRAMandanydevicethattheapplicationwasusingandawaitsacommandfromtheusertostartanotherprogram.
5History
Peoplehavebeenstoringsequencesofinstructionsintheformofaprogramforseveralcenturies.Musicboxesofthe18thcenturyandplayerpianosofthelate19thandearly20thcenturiesplayedmusicalprogramsstoredasseriesifmetalpins,orholesinpaper,witheachlinerepresentingwhenanotewastobeplayed,andthepinorholeindicatingwhatnotewastobeplayedatthattime.Moreelaboratecontrolofphysicaldevicesbecamecommonintheearly1800swithFrenchinventorJosephMarieJacquard’sinventionofthepunch-cardcontrolledweavi