计算机专业英语4.docx
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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
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