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THE SECRET LIFE OF PROGRAMS Understand Computers Craft Better Code
by Jonathan E. Steinhart
SanFrancisco
THE SECRET LIFE OF PROGRAMS. Copyright©2019byJonathanE.Steinhart.
Allrightsreserved.Nopartofthisworkmaybereproducedortransmittedinanyformorbyany means,electronicormechanical,includingphotocopying,recording,orbyanyinformationstorage orretrievalsystem,withoutthepriorwrittenpermissionofthecopyrightownerandthepublisher.
ISBN-10:1-59327-970-1
ISBN-13:978-1-59327-970-7
Publisher:WilliamPollock
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Thefollowingimagesareattributedasfollows:CompositioninFigure6-36courtesyofHanalei Steinhart.BrickwallinFigures11-3,11-4,11-7and11-8from www.cadhatch.com Figure1425from The Rocky Horror Picture Show (1975).
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Library of Congress Cataloging-in-Publication Data
Names:Steinhart,JonathanE.,author.
Title:TheSecretLifeofPrograms/JonathanE.Steinhart.
Description:SanFrancisco:NoStarchPress,Inc.,[2019]
Identifiers:LCCN2019018295(print)|LCCN2019021631(ebook)|ISBN 9781593279714(epub)|ISBN159327971X(epub)|ISBN9781593279707(print) |ISBN1593279701(print)
Subjects:LCSH:Computerprogramming.|Programminglanguages(Electronic computers)|Browsers(Computerprograms)
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ACKNOWLEDGMENTS Therearealotofinfluencesthatcontributedtomakingthisbookpossible.Thatstartswithmy parentsRobertandRosalynSteinhartformakingmepossibleandthenencouragingmyinterestin science,atleastuptothepointatwhichitstartedtoscarethem.Manyawesometeacherstookit fromthereincludingBeatriceSeagal,WilliamMulvahill,andMillerBugliari.MuchthankstoPaul RubenfieldfortellingmeaboutbothCivilDefenseandtheBellLabsExplorerScoutpost. It’simpossibletogiveenoughcredittomyExplorerScoutadvisorsCarlChristiansenandHeinz Lycklama.Theychangedmylife.ThroughthemImetmanyamazingpeopleatBellTelephone LaboratoriesincludingJoeCondon,SandyFraser,DaveHagelbarger,DickHause,JimKaiser,Hank McDonald,MaxMathews,DennisRitchie,KenThompson,andDaveWeller.Ilearnedalotfromeach ofthem.
ThankstoAubreyAnderson,ClemCole,LeeJalovec,A.C.Mendiones,EdPost,andBetsyZellerfor makingitthroughthewholethingatleastonce.AndespeciallytoAubreyfortechnicalediting. ThanksalsotoMattBlaze,AdamCecchetti,SandyClark,TomDuff,NatalieFreed,FrankHeidt,DV Henkel-Wallace(a.k.aGumby),LouKatz,Sara-JayeTerp,Talin,andPaulVixieforproviding feedbackonparticularchapters.
AndthankstoallofthepeoplewhoansweredthephonewhenIcalledwithgeneralquestions includingWardCunningham,JohnGilmore,EvelynMast,MikePerry,AlexPolvi,AlanWirfs-Brock, andMikeZuhl.Andofcourse,RakelHellberg,thegirlontheskilift,forprovidingoneofthenudges thatmotivatedmefinishthisproject.
Thisbookwouldnothavebeenpossiblewithoutthesupportandencouragementofpeoplein variousgeekcommunitiesincludingAMW,Hackers,andTUHS.
ThankstoHanaleiSteinhartforthecompositioninFigure6-36andtoJulieDonnellyforthescarf inFigure11-41
ThankstoTonyCatforallowingmetousehisimageandforkeepingmykeyboardfulloffur.
PREFACE Iwasbornageek.Accordingtomyfather,Iusedanimaginaryswitchtoturnontheswingsbefore usingthem,andIwouldturnthemoffwhenIwasdone.Machineryjustspoketomeaboutitsinner workings.IresembledC-3POunderstanding“thebinarylanguageofmoisturevaporators.”Iwas fortunatetogrowupinatimeinwhichonecouldexaminetheworkingsofmostthingswithouta microscope.
Inhindsight,IhadaverysurrealchildhoodgrowingupinNewJersey.Itinkeredwitheverything, oftentothedetrimentofmymom’snervoussystem.Myparentsgavemelotsof“50-in-one”project kitsbutbecameuncomfortablewhenIstartedhookingthemtogetherforprojectsthatweren’tin thebooks.ThisculminatedwiththePillowBurglarAlarm,whichcaughtthetoothfairyinaction a pooreconomicchoicethatwasneverthelessemotionallysatisfying.Icollectedbrokentelevisions andotherappliancesthatpeoplewouldleaveoutongarbagedaysothatIcouldtakethemapart, learnhowtheyworked,andbuildthingsoutoftheparts.Oneofmyfavoritetoyswasmydad’s 1929ErectorSet.Thespaceprogrammadeitagreattimetobeinterestedtechnology;Iremember standinginourfrontyardwithmyfatheronenightwatchingEcho1flyover. Mostkidshadpaperroutes;Irepairedtelevisionsandstereos.MyfatherworkedforIBM,andI occasionallywentwithhimtoworkandwasawedbythebigcomputers.Hetookmewithhimto theElectroShowinAtlanticCitywhenIwaseight,andIrememberplayingwithanIBM1620.Ialso rememberbeingfascinatedbytheequipmentattheTektronixbooth,whichmayhaveinfluenced mylaterchoicetoworkforthem.AyearlaterIwenttotheWorld’sFairinNewYorkandwasawed bytheBellSystemexhibit;bychanceIlaterworkedwithoneofitsdesigners.
Ireceivedanamazingpost-Sputnikpublicschooleducation,thelikesofwhichnolongerexistsin America.Wepassedaroundthejugofmercuryinfifthgrade.Iblewupthechemistrylabinsixth gradeandlearnedfromtheexperienceinsteadofbeinglockedaway.(Icanstillquotetherecipefor makingnitrogentriiodide.)Iremembermyeighth-gradescienceteachermarchingusoutofthe classroomandtakingusintoNewYorkCitytoseethemovie 2001: A Space Odyssey becausehe thoughtthatitwasimportant.Hedidthiswithnoparentalnotesorpermissionsslips;ateacher whodidthattodaywouldlikelylosetheirjob,orworse.Wemadegunpowderinhighschool chemistry,shotrocketsateachotheronthefootballfieldinphysicsclass,lancedourownfingersto dobloodtypinginbiology.Afarcryfromtodaywhensomanymillionsaredrowninginfive-gallon bucketsthatwarningsarerequired,wetfloorsstrikefearintotheheartsofman,andgovernment officialsaredismissiveofscienceandunabletodistinguishtinkeringfromterrorism. Outsideofschool,myparentssignedmeupfortheBoyScouts,whichIloved,andLittleLeague, whichIhated.Scoutingtaughtmealotaboutthephysicalworldfromhorsebackridingtosafely playingwithfiretooutdoorsurvival.LittleLeaguetaughtmethatIdidn’tliketeamsports. Hamradiowasbiginthosedays;itwaswheretinkeringhappened.IvolunteeredforthelocalCivil DefenseemergencyradiocommunicationsgroupjustsoIcouldplaywiththeequipment.Theyhad
aprimitiveradioteletypesystem,whichIredesignedandendedupbuildingunitsforother municipalities.Ilovedthethree-dimensionalmechanicalcontraptionthatwasaTeletype. WhenIwasinhighschool,afriendtoldmeaboutanExplorerScoutpostthatmeteveryMonday eveningatBellTelephoneLaboratoriesinnearbyMurrayHill.Ijoinedandgottoplaywith computersbackwhentheywerethesizeoflargehouses.Iwashooked.Itwasn’tlongbeforeIwas leavingschoolearly,hitchhikinguptotheLabs,andtalkingpeopleintolettingmein.Thisturned intoaseriesofamazingsummerjobsworkingwithincrediblepeoplethatchangedmylife.Ilearned alotjustbypokingmyheadintopeople’slabsandaskingthemwhattheyweredoing.Iendedup writingsoftwareforthemeventhoughIplannedtostudyelectricalengineering,becausehardware projectsjustcouldn’tbefinishedinasummer.
Ifeltthatthebestwaytohonormyscoutadvisorswastofollowintheirfootstepsbytryingtohelp newergenerationsofbuddingyoungtechnologistsalongtheirpathonceIwasinapositiontodo so.Thisturnedouttobedifficult,astheheydayofAmericanresearchhasgivenwaytoincreasing shareholdervalue;productsthemselvesarenotvaluedashighlyastheprofitstheygenerate,which makesresearchhardtojustify.Companiesrarelyletkidsrunwildontheirpremisesanymore,for liabilityreasons.IhadoriginallythoughtIwouldworkthroughscouting,butrealizedIcouldn’t becausescoutinghadadoptedsomepolicesthatIcouldn’tsupport,asIhadnevergottenmysexual discriminationmeritbadge.Instead,Ivolunteeredinmylocalschoolsystem.
IstartedwritingthisbooktosupplementaclassthatIvolunteeredtoteach.Ididthisbeforethe internetwasasreadilyaccessibleasitistoday.Icurrentlyliveinafairlypoorruralfarming community,sotheoriginaldraftofthisbooktriedtobeall-inclusiveundertheassumptionthat studentswouldn’tbeabletoaffordsupplementarymaterials.Thatturnedouttobeanimpossible task.
Lotsofmaterialaboutdifferentprogramminglanguagesandconceptsisnowavailableonline,and mostpeoplehaveinternetaccessathomeorattheirschoolorlibrary.I’verewrittenthematerial withtheexpectationthatreaderscannowmuchmoreeasilyfindadditionalinformationonline.So, ifsomethingisn’tclearoryouwantmoreinformation,lookitup.
Recently,anumberofstudentsIknowhaveexpressedfrustrationwiththewaythey’rebeing taughtprogramming.Althoughtheycanfindinformationonline,theykeepaskingwheretheycan findeverythingtheyneedinoneplace.Thisbookiswrittentobethatresource.
Iwasluckytogrowupcontemporaneouslywithcomputers.Wedevelopedtogether.Ihaveahard timeimaginingwhatitmustbeliketojumpintothematurefieldthatcomputingistodaywithout havingthebackground.Themostchallengingpartsofwritingthisbookweredecidinghowfarto reachbackintothepastforexamplesandchoosingelementsofmoderntechnologytodiscuss.I settledonsortofaretrofeel,asonecanlearnmostofwhat’snecessaryfromolder,simpler technologythat’seasiertounderstand.Newer,morecomplextechnologiesarebuiltusingthesame buildingblocksastheolderones;knowingthoseblocksmakesunderstandingnewtechnologies mucheasier.
It’sadifferentagenow.Gadgetsaremuchhardertotakeapart,repair,andmodify.Companiesare abusinglawssuchastheDigitalMillenniumCopyrightAct(DMCA)topreventpeoplefrom repairingdevicestheyown,whichisfortunatelyresultingin“righttorepair”lawsinsomeplaces. AsAmericans,wegetmixedmessagesfromourgovernment;ontheonehandwe’reencouragedto gointoSTEMcareers,whileontheotherhandweseesciencedenigratedandSTEMjobs outsourced.It’snotclearthattheUSwouldhaveeverbecomeatechnologypowerhouseifthis environmenthadexistedahalf-centuryago.
Thenagain,therearebrightspots.Makerspacesareproliferating.Somekidsarebeingallowedto buildthingsandarediscoveringthatit’sfun.Electronicpartsarecheaperthanthey’veeverbeen,as longasyoudon’twantoneswithwiresonthem.Smartphoneshavemoreprocessingpowerthanall
thecomputersintheworldcombinedwhenIwasakid.Computersarecheaperthananybodyever imagined;smallcomputerssuchastheRaspberryPiandArduinocostlessthanapizzaandhavea hugevarietyofavailabletoppings. Withsuchpoweravailable,it’stemptingtojustplaywiththehigh-levelfunctionality.It’slike playingwithLEGO.MyparentsgavemeoneofthefirstLEGOsetsmade;itprettymuchjusthad rectangularblocks.ButIhadmyimaginationandIcouldbuildanythingIwanted.Today,youcan geta Star Wars LEGOsetanddeployaprefabricatedYoda.It’smuchhardertoinventnew characters.Thefancypieceshampertheimagination.
There’sagreatsceneintheclassic1939movie The Wizard of Oz inwhichthewizardisexposedand bellows,“Paynoattentiontothatmanbehindthecurtain.”Thisbookisforthoseofyouwhoaren’t goingtolistentothatandwanttoknowwhat’sbehindthecurtain.Myintentistoshinelightonthe fundamentalbuildingblocksonwhichhigh-levelfunctionalityisbuilt.Thisbookisforthosewhose imaginationisn’tsatisfiedbyhigh-levelfunctionalityalone;it’sforthosewhoaredrawnto creating new high-levelfunctionality.Ifyou’reinterestedinbecomingawizardinadditiontobeing amerewielderofmagicitems,thenthisbookisforyou.
INTRODUCTION Afewyearsago,IwasridingonaskiliftwithourSwedishexchangestudent.Iaskedherifshehad thoughtaboutwhatshewasgoingtodoafterhighschool.Shesaidthatshewasconsidering engineeringandhadtakenaprogrammingclassthepreviousyear.Iaskedherwhattheytaught. Shereplied,“Java.”Iinstinctivelyrespondedwith“That’stoobad.”
WhydidIsaythat?Tookmeawhiletofigureitout.It’snotthatJavaisabadprogramming language;it’sactuallyprettydecent.IsaiditbecauseofthewayinwhichJava(andotherlanguages) aretypicallyusedtoteachprogrammingtoday without teaching anything about computers.Ifthis strikesyouasabitodd,thenthisbookisforyou.
TheJavaprogramminglanguagewasinventedbyJamesGosling,MikeSheridan,andPatrick Naughtoninthe1990satSunMicrosystems.ItwasmodeledinpartaftertheCprogramming language,whichwaswidelyusedatthetime.Cdoesn’tincludeautomaticmanagementofmemory, andmemorymanagementerrorswereacommonheadacheatthetime.Javaeliminatedthatclassof programmingerrorsbydesign;ithidtheunderlyingmemorymanagementfromtheprogrammer. That’spartofwhatmakesitsuchagoodprogramminglanguageforbeginners.Butittakesmuch morethanagoodprogramminglanguagetoproducegoodprogrammersandprograms.Andit turnedoutthatJavaintroducedawholenewclassofharder-to-debugprogrammingproblems, includingpoorperformanceresultingfromthehiddenmemorymanagementsystem.
Asyou’llseeinthisbook,understandingmemoryisakeyskillforprogrammers.Whenyou’re learningtoprogram,it’seasytodevelophabitsthatbecomehardtobreak.Studieshaveshownthat childrenwhogrewupplayingatso-called“safe”playgroundshaveahigherrateofinjurieslaterin lifethanthosewhodidn’t,presumablybecausetheydidn’tlearnthatfallinghurts.Programmingis ananalogoussituation.Safeprogrammingenvironmentsmakegettingstartedlessscary,butyou alsoneedtopreparefortheoutsideworld.Thisbookhelpsyoumakethattransition.
WHY GOOD PROGRAMMING IS IMPORTANT Tounderstandwhyit’sproblematictoteachcomputerprogrammingwithoutalsoteachingabout computers,firstconsiderhowubiquitouscomputershavebecome.Thepriceofcomputershas fallensodramaticallythatusingthemisnowthecheapestwaytobuildmanythings.Forexample, usingacomputertodisplayanimageofanold-fashionedanalogclockonacardashboardcosts muchlessthanamechanicalclock.Thisisaresultofhowcomputerchipsaremanufactured;they’re moreorlessprinted.It’snolongerabigdealtostampoutachipthatcontainsbillionsof components.NotethatI’mtalkingaboutthepriceofcomputersthemselves,notthepriceofthings thatcontaincomputers.Ingeneral,acomputerchiptodaycostslessthanthepackaginginwhichit’s shipped.Computerchipsareavailablethatcostpennies.Therewilllikelycomeatimewhenitwill bedifficulttofindanythingthatdoesn’tcontainacomputer.
Lotsofcomputersdoinglotsofthingsmeanslotsofcomputerprograms.Becausecomputersareso ubiquitous,thefieldofcomputerprogrammingisincrediblydiverse.Asinmedicine,many
programmersbecomespecialists.Youcanspecializeinareassuchasvision,animation,webpages, phoneapps,industrialcontrol,medicaldevices,andmore.
Butthestrangethingaboutcomputerprogrammingisthatunlikeinmedicine,inprogrammingyou canbecomeaspecialistwithouteverbeingageneralist.Youprobablywouldn’twantaheart surgeonwhoneverlearnedanatomy,buttheequivalenthasbecomenormalformany programmerstoday.Isthisreallyaproblem?Infact,there’splentyofevidencethatthisisn’t workingverywell,withalmostdailyreportsofsecuritybreachesandproductrecalls.Therehave beencourtcasesinwhichpeopleconvictedofdrunkdrivingbybreathalyzerhavewontherightto havethebreathalyzercodereviewed.Itturnedoutthatthecodewasfullofbugs,whichresultedin overturnedconvictions.Recently,apieceofantivirussoftwarecrashedapieceofmedical equipmentinthemiddleofaheartsurgery.LiveswerelostduetodesignissuesintheBoeing737 MAXairplane.Thelargenumberofincidentslikethesedon’tinspirealotofconfidence.
LEARNING TO CODE IS ONLY A STARTING PLACE Partofthereasonforthisstateofaffairsisthatit’snotallthatdifficulttowritecomputerprograms that appear towork,orworkmuchofthetime.Let’susethechangesinmusic(notdisco!)inthe 1980sasananalogy.Peopleusedtohavetodevelopafoundationinordertomakemusic.This includedlearningmusictheory,composition,andhowtoplayaninstrument;eartraining;andlots ofpracticing.ThentheMusicalInstrumentDigitalInterface(MIDI)standard,originallyproposedby IkutaroKakehashiofRoland,camealong,whichletanyonemake“music”fromtheircomputer withouteverhavingtodevelopcalluses.It’smyopinionthatonlyasmallpercentageofcomputergenerated“music”isactuallymusic;it’smostlynoise. Music isproducedbyactualmusicians who mayormaynotuseMIDItobuildontheirfoundation.Programmingthesedayshasbecomealot likeusingMIDI.Younolongerhavetosweatmuchorspendyearspracticingorevenlearntheoryin ordertowriteprograms.Butthatdoesn’tmeanthesearegoodorreliableprograms. Thissituationislikelytogetworse,atleastintheUnitedStates.Wealthypeoplewithvested interests,likethosewhoownsoftwarecompanies,havebeenlobbyingforlegislationmandating thateverybodylearntocodeinschool.Thissoundsgreatintheory,butit’snotagreatideain practicebecausenoteverybodyhastheaptitudetobecomeagoodprogrammer.Wedon’tmandate thateverybodylearntoplayfootballbecauseweknowthatit’snotforeverybody.Thelikelygoalof thisinitiativeisnottoproducegreatprogrammersbutrathertoincreasesoftwarecompanyprofits byfloodingthemarketwithlargenumbersofpoorprogrammers,whichwilldrivedownwages. Thepeoplebehindthispushdon’tcareverymuchaboutcodequality theyalsopushfor legislationthatlimitstheirliabilityfordefectiveproducts.Ofcourse,youcanprogramforfunjust likeyoucanplayfootballforfun.Justdon’texpecttobedraftedfortheSuperBowl.
In2014,PresidentObamasaidthathehadlearnedtocode.Hediddragafewthingsaroundinthe excellentvisualprogrammingtoolBlockly,andheeventypedinonelineofcodeinJavaScript(a programminglanguageunrelatedtoJava,whichwasinventedatNetscape,thepredecessortothe MozillaFoundationthatmaintainsnumeroussoftwarepackages,includingthe Firefox web browser.)Now,doyouthinkthatheactuallylearnedtocode?Here’sahint:ifyoudo,youshould probablyworkonhoningyourcriticalthinkingskillsinadditiontoreadingthisbook.Sure,hemay havelearnedateensybitaboutprogramming,but no, he didn’t learn to code.Ifhecouldlearnto codeinanhour,thenitfollowsthatcodingissotrivialthattherewouldn’tbeaneedtoteachitin schools.
IMPORTANCE OF LOW-LEVEL KNOWLEDGE Aninterestingandsomewhatcontraryviewabouthowtoteachprogrammingwasexpressedina blogposttitled“HowtoTeachComputationalThinking”byStephenWolfram,thecreatorof MathematicaandtheWolframlanguage.Wolframdefinescomputationalthinkingas“formulating thingswithenoughclarity,andinasystematicenoughway,thatonecantellacomputerhowtodo them.”Icompletelyagreewiththisdefinition.Infact,it’sinlargepartmymotivationforwriting thisbook.
ButIstronglydisagreewithWolfram’spositionthatthoselearningtoprogramshoulddevelop computationalthinkingskillsusingpowerfulhigh-leveltools,suchasthosethathe’sdeveloped, insteadoflearningtheunderlyingfoundationaltechnologies.Forexample,it’sclearfromtherising interestinstatisticsovercalculusthat“datawrangling”isagrowingfield.Butwhathappenswhen peoplejustfeedgiantpilesofdataintofancyprogramsthatthosesamepeopledon’tintimately understand?
Onepossibilityisthattheygenerateinteresting-lookingbutmeaninglessorincorrectresults.For example,arecentstudy(“GeneNameErrorsAreWidespreadintheScientificLiterature”byMark Ziemann,YotamEren,andAssamEl-Osta)showedthatone-fifthofpublishedgeneticspapershave errorsduetoimproperspreadsheetusage.Justthinkofthekindsoferrorsandramificationsthat morepowerfultoolsinthehandsofmorepeoplecouldproduce!Gettingitrightiscrucialwhen people’slivesareaffected.
Understandingunderlyingtechnologieshelpsyoudevelopasenseofwhatcangowrong.Knowing justhigh-leveltoolsmakesiteasytoaskthewrongquestions.It’sworthlearningtouseahammer beforegraduatingtoanailgun.Anotherreasonforlearningunderlyingsystemsandtoolsisthatit givesyouthepowertobuildnewtools,whichisimportantbecausetherewillalwaysbeaneedfor toolbuilders,eveniftoolusersaremorecommon.Learningaboutcomputerssothatthebehavior ofprogramsisn’tamysteryenablesyoutocraftbettercode.
WHO SHOULD READ THIS BOOK? Thisbookisforpeoplewhowanttobecomegoodprogrammers.Whatmakesagoodprogrammer? Firstandforemost,agoodprogrammerhasgoodcriticalthinkingandanalysisskills.Tosolve complexproblems,aprogrammerneedstheabilitytoevaluatewhetherornotprogramsactually solvetherightproblemcorrectly.Thisismoredifficultthanitsounds.It’snotuncommonforan experiencedprogrammertolookatsomeoneelse’sprogramandsnarkilycomment,“Why,that’sa complexnonsolutiontoasimplenonproblem.”
Youmaybefamiliarwithaclassicfantasytropeofwizardsacquiringpoweroverthingsbylearning theirtruenames.Andwoebetothewizardwhoforgetsadetail.Goodprogrammersarelikethese wizardswhocanholdtheessenceofthingsintheirmindswithoutdroppingdetails. Goodprogrammersalsohavesomedegreeofartistry,likeskilledcraftspeople.It’snotuncommon tofindcodethatiscompletelyincomprehensible,justlikemanyEnglishspeakersarebaffledby JamesJoyce’snovel Finnegans Wake.Goodprogrammerswritecodethatnotonlyworksbutisalso easyforotherstounderstandandmaintain. Finally,goodprogrammersneedadeepunderstandingofhowcomputerswork.Youcan’tsolve complexproblemswellusingashallowbaseofknowledge.Thisbookisforpeoplewhoarelearning programmingbutareunsatisfiedwiththelackofdepth.It’salsoforpeoplewhoarealready programmingbutwantmore.
WHAT ARE COMPUTERS? Acommonansweristhatcomputersareappliancesthatpeopleusefortaskssuchaschecking email,shoppingonline,writingpapers,organizingphotos,andplayinggames.Thisdefinitionis partlytheresultofsloppyterminologythatbecamecommonplaceasconsumerproductsbeganto incorporatecomputers.Anothercommonansweristhatcomputersarethebrainsthatmakeour high-techtoys,suchascellphonesandmusicplayers,work.Thisisclosertothemark. Sendingemailandplayinggamesaremadepossiblebyprogramsrunningoncomputers.The computeritselfislikeanewbornbaby.Itdoesn’treallyknowhowtodomuch.Wehardlyeverthink aboutthebasicmachineryofhumanbeings,becausewemostlyinteractwiththepersonalitiesthat arerunningonthatbasicmachinery,justlikeprogramsrunningoncomputers.Forexample,when you’reonawebpage,you’renotreadingitusingjustthecomputeritself;you’rereadingitusing programssomeoneelsewrotethatarerunningonyourcomputer,thecomputerhostingtheweb page,andallofthecomputersinbetweenthatmaketheinternetfunction.
WHAT IS COMPUTER PROGRAMMING? Teachersarepeoplewhotrainthebasichumanmachinerytoperformcertaintasks.Similarly, programmingisaboutbecomingateacherofcomputers.Programmersteachcomputerstodowhat theprogrammerswantthemtodo.
Knowinghowtoteachcomputersisuseful,especiallywhenyouwantacomputertodosomething thatitdoesn’tknowhowtodoandyoucan’tjustgobuyaprogramforitbecausenobodyhas createdoneyet.Forexample,youprobablytaketheWorldWideWebforgranted,butitwas inventednotlongago,whenSirTimBerners-LeeneededabetterwayforscientistsattheEuropean OrganizationforNuclearResearch(ConseilEuropéenpourlaRechercheNucléaire,orCERN)to shareinformation.Andhegotknightedforit.Howcoolisthat?
Teachingcomputersiscomplicated,butit’seasierthanteachingpeople.Weknowalotmoreabout howcomputerswork.Andcomputersarealotlesslikelytothrowuponyou.
Computerprogrammingisatwo-stepprocess:
1. Understandtheuniverse.
2. Explainittoathree-year-old.
Whatdoesthismean?Well,youcan’twritecomputerprogramstodothingsthatyouyourselfdon’t understand.Forexample,youcan’twriteaspellcheckerifyoudon’tknowtherulesforspelling,and youcan’twriteagoodactionvideogameifyoudon’tknowphysics.So,thefirststepinbecominga goodcomputerprogrammeristolearnasmuchasyoucanabouteverythingelse.Solutionsto problemsoftencomefromunexpectedplaces,sodon’tignoresomethingjustbecauseitdoesn’t seemimmediatelyrelevant.
Thesecondstepoftheprocessrequiresexplainingwhatyouknowtoamachinethathasavery rigidviewoftheworld,likeyoungchildrendo.Thisrigidityinchildrenisreallyobviouswhen they’reaboutthreeyearsold.Let’ssayyou’retryingtogetoutthedoor.Youaskyourchild,“Where areyourshoes?”Theresponse:“There.”She did answeryourquestion.Theproblemis,shedoesn’t understandthatyou’rereallyaskinghertoputhershoesonsothatyoubothcangosomewhere. Flexibilityandtheabilitytomakeinferencesareskillsthatchildrenlearnastheygrowup.But computersarelikePeterPan:theynevergrowup.
Computersarealsolikeyoungchildreninthattheydon’tknowhowtogeneralize.They’restill usefulbecauseonceyoufigureouthowtoexplainsomethingtothem,they’reveryfastandtireless aboutdoingit,thoughtheydon’thaveanycommonsense.Acomputerwilltirelesslydowhatyou askwithoutevaluatingwhetherit’sthewrongtask,muchliketheenchantedbroomsticksin“The
Sorcerer’sApprentice”segmentofthe1940movie Fantasia.Askingacomputertodosomethingis likeaskingthegeniefromamagiclantern(nottheFBIversion)tograntawish.Youhavetobe reallycarefulhowyouphraseit!
YoumaydoubtwhatI’msayingherebecausecomputersseemmorecapablethantheyare.When youuseacomputer,forexample,itknowshowtodrawpictures,correctyourspelling,understand whatyou’resaying,playmusic,andsoon.Butkeepinmind,that’snotthecomputer it’sa complicatedsetofcomputerprogramsthatsomeoneelsewrotethatallowsthecomputertodoall ofthosetasks.Computersareseparatefromtheprogramsthatrunonthem.
It’slikewatchingacarontheroad.Itseemsprettygoodatstoppingandstartingattherighttimes, avoidingobstacles,gettingwhereit’sgoing,eatingwhenitgetshungry,andsoon.Butit’snotjust thecar.It’sthecarandthedriverpackagedtogether.Computersarelikethecars,andprogramsare likethedrivers.Withoutknowledge,youcan’ttellwhat’sdonebythecarandwhat’sdonebythe driver.(See“SouthboundontheFreeway”byMaySwenson.Youmightchangeyouranswertothe questionposedattheendofthepoemduringyourlifetime.)
Insum,computerprogramminginvolveslearningwhatyouneedtoknowtosolveaproblemand thenexplainingittoayoungchild.Becausetherearelotsofwaystosolveaproblem,programming isjustasmuchanartasitisascience.Itinvolvesfindingelegantsolutionsasopposedto usingbruteforce.Yes,you can getoutofahousebybashingaholeinthewall,butit’sprobablyalot easiertogooutthedoor.Manycanwritesomethinglike HealthCare.gov inmillionsoflinesofcode, butittakesskilltodoitinthousandsoflines.
Beforeyoucaninstructathree-year-old,though,youneedtolearnaboutthree-year-oldsandwhat theyunderstand.Andthisisn’tanyordinarythree-year-old it’sanalienlifeform.Acomputer doesn’tplaybythesamerulesthatwedo.Youmayhaveheardofartificialintelligence(AI),which triestogetcomputerstoactmorelikepeople.Progressinthatfieldhasmovedmuchslowerthan originallyanticipated.That’smainlybecausewedon’treallyunderstandtheproblem;wedon’t knowenoughabouthowhumansthink.Asyoucanimagine,it’sprettyhardtoteachanaliento thinkaswedowhenweourselvesdon’tknowhowexactlyit’sdone.
Thehumanbrainletsyoudothingswithoutconsciouslythinkingaboutthem.Yourbrainstarted outasjustapieceofhardware,whichthengotprogrammed.Forexample,youlearnedtomove yourfingersandthenyoulearnedtograbthings.Afterpractice,youcanjustgrabthingswithout thinkingaboutallthestepsthatmakeitpossible.PhilosopherssuchasJeanPiaget(French psychologist,1898–1980)andNoamChomsky(Americanlinguistbornin1928)havedeveloped differenttheoriesabouthowthisprocessoflearningworks.Isthebrainjustageneralpieceof equipment,ordoesithavespecialhardwareforfunctionslikelanguage?Thisissueisstillbeing studied.
Ourincredibleabilitytoperformtasksunconsciouslymakeslearninghowtoprogramdifficult, becauseprogrammingrequiresbreakingdowntasksintosmallerstepsthatacomputercanfollow. Forexample,youprobablyknowhowtoplaytic-tac-toe.Getagroupofpeopletogetherandhave eachofyouindependentlylistthestepsaplayershouldtakeinordertomakeagoodmoveforany configurationoftheboard.(I’msureyoucanfindthisonline,butdon’tlookitup.)Aftereverybody hasmadetheirlists,holdaplayoff.Findoutwhoserulesrule!Howgoodwereyourrules?What sortofthingsdidyoumiss?Doyouactuallyknowwhatyou’redoingwhenyouplaythegame? Chancesare,therewereanumberoffactorsthatyoudidn’tthinktospelloutbecauseyou understandthemintuitively.
Incaseit’snotobvious,thefirststep,understandingtheuniverse,ismuchmoreimportantthanthe second,explainingittoathree-year-old.Thinkaboutit:whatgoodisittoknowhowtotalkifyou don’tknowwhattosay?Despitethis,currenteducationfocusesonthesecondstep.Thisisbecause it’smucheasiertoteachandgradethemechanicalaspectsofthetaskthanthecreativeelements.
Andingeneral,teachershavelittletraininginthefieldandareworkingfromcurriculadeveloped elsewherethattheyweregiventouse.Thisbook,however,focusesonthefirststep.Whileitcan’t covertheuniverseingeneral,itexaminesproblemsandtheirsolutionsinthecomputeruniverse insteadofdwellingontheexactprogrammingsyntaxneededtoimplementthosesolutions.
CODING, PROGRAMMING, ENGINEERING, AND COMPUTER SCIENCE Anumberofdifferenttermsareusedtodescribeworkingwithsoftware.Thesetermshavenoexact definitions,althoughtheyhaveacquiredsomeroughmeanings.
Coding,afairlyrecenttermpopularizedaspartof“learningtocode,”canbeviewedasthe somewhatmechanicalworkoftranslation.Let’scompareittothejobofmedicalcoding.Whenyou visitadoctor,gettingadiagnosisistheeasypart.Thehardpartistranslatingthatdiagnosisinto oneoftheover100,000codesintheICDstandards,ICD-10atthetimeofwriting.ACertified ProfessionalCoderwhohaslearnedthesecodesknowsthat,whenadoctorcomesupwitha diagnosisof“struckbycow,”itshouldbeassignedcodeW55.2XA.Thisisactuallyharderthanmany codingjobsintheprogrammingspaceduetothesheernumberofcodes.Buttheprocessissimilar towhatacoderwoulddoifdirectedto“makethattextbold”onawebpage;acoderknowswhich codetousetomakethathappen.
TheICD-10standardissocomplicatedthatfewcodersknowitall.Instead,medicalcodersget certifiedinspecialtyareassuchas“Diseasesofthenervoussystem”or“Mentalandbehavioral disorders.”ThisisanalogoustoacoderbeingproficientinalanguagesuchasHTMLorJavaScript. But programming thatis,beingaprogrammer meansknowingmorethanaspecialtyareaor two.Thedoctorinthisscenarioisanalogoustoaprogrammer.Thedoctordeterminesadiagnosis byevaluatingthepatient.Thiscanbeprettycomplex.Forexample,ifapatienthasburnsandis soakingwet,isita“bizarrepersonalappearance”(R46.1)ora“burnduetowaterskisonfire,initial encounter”(V91.07XA)?Oncethedoctorhasadiagnosis,atreatmentplancanbedevised.The treatmentplanmustbeeffective;thedoctorprobablydoesn’twanttoseethesamepatient sufferingfromabadcaseof“parentaloverprotection”(Z62.1).
Justlikethedoctor,aprogrammerevaluatesaproblemanddeterminesasolution.Forexample, maybethere’saneedforawebsitethatallowspeopletorankICD-10codesintermsofsilliness.A programmerwoulddeterminethebestalgorithmsforstoringandmanipulatingthedata,the structureofthecommunicationbetweenthewebclientandserver,theuserinterface,andsoon. It’snotasimple“pluginthecode”sortofthing.
Engineering isthenextstepupincomplexity.Ingeneral,engineeringistheartoftakingknowledge andusingittoaccomplishsomething.YoucouldconsiderthecreationoftheICDstandardstobe engineering;ittookthelargefieldofmedicaldiagnosesandreducedthemtoasetofcodesthat couldbemoreeasilytrackedandanalyzedthandoctor’snotes.It’samatterofopinionasto whetherornotsuchacomplexsystemrepresents good engineering.Asanexampleofcomputer engineering,manyyearsagoIworkedonaprojecttobuildalow-costmedicalmonitorsuchas thosethatyouseeinhospitals.ThechargeIwasgivenwastomakeasystemthatadoctorornurse couldfigureouthowtouseinlessthan5minuteswithoutanydocumentation.Asyoumight imagine,thisrequiredmuchmorethanjustknowledgeofprogramming.AndIbeatthegoal my solutionendeduptakingabout30secondstolearntouse.
Programmingisoftenconfusedwithcomputerscience.Whilemanycomputerscientistsprogram, mostprogrammersaren’tcomputerscientists. Computer science isthestudyofcomputing. Computersciencediscoveriesareusedbyengineersandprogrammers.
Coding,programming,engineering,andcomputerscienceareindependentbutrelateddisciplines thatdifferinthetypeandamountofknowledgerequired.Beingacomputerscientist,engineer,or coderdoesn’tautomaticallymakesomeoneagoodprogrammer.Whilethisbookgivesyouataste ofhowengineersandcomputerscientiststhink,it’snotgoingtomakeyouone;thattypically requiresacollegeeducationcombinedwithsomehard-earnedrelevantexperience.Engineering andprogrammingaresimilartomusicorpainting they’repartskillandpartart.Theexpositionof bothaspectsinthisbookshouldhelpyoutoimproveyourskillsasaprogrammer.
THE LANDSCAPE Computerdesignandprogrammingisahugefieldofstudy,whichIwon’tbeabletocoverhere.You canvisualizeitinlayers,asshowninFigure1.
KeepinmindthatFigure1isasimplificationandthatthelinesdividingthevariouslayersarenot ascleaninreality.
Themajorityofpeopleare users ofcomputersystems.You’reprobablyinthatcamprightnow. Therearespecializedsortsofuserscalled system administrators whosejobistokeepcomputer systemsworking.Theyinstallsoftware,manageuseraccounts,dobackups,andsoon.They typicallyhavespecialpowersnotgrantedtonormalusers.
Thepeoplewhowriteprogramslikewebpages,phoneapps,andmusicplayersare called application programmers.Theywritethesoftwarethatusersusetointeractwiththe computers,usingblocksthatothershavecreated.Applicationprogrammingisbeingtaughtinmost “learntocode”classesasifallprogrammershavetolearnishowtoimporttheseotherblocksand gluethemtogether.Althoughyoucangetawaywiththatalotofthetime,it’smuchbetterto actuallyunderstandboththoseblocksandtheglue.
Figure 1: Computer landscape
Applicationprogramsdon’ttalktothecomputerhardwaredirectly;that’swhere system programming comesintoplay.Systemprogrammersmakethebuildingblocksusedbyapplication programmers.Systemprogrammersneedtoknowabouthardwarebecausetheircodeinteracts withit.Oneofthegoalsofthisbookistoteachyouthingsyouneedtoknowinordertobeagood systemprogrammer.
Computerhardwareincludesnotonlythepartthatdoestheactualcomputingbutalsohowthat partconnectstotheworldoutside.Computerhardwareisexpressedas logic.It’sthesamelogic usedtowritecomputerprograms,andit’skeytounderstandingtheworkingsofthecomputer.The logicisconstructedfromvarioustypesofelectronic circuits.Circuitdesignisbeyondthescopeof thisbook,butyoucanlearnmoreaboutitbystudyingelectricalengineering.Consideradouble majorinelectricalengineeringandcomputerscienceifyouwanttoruletheworld.
Ofcourse,basicscienceunderpinsitall,providingeverythingfromourunderstandingofelectricity tothechemistryneededtocreatechips.
AsshowninFigure1,eachlevelbuildsontheonebeneathit.Thismeansthatpoordesignchoices orerrorsatlowerlevelsaffecteverythingabove.Forexample,adesignerrorinIntelPentium processorscirca1994causedsomedivisionoperationstoproduceincorrectresults.Thisaffected allsoftwarethatusedfloating-pointdivisionintheseprocessors.
Asyoucansee,systemprogrammingisatthebottomofthesoftwarehierarchy.It’ssimilarto infrastructure,likeroads,electricity,andwater.Beingagoodprogrammeralwaysmatters,butit mattersmoreifyou’reasystemprogrammer,becauseothersrelyonyourinfrastructure.Youcan alsoseethatsystemprogrammingissandwichedbetweenapplicationprogrammingandcomputer hardware,whichmeansyouneedtolearnsomethingaboutbothofthose.TheSanskrit word yoga translatesto“union,”andjustasyogapractitionersseektounifytheirmindandbody, systemprogrammersaretechno-yogiswhounifythehardwareandsoftware.
Youdon’thavetolearnsystemprogramminginordertoworkatoneoftheotherlevels.Butifyou don’t,you’llhavetofindsomeoneelsetohelpyoudealwithissuesoutofyourdomainratherthan beingabletofigurethemoutforyourself.Anunderstandingofthecoretechnologyalsoleadsto bettersolutionsathigherlevels.Thisisn’tjustmyopinion;checkoutthe2014blogpost“The ResourceLeakBugofOurCivilization”byVille-MatiasHeikkiläforasimilarview.
Thisbookalsoaimstocoveralotofretrohistory.Mostprogrammersaren’tlearningthehistoryof theircraftbecausethereissomuchmaterialtocover.Theresultisthatalotofpeoplearemaking mistakesthathavealreadybeenmade.Knowingsomeofthehistoryallowsyoutoatleastmake newandbettermistakesratherthanrepeatpastones.Bearinmindthatthehotnewtechnology you’reusingtodaywillquicklybecomeretrotomorrow.
Speakingofhistory,thisbookisjam-packedwithinterestingtechnologiesandthenamesoftheir inventors.Takesometimetolearnmoreaboutboththetechnologiesandthepeople.Mostofthe peoplementionedsolvedatleastoneinterestingproblem,andit’sworthlearningabouthowthey perceivedtheirworldandthewayinwhichtheyapproachedandsolvedproblems.There’sagreat exchangeinNealStephenson’s2008novel Anathem:
“Ouropponentisanalienstarshippackedwithatomicbombs.Wehaveaprotractor.”
“Okay,I’llgohomeandseeifIcanscroungeuparulerandapieceofstring.”
Notetherelianceonfundamentals.It’snot“Let’slookupwhattodoonWikipedia”or“I’llposta questiononStackOverflow”or“I’llfindsomepackageonGitHub.”Learningtosolveproblemsthat nobodyelsehassolvedisacrucialskill.
Manyoftheexamplesinthisbookarebasedonoldtechnologysuchas16-bitcomputers.That’s becauseyoucanlearnalmosteverythingyouneedtoknowfromthemandthey’reeasiertofitona page.
WHAT’S IN THIS BOOK Thebookisconceptuallydividedintothreeparts.Thefirstpartexplorescomputerhardware,both whatitisandhowit’sbuilt.Thesecondpartexaminesthebehaviorofsoftwarerunningon hardware.Thelastpartcoverstheartofprogramming workingwithotherstoproducegood programs.
Chapter 1: The Internal Language of Computers
Thischapterstartsexploringthethree-year-oldmentality.Computersarebitplayers;theyherd bitsforaliving.You’lllearnwhattheyareandwhatcanbedonewiththem.We’llplaymake-believe toascribemeaningtobitsandtocollectionsofbits.
Chapter 2: Combinatorial Logic
Thischapterexaminestherationaleforusingbitsinsteadofdigitsandexploresthejustificationfor digitalcomputers.Thisincludesadiscussionofsomeoftheoldertechnologiesthatpavedtheway forwhatwehavetoday.Itcoversthebasicsofcombinatoriallogic.You’lllearnhowtobuildmore complicatedfunctionalityfrombitsandlogic.
Chapter 3: Sequential Logic
Hereyou’lllearnhowtouselogictobuildmemory.Thisincludeslearninghowtogeneratetime, becausememoryisnothingbutstatethatpersistsovertime.Thischaptercoversthebasicsof sequentiallogicanddiscussesvariousmemorytechnologies.
Chapter 4: Computer Anatomy
Thischaptershowshowcomputersareconstructedfromthelogicandmemoryelementsdiscussed intheearlierchapters.Anumberofdifferentimplementationmethodologiesareexamined.
Chapter 5: Computer Architecture
Inthischapter,we’llexploresomeoftheadd-onstothebasiccomputerthatwesawinChapter4. You’lllearnhowtheyprovideessentialfunctionalityandefficiency.
Chapter 6: Communications Breakdown
Computersneedtointeractwiththeoutsideworld.Thischaptercoversinputandoutput.Italso revisitsthedifferencebetweendigitalandanalogandhowwegetdigitalcomputerstoworkinan analogworld.
Chapter 7: Organizing Data
Nowthatyou’veseenhowcomputerswork,we’lllookathowtousethemeffectively.Computer programsmanipulatedatainmemory,andit’simportanttomapthewaymemoryisusedtothe problembeingsolved.
Chapter 8: Language Processing
Languageshavebeeninventedtomakeprogrammingcomputerseasierforpeople.Thischapter looksattheprocessofconvertinglanguagesintosomethingthatactuallyrunsoncomputers.
Chapter 9: The Web Browser
Alotofprogrammingisdoneforwebbrowsers.Thischapterlooksathowawebbrowserworks andteasesoutitsmaincomponents.
Chapter 10: Application and System Programming
Inthischapter,we’llwritetwoversionsofaprogramthatrunsattwoofthedifferentlevels fromFigure1.Thechapterexposesmanyofthedifferencesbetweenapplication-andsystem-level programming.
Chapter 11: Shortcuts and Approximations Makingprogramsefficientisimportant.Thischapterexploressomeofthewaysinwhichwecan makeprogramsmoreefficientbyhavingthemavoidunnecessarywork.
Chapter 12: Deadlocks and Race Conditions
Manysystemsincludemorethanonecomputer.Thischapterexaminessomeoftheproblemsthat canoccurwhenwe’retryingtogetcomputerstocooperate.
Chapter 13: Security
Computersecurityisanadvancedtopic.Thischaptercoversthebasicswhilepuntingontheheavy math.
Chapter 14: Machine Intelligence
Thischapteralsocoversanadvancedtopic.Newapplicationsresultfromthecombinationofbig data,artificialintelligence,andmachinelearning fromdrivingyourcartodrivingyounutswith advertisements.
Chapter 15: Real-World
Considerations Programmingisaverymethodicalandlogicalprocess.Buthumansareinvolvedinthe determinationofwhatandhowtoprogram,andhumansareoftenlackinginlogic.Thischapter discussessomeoftheissuesofprogrammingintherealworld.
Whenreadingthisbook,bearinmindthatmanyoftheexplanationsaresimplifiedandtherefore notcorrectdowntothesmallestdetail.Makingtheexplanationsperfectwouldrequiretoomuch distractingdetail.Don’tbesurprisedifyoudiscoverthisasyoulearnmore.Youcanconsiderthis booktobeaglossytravelbrochureforatriptocomputer-land.Itcan’tcovereverythingindetail, andwhenyougovisit,you’llfindplentyofsubtledifferences.
1 THEINTERNALLANGUAGEOFCOMPUTERS Thewholepointoflanguageistobeabletocommunicateinformation.Yourjobasaprogrammeris togiveinstructionstocomputers.Theydon’tunderstandourlanguage,soyouhavetolearntheirs. Humanlanguageistheproductofthousandsofyearsofevolution.Wedon’tknowalotabouthowit evolved,sinceearlylanguagedevelopmentwasn’tadvancedtothepointthathistorycouldbe recorded.(Apparentlynobodywroteballadsaboutlanguagedevelopment.)Computerlanguages areadifferentstory,asthey’reafairlyrecentinventionthatoccurredlongafterthedevelopmentof humanlanguage,whichenablesustowriteaboutthem.
Humanandcomputerlanguagessharemanyofthesameelements,suchaswrittensymbolsand rulesfortheirproperarrangementandusage.Onethingthattheydon’tshareisnonwritten languageforms;computershavewrittenlanguageonly.
Inthischapter,you’llstarttolearnthelanguageofcomputers.Thisprocesshappensinstagesjust likewithhumanlanguage.Wehavetostartwithlettersbeforebuildinguptowordsandsentences. Fortunately,computerlanguagesaremuchsimplerthantheirhumancounterparts.
WHAT IS LANGUAGE? Languageisaconvenientshortcut.Itallowsustocommunicatecomplexconceptswithouthavingto demonstratethem.Italsoallowsconceptstobeconveyedatadistance,evenviaintermediaries. Everylanguage whetherwritten,spoken,orexpressedinaseriesofgesturesorbybangingtwo rockstogether ismeaning encoded asasetofsymbols.Encodingmeaningassymbolsisn’tenough, though.Languageonlyworksifallcommunicatingpartieshavethesame context,sotheycanassign thesamemeaningtothesamesymbols.Forexample,theword Toto mightsuggestthedogfrom The Wizard of Oz tomanypeople,whileothersmightthinkoftheJapanesemanufacturerofheated toiletseats.IrecentlyencounteredmuchconfusionwhilediscussingclothingwithmyFrench exchangestudent.Itturnsoutthatthecommoninterpretationoftheword camisole inAmericais undershirt,butinFranceit’sstraitjacket!Inbothoftheseexamples,thesamesymbolscanbe distinguishedonlybycontext,andthatcontextisnotalwaysreadilydiscernible.Computer languageshavethisissuetoo.
WRITTEN LANGUAGE Writtenlanguageisasequenceofsymbols.Weformwordsbyplacingsymbolsinaparticularorder. Forexample,inEnglishwecanformtheword yum byplacingthreesymbols(thatis,letters)in orderfromlefttorightasfollows: y u m.
Therearemanypossiblesymbolsandcombinations.Thereare26basicsymbols(A–Z)inEnglish ifweignorethingslikeupper-andlowercase,punctuation,ligatures,andsoon whichnative Englishspeakerslearnastoddlers.Otherlanguageshavedifferenttypesandnumbersofsymbols. Somelanguages,suchasChineseandJapaneseaswritteninkanji,haveaverylargenumberof symbolswhereeachsymbolisaworduntoitself.
Languagesalsousedifferentordering,suchasreadingrighttoleftinHebrewandverticallyin Chinese.Symbolorderisimportant: d o g isnotthesameas g o d
Althoughstylecaninsomewaysbeconsideredalanguageuntoitself,wedon’tdistinguishsymbols basedontypeface:a, a,and a areallthesamesymbol.
Threecomponentsframethetechnologyofwrittenlanguage,includingcomputerlanguage:
• Thecontainersthatholdsymbols
• Thesymbolsthatareallowedinthecontainers
• Theorderingofthecontainers
Somelanguagesincludemorecomplicatedrulesthatconstrainthepermittedsymbolsincontainers basedonthesymbolsinothercontainers.Forexample,somesymbolscan’toccupyadjacent containers.
THE BIT We’llbeginwiththecontainer.Thismightbecalleda character inahumanlanguageanda bit for computers.Theterm bit isanawkwardmarriagebetween binary and digit.It’sawkward because binary isawordforsomethingwithtwoparts,whereas digit isawordforoneofthe10 symbols(0–9)thatmakeupoureverydaynumbersystem.You’lllearnwhyweusebitsinthenext chapter;fornow,it’senoughtoknowthatthey’recheapandeasytobuild.
Abitisbinary,whichmeansabitcontainercanholdonlyoneoftwosymbols,kindoflikethedot anddashfromMorsecode.Morsecodeusesjusttwosymbolstorepresentcomplexinformationby stringingthosesymbolstogetherindifferentcombinations.Theletter A isdot-dash,for example. B isdash-dot-dot-dot, C isdash-dot-dash-dot,andsoon.Theorderofthesymbolsis importantjustlikeinahumanlanguage:dash-dotmeans N,not A.
Theconceptofsymbolsisabstract.Itreallydoesn’tmatterwhattheystandfor;theycouldbeoff andon,dayandnight,orduckandgoose.Butremember,languagedoesn’tworkwithoutcontext. Thingswouldgetweirdfastifasenderthoughttheyweresaying U (dot-dot-dash),buttherecipient heard duck-duck-goose.
Intheremainderofthischapter,you’lllearnaboutsomeofthecommonwaysinwhichmeaningis assignedtobitsforcomputing.Keepinmindthatthereisalotofmake-believeinvolved for example,youmayrunintothingslike,“Let’spretendthatthisbitmeansblue.”Programming actuallyworksthatway,soeventhoughyou’llbelearningsomestandardbituses,don’tbeafraidto inventyourownwhenappropriate.
LOGIC OPERATIONS Oneuseofbitsistorepresenttheanswerstoyes/noquestionssuchas“Isitcold?”or“Doyoulike myhat?”Weusetheterms true foryesand false forno.Questionslike“Where’sthedogparty?” don’thaveayes/noanswerandcan’tberepresentedbyasinglebit.
Inhumanlanguage,weoftencombineseveralyes/noclausesintoasinglesentence.Wemightsay, “Wearacoatifitiscoldorifitisraining”or“Goskiingifitissnowingandit’snotaschoolday.”
Anotherwayofsayingthosethingsmightbe“Wearcoatistrueifcoldistrueorrainingistrue”and
“Skiingistrueifsnowingistrueandschooldayisnottrue.”Theseare logic operationsthateach produceanewbitbasedonthecontentsofotherbits.
BooleanAlgebra Justasalgebraisasetofrulesforoperatingonnumbers, Boolean algebra,inventedinthe1800sby EnglishmathematicianGeorgeBoole,isasetofrulesthatweusetooperateonbits.Aswithregular algebra,theassociative,commutative,anddistributiverulesalsoapply.
TherearethreebasicBooleanoperations, NOT, AND,and OR,aswellasonecomposite operation, XOR (shortfor“exclusive-or”),asdescribedhere:
NOT Thisoperationmeans“theopposite.”Forexample,ifabitisfalse, NOT thatbitwouldbetrue. Ifabitistrue, NOT thatbitwouldbefalse.
AND Thisoperationinvolves2ormorebits.Ina2-bitoperation,theresultistrueonlyifboththe first AND secondbitaretrue.Whenmorethan2bitsareinvolved,theresultistrueonlyif allbits aretrue.
OR Thisoperationalsoinvolves2ormorebits.Ina2-bitoperation,theresultistrueifthe first OR secondbitistrue;otherwise,theresultisfalse.Withmorethan2bits,theresultistrueif anybitistrue.
XOR Theresultofan exclusive-or operationistrueifthefirstandsecondbitshavedifferentvalues. It’seitherbutnotboth.Because“exclusive-or”isamouthful,weoftenusetheabbreviationXOR (pronounced“ex-or”).
Figure1-1summarizestheseBooleanoperationsgraphicallyinwhatareknownas truth tables. The inputs areoutsideoftheboxesandthe outputs areinside.Inthesetables,TstandsforTrueand FstandsforFalse.
Figure 1-1: Truth tables for Boolean operations
Figure1-2showshowthisworksfortheNOTandANDoperations.Wecanfindtheoutputby tracingapathfromtheinputorinputs.
Figure 1-2: Using truth tables
Asyoucansee,theNOToperationsimplyreversesthestateoftheinput.Ontheotherhand,the ANDoperationreturnstrueonlywhenbothinputsaretrue.
NOTE
The XOR operation is built from other operations. For example, the XOR of 2 bits,a and b, is the same thing as (a OR b) AND NOT (a AND b). This shows that basic Boolean operations can be combined in different ways to yield the same result.
DeMorgan’sLaw Inthe1800s,BritishmathematicianAugustusDeMorganaddedalawthatappliesonlytoBoolean algebra,theeponymous De Morgan’s law.Thislawstatesthattheoperation a AND b isequivalentto theoperation NOT(NOT a OR NOT b),asshowninFigure1-3.
Figure 1-3: The truth table for De Morgan’s law
Noticethattheresultsof a AND b inthesecondcolumnareidenticaltotheresultslistedinthe final NOT(NOT a OR NOT b)column.ThismeansthatwithenoughNOToperations,wecanreplace ANDoperationswithORoperations(andviceversa).Thisisusefulbecausecomputersoperateon real-worldinputthat’snotundertheircontrol.Whileitwouldbeniceifinputswereofthe form cold or raining,they’reoften NOT cold or NOT raining.Similartodoublenegativesinlanguages suchasEnglish(“Wedidn’tnotgoskiing”),DeMorgan’slawisatoolthatletsusoperateon these negative logic propositionsinadditiontothe positive logic thatwe’vealreadyseen.Figure14illustratesthecoat-wearingdecisionforbothpositiveandnegativelogicforms.
Figure 1-4: Positive and negative logic
Ontheleft(positivelogic)side,wecanmakeourdecisionusingasingleORoperation.Ontheright (negativelogic)side,DeMorgan’slawallowsustomakeourdecisionusingasingleANDoperation. WithoutDeMorgan’slaw,we’dhavetoimplementthenegativelogiccaseas NOTnot-cold OR NOT not-raining.Althoughthatworks,thereisacostinpriceandperformancetoeachoperation,so minimizingoperationsminimizescosts.ThehardwarethatperformstheNOToperationcostsreal moneyand,asyou’lllearninthenextchapter,cascadingoperationsslowsthingsdown. DeMorgantellsusthatthisisequivalentto“coldandraining,”whichismuchsimpler.
REPRESENTING INTEGERS USING BITS Let’smoveupthefoodchainandlearnhowtousebitstorepresentnumbers.Numbersaremore complicatedthanlogicbutmuchsimplerthanwords.
RepresentingPositiveNumbers Wecommonlyusethe decimal numbersystembecauseitcorrespondstoouranatomy.Ten differentsymbolscalled digits cangointothecontainers:0123456789.Containersarestackedright toleft.Eachcontainerhasanamethatisseparatefromitscontents;wecalltherightmostcontainer theones,nextisthetens,thenthehundreds,thousands,andsoon.Thesenamesarealiasesfor powersof10;100 isone,101 isten,102 isonehundred,103 isonethousand.Thissystemis called base-10 since10isthebaseholdinguptheexponents.Thevalueofanumberisderivedfrom thesumoftheproductofeachcontainervalueandthevalueofitscontents.Forexample,the number5,028isthesumof5thousands,0hundreds,2tens,and8ones,or5×103 +0×102 +2× 101 +8×100,asshowninFigure1-5
Figure 1-5: The number 5,028 in decimal notation
Wecanuseasimilarapproachtomakenumbersusingbits.Sincewe’reusingbitsinsteadofdigits, weonlyhavetwosymbols:0and1.Butthat’snotaproblem.Indecimal,weaddacontainer wheneverwerunoutofroom;wecanfita9inasinglecontainerbutneedtwocontainersfor10. Thatworksinbinarytoo;wejustneedanewcontainerforanythinggreaterthana1.Therightmost containerwouldstillbetheones,butwhat’sthenextone?It’sthe twos.Thevalueofacontainerin decimalwherethereare10symbolsis10timesthatoftheoneontheright.Thus,inbinarywhere therearetwosymbols,thecontainervalueistwotimesthatoftheoneontheright.That’sallthere istoit!Thecontainervaluesarepowersof2,whichmeansit’sa base-2 systeminsteadofbase-10.
Table1-1listssomeofthepowersof2.Wecanuseitasareferencetounderstandthebinary representationofthenumber5,028.
Table 1-1: Powersof2
Eachnumberinthefar-rightcolumnofTable1-1representsthevalueofabinarycontainer.Figure 1-6showshowthenumber5,028canbewritteninbinary,usingessentiallythesameprocessthat weusedearlierfordecimalnotation.
Figure 1-6: The number 5,028 in binary
Theresultoftheconversiontobinaryis:
Asyoucansee,thenumber5,028inbinaryhasone4,096(212),zero2,048s(211),zero1,024s(210), one512(29),one256(28),andsoontomakeup1001110100100.Performingthesamesortof calculationthatwedofordecimalnumbers,wewrite1×212 +0×2
Substitutingthedecimalnumbers fromTable1-1,weget4,096+512+256+128+32+4,whichisequalto5,028. Wewouldsaythat5,028isafour-digitnumberindecimal.Inbinaryit’sa13-bitnumber. Thenumberofdigitsdeterminestherangeofvaluesthatwecanrepresentindecimal.Forexample, 100differentvaluesintherange0–99canberepresentedbytwodigits.Likewise,thenumberof bitsdeterminestherangeofvalueswecanrepresentinbinary.Forexample,2bitscanrepresent fourvaluesintherange0–3.Table1-2summarizesboththenumberandrangeofvaluesthatwe canrepresentwithdifferentnumbersofbits.
Table 1-2: RangesofBinaryNumberValues Therightmostbitinabinarynumberiscalledthe least significant bit andtheleftmostbitiscalled the most significant bit,becausechangingthevalueoftherightmostbithasthesmallesteffecton thevalueofthenumberandchangingthevalueoftheleftmostbithasthegreatesteffect.Computer peoplearefondofthree-letteracronyms,orTLAsaswecallthem,sothesearecommonlyreferred toasthe LSB and MSB.Figure1-7showsanexampleofthenumber5,028heldin16bits.
You’llnoticethatwhilethebinaryrepresentationof5,028takes13bits,Figure1-7showsitin16 bits.Justlikeindecimal,wecanalwaysusemorecontainersthantheminimumrequiredby adding leading zeros ontheleft.Indecimal,05,028hasthesamevalueas5,028.Binarynumbersare oftenrepresentedthiswaybecausecomputersarebuiltaroundblocksofbits.
BinaryAddition Nowthatyouknowhowtorepresentnumbersusingbinary,let’slookathowtodosimple arithmeticwithbinarynumbers.Indecimaladdition,weaddupeachdigitfromright(least significantdigit)toleft(mostsignificantdigit),andiftheresultisgreaterthan9,wecarrythe1. Similarly,weaddtogethereachbitinabinarynumber,goingfromtheleastsignificanttothemost significantbit,andiftheresultisgreaterthan1,wecarrythe1. Additionisactuallyabiteasierinbinarybecausethereareonly4possiblecombinationsof2bits comparedto100combinationsof2digits.Forexample,Figure1-8showshowtoadd1and5using binarynumbers,showingthenumberbeingcarriedaboveeachcolumn.
Figure 1-7: MSB and LSB
Figure 1-8: Binary addition
Thenumber1is001inbinary,whilethenumber5is101because(1×4)+(0×2)+(1×1)=5.To addthebinarynumbers001and101together,westartwiththeleastsignificantbitinthe rightmostcolumn.Addingthebinarynumbers1and1inthatcolumngivesus2,butwedon’thave asymbolfor2inbinary.Butweknowthat2isactually10inbinary([1×2]+[0×1]=2),soweput 0asthesumandcarrythe1tothenextdigit.Becausethemiddlebitsarezeros,weonlyhave1, whichwecarriedoverfrombefore,asthesum.Thenweaddthedigitsintheleftmostcolumn:0 plus1issimply1inbinary.Thefinalresultisthebinary110,or6indecimalnotation,whichis whatyouwouldgetbyadding1and5.
Youmightnoticethattherulesforbinaryadditioncanbeexpressedintermsofthelogical operationsthatwediscussedpreviously,asFigure1-9illustrates.We’llseeinChapter2thatthisis infacthowcomputerhardwaredoesbinaryaddition.
Figure 1-9: Binary addition using logical operations
Whenweadd2bitstogether,thevalueoftheresultistheXORofthe2bits,andthevalueofthe carryistheANDofthe2bits.YoucanseethatthisistrueinFigure1-9,whereadding1and1in binaryresultsin10.Thismeansthatthecarryvalueis1,whichiswhatyougetbyperformingthe expression(1 AND 1).Likewise,theexpression(1 XOR 1)yields0,whichisthevaluethatweassign tothebitpositionitself.
Adding2bitsisanoperationthatrarelyhappensinisolation.ReferringbacktoFigure1-8,it appearsthatwe’readding2bitstogetherineachcolumn,butwe’rereallyadding3becauseofthe carry.Fortunately,wedon’tneedtolearnanythingnewtoadd3bitstogether(becauseA+B+Cis thesameas(A+B)+C,accordingtotheassociativerule),sowecanadd3bitstogetherusingapair of2-bitadds.
Whathappenswhentheresultofadditiondoesn’tfitinthenumberofbitsthatwehave?This resultsin overflow,whichhappenswheneverwehaveacarryfromthemostsignificantbit.For example,ifwehave4-bitnumbers,andadd1001(910)to1000(810),theresultshouldbe10001 (1710),butitwillendupbeing0001(110)becausethere’snoplaceforthemostsignificantbit.As we’llseeinmoredetaillater,computershavea condition code register,whichisaplacethatholds oddpiecesofinformation.Oneoftheseisan overflow bit,whichholdsthecarryvaluefromthemost significantbit.Wecanlookatthisvaluetodeterminewhetherornotoverflowoccurred.
You’reprobablyawarethatyoucansubtractonenumberfromanotherbyaddingthenegativeof thatnumber.We’lllearnhowtorepresentnegativenumbersinthenextsection.Borrowingbeyond themostsignificantbitiscalled underflow.Computershaveaconditioncodeforthistoo.
RepresentingNegativeNumbers Allofthenumberswerepresentedusingbinaryinthelastsectionwerepositive.Butlotsofrealworldproblemsinvolvebothpositiveandnegativenumbers.Let’sseehowwecanusebitsto representnegativenumbers.Forexample,let’sassumethatwehave4bitstoplaywith.Asyou learnedinthelastsection,4bitscanrepresent16numbersintherangeof0through15.Just becausewecanhold16numbersin4bitsdoesn’tmeanthatthosenumbershavetobe0through 15.Remember,languageworksthroughmeaningandcontext.Thatmeansthatwecandevisenew contextsinwhichwecaninterpretbits.
SignandMagnitude A sign iscommonlyusedtodistinguishnegativenumbersfrompositiveones.Thesignhastwo values,plusandminus,soitcanberepresentedusingabit.We’llarbitrarilyusetheleftmostbit (MSB)forthesign,leavingus3bitsthatcanrepresentanumberbetween0and7.Ifthesignbitis 0,wetreatthatnumberaspositive.Ifit’s1,wetreatitasnegative.Thisletsusrepresent15 differentpositiveandnegativenumbersintotal,not16,becausethereisbothapositive0anda negative0.Table1-3showshowthisallowsustorepresentthenumbersbetween–7and+7. Thisiscalled sign and magnitude representationbecausethere’sabitthatrepresentsasignand bitsthatrepresentthemagnitude,orhowfarthevalueisfromzero.
Signandmagnituderepresentationisnotusedmuchfortworeasons.First,bitscostmoneyto build,sowedon’twanttowastethembyhavingtwodifferentrepresentationsforzero;we’dmuch ratherusethatbitcombinationtorepresentanothernumber.Second,arithmeticusingXORand ANDdoesn’tworkusingthisrepresentation.
Table 1-3: SignandMagnitudeBinaryNumbers
Let’ssaythatwewanttoadd+1to–1.We’dexpecttoget0,butusingsignandmagnitude representationwegetadifferentresult,asshowninFigure1-10
Figure 1-10: Sign and magnitude addition
Asyoucansee,0001representspositive1inbinary,becauseitssignbitis0.The1001represents–1inbinary,becausethesignbitis1.AddingthesetogetherusingXORandANDarithmeticgivesus 1010.Thisevaluatesto–2indecimalnotation,whichisnotthesumof+1and–1. Wecouldmakesignandmagnitudearithmeticworkbyusingmorecomplicatedlogic,butthere’s valueinkeepingthingsassimpleaspossible.Let’sexploreafewotherwaysofrepresenting numberstofindabetterapproach.
One’sComplement Anotherwaytogetnegativenumbersistotakepositivenumbersandflipallthebits,whichis called one’s complement representation.Wepartitionthebitsinamannersimilartosignand magnitude.Inthiscontext,wegetacomplementusingtheNOToperation.Table1-4shows–7 through7usingone’scomplement.
Table 1-4: One’sComplementBinaryNumbers
Asyoucansee,flippingeachbitof0111(+7)yields1000(–7).
One’scomplementrepresentationstillhastheproblemoftwodifferentrepresentationsforzero.It stilldoesn’tletusperformadditioneasily,either.Togetaroundthis,weuse end-around carry to add1totheLSBifthereisacarryoutofthemostsignificantpositioninordertogetthecorrect result.Figure1-11illustrateshowthisworks.
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