计算机科学引论ppt课件.ppt

1 Introduction FoundationsofComputerScience CengageLearning 1 1 DefinetheTuringmodelofacomputer DefinethevonNeumannmodelofacomputer Describethethreecomponentsofacomputer hardware data andsoftware Listtopicsrelatedtocomputerhardware Listtopicsrelatedtodata Listtopicsrelatedtosoftware Discusssomesocialandethicalissuesrelatedtotheuseofcomputers Giveashorthistoryofcomputers Objectives Afterstudyingthischapter thestudentshouldbeableto 1 2 1 1TURINGMODEL TheideaofauniversalcomputationaldevicewasfirstdescribedbyAlanTuringin1937 Heproposedthatallcomputationcouldbeperformedbyaspecialkindofamachine nowcalledaTuringmachine Hebasedthemodelontheactionsthatpeopleperformwheninvolvedincomputation Heabstractedtheseactionsintoamodelforacomputationalmachinethathasreallychangedtheworld 1 3 Dataprocessors BeforediscussingtheTuringmodel letusdefineacomputerasadataprocessor Usingthisdefinition acomputeractsasablackboxthatacceptsinputdata processesthedata andcreatesoutputdata Figure1 1 Althoughthismodelcandefinethefunctionalityofacomputertoday itistoogeneral Inthismodel apocketcalculatorisalsoacomputer whichitis inaliteralsense Figure1 1Asinglepurposecomputingmachine 1 4 Programmabledataprocessors TheTuringmodelisabettermodelforageneral purposecomputer Thismodeladdsanextraelementtothespecificcomputingmachine theprogram Aprogramisasetofinstructionsthattellsthecomputerwhattodowithdata Figure1 2showstheTuringmodel Figure1 2AcomputerbasedontheTuringmodel 1 5 Figure1 3Thesameprogram differentdata 1 6 Figure1 4Thesamedata differentprograms 1 7 TheuniversalTuringmachine AuniversalTuringmachine amachinethatcandoanycomputationiftheappropriateprogramisprovided wasthefirstdescriptionofamoderncomputer ItcanbeprovedthataverypowerfulcomputerandauniversalTuringmachinecancomputethesamething Weneedonlyprovidethedataandtheprogram thedescriptionofhowtodothecomputation toeithermachine Infact auniversalTuringmachineiscapableofcomputinganythingthatiscomputable 1 8 1 2VONNEUMANNMODEL ComputersbuiltontheTuringuniversalmachinestoredataintheirmemory Around1944 1945 JohnvonNeumannproposedthat sinceprogramanddataarelogicallythesame programsshouldalsobestoredinthememoryofacomputer 1 9 ComputersbuiltonthevonNeumannmodeldividethecomputerhardwareintofoursubsystems memory arithmeticlogicunit controlunit andinput output Figure1 5 Foursubsystems Figure1 5ThevonNeumannmodel 1 10 ThevonNeumannmodelstatesthattheprogrammustbestoredinmemory Thisistotallydifferentfromthearchitectureofearlycomputersinwhichonlythedatawasstoredinmemory theprogramsfortheirtaskwasimplementedbymanipulatingasetofswitchesorbychangingthewiringsystem Thememoryofmoderncomputershostsbothaprogramanditscorrespondingdata Thisimpliesthatboththedataandprogramsshouldhavethesameformat becausetheyarestoredinmemory Infact theyarestoredasbinarypatternsinmemory asequenceof0sand1s Thestoredprogramconcept 1 11 AprograminthevonNeumannmodelismadeofafinitenumberofinstructions Inthismodel thecontrolunitfetchesoneinstructionfrommemory decodesit thenexecutesit Inotherwords theinstructionsareexecutedoneafteranother Ofcourse oneinstructionmayrequestthecontrolunittojumptosomepreviousorfollowinginstruction butthisdoesnotmeanthattheinstructionsarenotexecutedsequentially SequentialexecutionofaprogramwastheinitialrequirementofacomputerbasedonthevonNeumannmodel Today scomputersexecuteprogramsintheorderthatisthemostefficient Sequentialexecutionofinstructions 1 12 1 3COMPUTERCOMPONENTS Wecanthinkofacomputerasbeingmadeupofthreecomponents computerhardware data andcomputersoftware 1 13 ComputerhardwaretodayhasfourcomponentsunderthevonNeumannmodel althoughwecanhavedifferenttypesofmemory differenttypesofinput outputsubsystems andsoon WediscusscomputerhardwareinmoredetailinChapter5 Computerhardware ThevonNeumannmodelclearlydefinesacomputerasadataprocessingmachinethatacceptstheinputdata processesit andoutputstheresult Data 1 14 ThemainfeatureoftheTuringorvonNeumannmodelsistheconceptoftheprogram Althoughearlycomputersdidnotstoretheprograminthecomputer smemory theydidusetheconceptofprograms Programmingthoseearlycomputersmeantchangingthewiringsystemsorturningasetofswitchesonoroff Programmingwasthereforeataskdonebyanoperatororengineerbeforetheactualdataprocessingbegan Computersoftware 1 15 Figure1 6Programanddatainmemory 1 16 Figure1 7Aprogrammadeofinstructions 1 17 1 4HISTORY Inthissectionwebrieflyreviewthehistoryofcomputingandcomputers Wedividethishistoryintothreeperiods 1 18 Mechanicalmachines before1930 Duringthisperiod severalcomputingmachineswereinventedthatbearlittleresemblancetothemodernconceptofacomputer Inthe17thcentury BlaisePascal aFrenchmathematicianandphilosopher inventedPascaline Inthelate17thcentury aGermanmathematiciancalledGottfriedLeibnitzinventedwhatisknownasLeibnitz Wheel ThefirstmachinethatusedtheideaofstorageandprogrammingwastheJacquardloom inventedbyJoseph MarieJacquardatthebeginningofthe19thcentury 1 19 In1823 CharlesBabbageinventedtheDifferenceEngine Later heinventedamachinecalledtheAnalyticalEnginethatparallelstheideaofmoderncomputers In1890 HermanHollerith workingattheUSCensusBureau designedandbuiltaprogrammermachinethatcouldautomaticallyread tally andsortdatastoredonpunchedcards 1 20 Thebirthofelectroniccomputers 1930 1950 Between1930and1950 severalcomputerswereinventedbyscientistswhocouldbeconsideredthepioneersoftheelectroniccomputerindustry Theearlycomputersofthisperioddidnotstoretheprograminmemory allwereprogrammedexternally Fivecomputerswereprominentduringtheseyears ABCZ1MarkI ColossusENIAC Earlyelectroniccomputers 1 21 ThefirstcomputerbasedonvonNeumann sideaswasmadein1950attheUniversityofPennsylvaniaandwascalledEDVAC Atthesametime asimilarcomputercalledEDSACwasbuiltbyMauriceWilkesatCambridgeUniversityinEngland ComputersbasedonthevonNeumannmodel 1 22 Computergenerations 1950 present Computersbuiltafter1950moreorlessfollowthevonNeumannmodel Theyhavebecomefaster smaller andcheaper buttheprincipleisalmostthesame Historiansdividethisperiodintogenerations witheachgenerationwitnessingsomemajorchangeinhardwareorsoftware butnotinthemodel Thefirstgeneration roughly1950 1959 ischaracterizedbytheemergenceofcommercialcomputers Firstgeneration 1 23 Second generationcomputers roughly1959 1965 usedtransistorsinsteadofvacuumtubes Twohigh levelprogramminglanguages FORTRANandCOBOLinventedandmadeprogrammingeasier Secondgeneration Theinventionoftheintegratedcircuitreducedthecostandsizeofcomputersevenfurther Minicomputersappearedonthemarket Cannedprograms popularlyknownassoftwarepackages becameavailable Thisgenerationlastedroughlyfrom1965to1975 Thirdgeneration 1 24 Thefourthgeneration approximately1975 1985 sawtheappearanceofmicrocomputers Thefirstdesktopcalculator theAltair8800 becameavailablein1975 Thisgenerationalsosawtheemergenceofcomputernetworks Fourthgeneration Thisopen endedgenerationstartedin1985 Ithaswitnessedtheappearanceoflaptopandpalmtopcomputers improvementsinsecondarystoragemedia CD ROM DVDandsoon theuseofmultimedia andthephenomenonofvirtualreality Fifthgeneration 1 25 1 5SOCIALANDETHICALISSUES Computersciencehascreatedsomeperipheralissues themostprevalentofwhichcanbecategorizedassocialandethicalissues 1 26 Socialissues Computershavecreatedsomearguments Weintroducesomeoftheseargumentshere Somepeoplethinkthatcomputershavecreatedakindofdependency whichmakespeople slivesmoredifficult Dependency Socialjusticeisanotherissueweoftenhearabout Theadvocatesofthisissuearguethatusingcomputersathomeisaluxurybenefitthatnotallpeoplecanafford Thecostofacomputer peripheraldevices andamonthlychargeforInternetaccessisanextraburdenonlow incomepeople Socialjustice 1 27 Theconceptofdigitaldividecoversboththeissuesofdependencyandsocialjusticediscussedabove Theconceptdividessocietyintotwogroups thosewhoareelectronicallyconnectedtotherestofsocietyandthosewhoarenot Digitaldivide 1 28 Ethicalissues Computershavecreatedsomeethicalissues Weintroducesomeofthesehere Computersallowcommunicationbetweentwopartiestobedoneelectronically However muchneedstobedonetomakethistypeofcommunicationprivate Societyispayingabigpriceforprivateelectroniccommunication Networksecuritymaycreatethistypeofprivacy butitneedseffortandcostsalot Privacy 1 29 Anotherethicalissueinacomputerizedsocietyiscopyright whoownsdata TheInternethascreatedopportunitiestoshareideas buthasalsobroughtwithitafurtherethicalissue electroniccopyright Copyright Computersandinformationtechnologyhavecreatednewtypesofcrime Hackershavebeenabletoaccessmanycomputersintheworldandhavestolenalotofmoney ViruscreatorsdesignnewvirusestobesentthroughtheInternetanddamagetheinformationstoredincomputers Althoughtherearemanyanti virusprogramsinusetoday societyispayingabigpriceforthistypeofcrime whichdidnotexistbeforethecomputerandInternetera Computercrime 1 30 1 6COMPUTERSCIENCEASADISCIPLINE Withtheinventionofcomputers anewdisciplinehasevolved computerscience Likeanyotherdiscipline computersciencehasnowdividedintoseveralareas Wecandividetheseareasintotwobroadcategories systemsareasandapplicationsareas Thisbookisabreadth firstapproachtoalltheseareas Afterreadingthebook thereadershouldhaveenoughinformationtoselectthedesiredareaofspecialty 1 31 1 7OUTLINEOFTHECOURSE Afterthisintroductorychapter thebookisdividedintofiveparts PartI Datarepresentationandoperation ThispartincludesChapters2 3 and4 Chapter2discussesnumbersystems howaquantitycanberepresentedusingsymbols Chapter3discusseshowdifferentdataisstoredinsidethecomputer Chapter4discussessomeprimitiveoperationsonbits 1 32 PartII Computerhardware Thispartinc