计算机专业英语4.docx

计算机专业英语4.docx

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计算机专业英语4

Unit4SoftwareDevelopment

SectionA

ComputerProgram

I.Introduction

Acomputerprogramisasetofinstructionsthatdirectsacomputertoperformsomeprocessingfunctionorcombinationoffunctions.Fortheinstructionstobecarriedout,acomputermustexecuteaprogram,thatis,thecomputerreadstheprogram,andthenfollowsthestepsencodedintheprograminapreciseorderuntilcompletion.Aprogramcanbeexecutedmanydifferenttimes,witheachexecutionyieldingapotentiallydifferentresultdependingupontheoptionsanddatathattheusergivesthecomputer.

Programsfallintotwomajorclasses:

applicationprogramsandoperatingsystems.Anapplicationprogramisonethatcarriesout（执行）somefunctiondirectlyforauser,suchaswordprocessingorgameplaying.Anoperatingsystemisaprogramthatmanagesthecomputerandthevariousresourcesanddevicesconnectedtoit,suchasRAM（randomaccessmemory）,harddrives,monitors,keyboards,printers,andmodems,sothattheymaybeusedbyotherprograms.ExamplesofoperatingsystemsareDOS',Windows95,OS/22,andUNIX.

II.ProgramDevelopment

Softwaredesignerscreatenewprogramsbyusingspecialapplicationsprograms,oftencalledutilityprogramsordevelopmentprograms.Aprogrammerusesanothertypeofprogramcalledatexteditortowritethenewprograminaspecialnotationcalledaprogramminglanguage.Withthetexteditor,theprogrammercreatesatextfile,whichisanorderedlistofinstructions,alsocalledtheprogramsourcefile.Theindividualinstructionsthatmakeup（整理，组成）theprogramsourcefilearecalledsourcecode.Atthispoint,aspecialapplicationsprogramtranslatesthesourcecodeintomachinelanguage,orobjectcode—aformatthattheoperatingsystemwillrecognizeasaproperprogramandbeabletoexecute.

Threetypesofapplicationsprogramstranslatefromsourcecodetoobjectcode:

compilers,interpreters（解释程序）,andassemblers.Thethreeoperatedifferentlyandondifferenttypesofprogramminglanguages,buttheyservethesamepurposeoftranslatingfromaprogramminglanguageintomachinelanguage.

Acompilertranslatestextfileswritteninahigh-levelprogramminglanguage—suchasFORTRAN,C,orPascal—fromthesourcecodetotheobjectcodeallatonce.ThisdiffersfromtheapproachtakenbyinterpretedlanguagessuchasBASIC,inwhichaprogramistranslatedintoobjectcodestatementbystatementaseachinstructionisexecuted.Theadvantageofinterpretedlanguagesisthattheycanbeginexecutingtheprogramimmediatelyinsteadofhavingtowaitforallofthesourcecodetobecompiled.Changescanalsobemadetotheprogramfairlyquicklywithouthavingtowaitforittobecompiledagain.Thedisadvantageofinterpretedlanguagesisthattheyareslowtoexecute,sincetheentireprogrammustbetranslatedoneinstructionatatime,eachtimetheprogramisrun.Ontheotherhand,compiledlanguagesarecompiledonlyonceandthuscanexecutedbythecomputermuchmorequicklythaninterpretedlanguages.Forthisreason,compiledlanguagesaremorecommonandarealmostalwaysusedinprofessionalandscientificapplications.

Anothertypeoftranslatoristheassembler,whichisusedforprogramsorpartsofprogramswritteninassemblylanguage.Assemblylanguageisanotherprogramminglanguage,butitismuchmoresimilartomachinelanguagethanothertypesofhigh-levellanguages.Inassemblylanguage,asinglestatementcanusuallybetranslatedintoasingleinstructionofmachinelanguage.Today,assemblylanguageisrarelyusedtowriteanentireprogram,butisinsteadmostoftenusedwhentheprogrammerneedstodirectlycontrolsomeaspectofthecomputer'sfunction.

Programsareoftenwrittenasasetofsmallerpieces,witheachpiecerepresentingsomeaspectoftheoverallapplicationprogram.Aftereachpiecehasbeencompiledseparately,aprogramcalledalinkercombinesallofthetranslatedpiecesintoasingleexecutableprogram.

Programsseldomworkcorrectlythefirsttime,soaprogramcalleddebuggerisoftenusedtohelpfindproblemscalledbugs.Debuggingprogramsusuallydetectaneventintheexecutingprogramandpointtheprogrammerbacktotheoriginoftheeventintheprogramcode.

Recentprogrammingsystems,suchasJava,useacombinationofapproachestocreateandexecuteprograms.AcompilertakesaJavasourceprogramandtranslatesitintoanintermediateform.SuchintermediateprogramsarethentransferredovertheInternetintocomputerswhereaninterpreterprogramthenexecutestheintermediateformasanapplicationprogram.

III.ProgramElements

Mostprogramsarebuiltfromjustafewkindsofstepsthatarerepeatedmanytimesindifferentcontextsandindifferentcombinationsthroughouttheprogram.Themostcommonstepperformssomecomputation,andthenproceedsto（向…进发）thenextstepintheprogram,intheorderspecifiedbytheprogrammer.

Programsoftenneedtorepeatashortseriesofstepsmanytimes,forinstanceinlookingthroughalistofgamescoresandfindingthehighestscore.Suchrepetitivesequencesofcodearecalledloops.

Oneofthecapabilitiesthatmakecomputerssousefulistheirabilitytomakeconditionaldecisionsandperformdifferentinstructionsbasedonthevaluesofdatabeingprocessed.If-then-elsestatementsimplementthisfunctionbytestingsomepieceofdataandthenselectingoneoftwosequencesofinstructionsonthebasisoftheresult.Oneoftheinstructionsinthesealternativesmaybeagotostatementthatdirectsthecomputertoselectitsnextinstructionfromadifferentpartoftheprogram.Forexample,aprogrammightcomparetwonumbersandbranchtoadifferentpartoftheprogramdependingontheresultofthecomparison:

Ifxisgreaterthany

thengotoinstruction#10

elsecontinue

Programsoftenuseaspecificsequenceofstepsmorethanonce.Suchasequenceofstepscanbegroupedtogetherintoasubroutine,whichcanthenbecalled,oraccessed,asneededindifferentpartsofthemainprogram.Eachtimeasubroutineiscalled,thecomputerrememberswhereitwasintheprogramwhenthecallwasmade,sothatitcanreturnthereuponcompletionofthesubroutine.Precedingeachcall,aprogramcanspecifythatdifferentdatabeusedbythesubroutine,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

IV.ProgramFunction

Modemcomputersusuallystoreprogramsonsomeformofmagneticstoragemediathatcanbeaccessedrandomlybythecomputer,suchastheharddrivediskpermanentlylocatedinthecomputer,oraportablefloppydisk.Additionalinformationonsuchdisks,calleddirectories,indicatesthenamesofthevariousprogramsonthedisk,whentheywerewrittentothedisk,andwheretheprogrambeginsonthediskmedia.Whenauserdirectsthecomputertoexecuteaparticularapplicationprogram,theoperatingsystemlooksthrough（浏览）thesedirectories,locatestheprogram,andreadsacopyintoRAM.TheoperatingsystemthendirectstheCPU（centralprocessingunit）tostartexecutingtheinstructionsatthebeginningoftheprogram.InstructionsatthebeginningoftheprogrampreparethecomputertoprocessinformationbylocatingfreememorylocationsinRAMtoholdworkingdata,retrievingcopiesofthestandardoptionsanddefaultstheuserhasindicatedfromadisk,anddrawinginitialdisplaysonthemonitor.

Theapplicationprogramrequestsacopyofanyinformationtheuserentersbymakingacalltoasystemroutine.Theoperatingsystemconvertsanydatasoenteredintoastandardinternalform.Theapplicationthenlistthisinformationtodecidewhattodonext—forexample,performsomedesiredprocessingfunctionsuchasreformattingapageoftext,orobtainsomeadditionalinformationfromanotherfileonadisk.Ineithercase,callstoothersystemroutinesareusedtoactuallycarryoutthedisplayofresultsortheaccessingofthefilefromthedisk.

Whentheapplicationreachescompletionorispromptedtoquit,makesfurthersystemcallstomakesurethatalldatathatneedstobesavedhasbeenwrittenbacktodisk.Itthenmakesafinalsystemcalltotheoperatingsystemindicatingthatitisfinished.TheoperatingsystemthenfreesuptheRAMandanydevicesthattheapplicationwasusingandawaitsacommandfromtheusertostartanotherprogram.

V.History

Peoplehavebeenstoringsequencesofinstructionsintheformofprogramforseveralcenturies.Musicboxesofthe18thcenturyandplayerpianosofthelate19thandearly20thcenturiesplayedmusicalprogramsstoredasseriesofmetalpins,orholesinpaper,witheachline（ofpinsholes）representing