Assembly language step by step programming with DOS and linux

AssemblyLanguageStepby-Step:Programming
withDOSandLinux,
SecondEdition
ISBN:0471375233
byJeff
Duntemann
JohnWiley&Sons©2000(613
pages)

A“LostWorld”journeyinto
16-bitassembler
programmingconceptsand
techniques.

TableofContents
AssemblyLanguageStep-by-Step—
ProgrammingwithDOSandLinux,
SecondEdition
Foreword
"WhyWouldYouWantto
Introduction DoThat?"
AnotherPleasantValley
SaturdayUnderstanding
Chapter1 WhatComputersReally
Do
AlienBasesGettingYour
Chapter2 - ArmsaroundBinaryand
Hexadecimal
LiftingtheHood

Chapter3

- DiscoveringWhat
ComputersActuallyAre
TheRighttoAssemble
TheProcessofMaking
Chapter4 AssemblyLanguage
Programs
NASM-IDE:APlaceto
StandGivemealever
Chapter5 - longenough,anda
placetostand,andIwill
movetheEarth.
AnUneasyAllianceThe
x86CPUandIts
Chapter6 SegmentedMemory
System
FollowingYour
InstructionsMeeting
Chapter7 MachineInstructionsup
CloseandPersonal
OurObjectAllSublime
Chapter8 - CreatingProgramsthat
Work
DividingandConquering
UsingProceduresand
Chapter9 MacrostoBattle
Complexity
Bits,Flags,Branches,
andTablesEasinginto

Chapter10 MainstreamAssembly
Programming
StringingThemUp
Chapter11 - ThoseAmazingString


Chapter12

Chapter13

Conclusion
AppendixA

AppendixB
AppendixC
AppendixD

Instructions
TheProgrammer'sView
ofLinuxToolsandSkills
- toHelpYouWrite
AssemblyCodeundera
True32-BitOS
CodingforLinux
ApplyingWhatYou've
- LearnedtoaTrue
ProtectedMode
OperatingSystem
NottheEnd,ButOnly
theBeginning

Partial8086/8088
- InstructionSet
Reference
SegmentRegister
- AssumptionsforReal
ModeSegmentedModel
WebURLsforAssembly
Programmers
SegmentRegister
Assumptions
- What'sontheCD-ROM?

AppendixE
Index
ListofFigures
ListofTables


BackCover
Thebestsellingguidetoassemblylanguage--now
updatedandexpandedtoincludecoverageofLinux.
Thisneweditionofthebestsellingguidetoassembly
programmingnowcoversDOSandLinux!TheSecond
Editionbeginswithahighlyaccessibleoverviewofthe
internaloperationsoftheIntel-basedPCand
systematicallycoversallthestepsinvolvedinwriting,
testing,anddebuggingassemblyprograms.
ExpertauthorJeffDuntemannthenpresentsworking
exampleprogramsforboththeDOSandLinux
operatingsystemsusingthepopularfreeassembler

NASM.Healsoinlcudesvaluableinfomationonhowto
useproceduresandmacros,plusrareexplanationsof
assembly-levelcodingforLinux,allofwhichcombine
toofferacomprehensivelookatthecomplexititesof
assemblyprogrammingforIntelprocessors.
Providingyouwiththefoundationtocreateexecutable
assemblylanguageprograms,thisbook:
ExplainshowtouseNASM-IDE,asimpleprogram
editorandassembly-orienteddevelopment
environment
Detailsthemostusedelementsofthe86-family
instructionset
TeachesaboutDEBUG,thesinglemostusefultool
youhaveasanassemblylanguageprogrammer
Examinestheoperationsthatmachineinstructions


forcetheCPUtoperform
Discussestheprocessofmemoryaddressing
CoverscodingforLinux
AbouttheAuthor
JeffDuntemannistheEditor-in-ChiefofVisual
Developermagazine,formereditorofTurboTechnix
andPCTechniques,the"StructuredProgramming"
columnistforDr.Dobb’sJournal,andhaswrittenand
editedmorethantwentyprogrammingbooks.


AssemblyLanguageStep-by-Step—
ProgrammingwithDOSandLinux,Second

Edition
JeffDuntemann

WileyComputerPublishing
JohnWiley&Sons,Inc.
NEWYORK•CHICHESTER•WEINHEIM•BRISBANE•SINGAPORE•
TORONTO
Publisher:RobertIpsen
Editor:CarySullivan
ManagingEditor:MichelineFrederick
TextDesign&Composition:NorthMarketStreetGraphics
Designationsusedbycompaniestodistinguishtheirproductsareoften
claimedastrademarks.InallinstanceswhereJohnWiley&Sons,Inc.,is
awareofaclaim,theproductnamesappearininitialcapitalorALL
CAPITALLETTERS.Readers,however,shouldcontacttheappropriate
companiesformorecompleteinformationregardingtrademarksand
registration.
Copyright©2000byJeffDuntemann.
Allrightsreserved.
PublishedbyJohnWiley&Sons,Inc.
PublishedsimultaneouslyinCanada.
Nopartofthispublicationmaybereproduced,storedinaretrieval
systemortransmittedinanyformorbyanymeans,electronic,


mechanical,photocopying,recording,scanningorotherwise,exceptas
permittedunderSections107or108ofthe1976UnitedStatesCopyright
Act,withouteitherthepriorwrittenpermissionofthePublisher,or
authorizationthroughpaymentoftheappropriateper-copyfeetothe
CopyrightClearanceCenter,222RosewoodDrive,Danvers,MA01923,

(978)750-8400,fax(978)750-4744.RequeststothePublisherfor
permissionshouldbeaddressedtothePermissionsDepartment,John
Wiley&Sons,Inc.,605ThirdAvenue,NewYork,NY10158-0012,(212)
850-6011,fax(212)850-6008,E-Mail:<PERMREQ@WILEY.COM>.
Thispublicationisdesignedtoprovideaccurateandauthoritative
informationinregardtothesubjectmattercovered.Itissoldwiththe
understandingthatthepublisherisnotengagedinprofessionalservices.
Ifprofessionaladviceorotherexpertassistanceisrequired,theservices
ofacompetentprofessionalpersonshouldbesought.
LibraryofCongressCataloging-in-PublicationData:
Duntemann,Jeff.
Assemblylanguagestep-by-step:programmingwithDOSandLinux/
JeffDuntemann.—2nded.
p.cm.
Rev.ed.of:Assemblylanguage,©1992.
ISBN0-471-37523-3(paper/CD-ROM:alk.paper)
1.Assemblerlanguage(Computerprogramlanguage)I.Duntemann,Jeff.
Assemblylanguage.II.Title.
QA76.73.A8D882000
005.265—dc2100-020611
10987654321
TotheeternalmemoryofKathleenM.Duntemann,Godmother
1920–1999whogavemebookswhenallIcoulddowasputteeth
marksonthem.TherearenowordsforhowmuchIoweyou!
Acknowledgments
Firstofall,totheauthorsofthesoftwarethatIamhonoredtoprovideby


theirpermissiononthisbook'sCD-ROM:
RobertAnderton,authorofNASM-IDE:

www.inglenook.co.uk/nasmide/
AnthonyWilliams,authorofALINK: />And,ofcourse,theNASMteam:Julian"Jules"Hall,Simon
Tatham,H.PeterAnvin,JohnFine,KendallBennet,GaryClark,
andAndrewCrabtree:www.web-sites.co.uk/nasm/
Becauseoftheirgenerosity,thereis"nothingelsetobuy."Everythingyou
needtoprograminassemblyisrighthereontheCD-ROMforthisbook.
AbundantthanksalsogotoBillSchindler,forteachingmethewaysof
theClibrary,andMichaelAbrash,whohasalwayshasbeentherock
uponwhommyknowledgeofassemblylanguageitselfhasbeen
anchored.
Finally,toCarol,asalways,forthesacramentalfriendshipthathas
challengedme,shapedme,andredeemedmeeverydayofour30years
togetherand23yearsasspouses.


Foreword
Timepasses.Itwasexactly10yearsagothissummer,backinJuly1989,
whenIturnedinthemanuscriptofabookcalledAssemblyLanguage
fromSquareOne.Thebookwaswellreceived,butitspublisherwent
belly-uponlyafewmonthsafteritsintroduction.Thatmayhavebeena
blessing,becausethebookwastooshort,hadafewmoreerrorsinit
thanitshouldhavehad,andwasprintedonhorriblecheappaperthat
rippedwithalmostnoprovocationandisnowturningsicklyyellow.
So,Ileaptonthechancetodothebookoverandpublishitwithareal
publisher,themostvenerableJohnWiley&Sons,who(astheirT-shirts
say)hasbeentotallyawesomesince1809.Itwasthoroughlyrewritten
andbecameanewbookwithanewtitle,andwentontheshelvesin
Septemberof1992.Timepasses,butinaworldwherethelifeofa
computerbookmaywellbeeightmonthsorless,Wileykeptthefirst
editionofAssemblyLanguageStep-by-Stepinprintforeightyears,from

1992to2000.
Inthattimeithasprobablysoldmorecopiesthananyothersingle
assemblylanguagebook,andI'vereceivedhundredsoflettersofadvice,
corrections,suggestions,andsimple,"Hey,thisiscool!"compliments.
Thankstoyouallfortakingthetimetowrite.Itmeansalottome.It's
unclearhowlongthissecondeditionwillremaininprint,butaslongas
peoplekeepbuyingit(andtellingmeit'sbeenusefultothem),Isuspect
thateitherthiseditionoronetofollowwillremainavailable.
Timepasses.Andbeforewegetintothebookproper,there'ssomething
elseIwantedtorelate.OnJuly8,1999,mysisterGretchenDuntemann
RoperfoundthatKathleenDuntemannhaddiedpeacefullyinhersleepin
Chicago,almost10yearstothedaysinceIhadcompletedAssembly
LanguagefromSquareOne,whichwasalsodedicatedtoher.Shekept
bothbooksonhercoffeetableandwouldshowthemtoanyonewho
cametovisit,eventhoughsheneverhadacomputerandprobablynever
understoodwhatassemblylanguagewas.Shewasmyauntand
godmother,myfather'ssolesibling,whosangmyABCstomeand
demandedthatIbeadmittedtoAdlerPlanetariuminChicagowhenIwas


six,eventhoughtherulesatthattimedemandedthatchildrenbeseven
toattendtheskyshow."Nametheplanetsfortheniceman,"shetoldme,
andIdid,andwhenIhadgonethroughalltheplanetsIstartedinonthe
constellations.Igotin,becauseshebelievedinme.Andshewasthere
througheveryothermajormilestoneinmylife:FirstCommunion,
Confirmation,wedding,myfather'sillnessanddeath,yearsandyearsof
ChristmasesandThanksgivingsandbirthdays,alwayswithtreatsforthe
dogandstoriestotell,withaquickIrishwitandagenerousheart-and
trulyIcannotandwillnoteverforgether.
IsaythisonlybecausesomanyofyouareconsiderablyyoungerthanI,

andmayforgetinthefeverofyounglife:Timepasses,andsodothe
peoplewhobelieveinus,andurgeusthroughthewallsaswehitthem
sothatwemayarriveatmidlifewithsomethingtoshowforit.Fathers
andmothers,grandparents,auntsandunclescanaddimmeasurablyto
ourlives,andoftendo,evenwhenwe'retoobusytonotice.Cherishthem
whileyouhavethem,becausecherishingthemafterthey'regoneisa
lonelybusinessindeed.
Inthemeantime,havingbeentalkingaboutassemblylanguageinone
bookoranotherfor10years,I'vedecidedtomakeit20.Aslongasthere
willbePCs,therewillbeassemblylanguage.Staytuned.Theyear2009
willbeherebeforeyouknowit.


Introduction:"WhyWouldYouWanttoDo
That?"
Itwas1985,andIwasinacharteredbusinNewYorkCity,headingfora
pressreceptionwithabunchofotherrestlessmediaegomaniacs.Iwas
onlybeginningmymediacareer(astechnicaleditorforPCTechJournal)
andmyfirstbookwasstillmonthsinthefuture.Ihappenedtobesitting
nexttoanestablishedprogrammingwriter/guru,withwhomIwas
impressedandtowhomIwasbabblingaboutonethingoranother.I
won'tnamehim,ashe'sdonealotforthefield,andwilldoalotmore
beforehe'sthroughifhedoesn'tkillhimselfsmokingfirst.
ButIhappenedtoletslipthatIwasaTurboPascalfanatic,andwhatI
reallywantedtodowaslearnhowtowriteTurboPascalprogramsthat
madeuseofthebrandnewMicrosoftWindowsuserinterface.He
wrinkledhisnoseandgrimacedwryly,beforespeakingtheInfamous
Question:
"Whywouldyouwanttodothat?"
Ihadneverheardthequestionbefore(thoughIwouldhearitmanytimes

thereafter),andittookmeaback.Why?Because,well,because…I
wantedtoknowhowitworked.
"Heh.That'swhatC'sfor."
FurtherdiscussiongotmenowhereinaPascaldirection.Butsome
probingledmetounderstandthatyoucouldn'twriteWindowsappsin
TurboPascal.Itwasimpossible.Or...theprogrammingwriter/gurudidn't
knowhow.Maybeboth.Ineverlearnedthetruth.ButIdidlearnthe
meaningoftheInfamousQuestion.
Notewell:Whensomebodyasksyou,"Whywouldyouwanttodothat?"
whatitreallymeansisthis:"You'veaskedmehowtodosomethingthat
iseitherimpossibleusingtoolsthatIfavororcompletelyoutsidemy
experience,butIdon'twanttolosefacebyadmittingit.So,...how'bout
thoseBlackhawks?"


Ihearditagainandagainovertheyears:
Q:HowcanIsetupaCstringsothatIcanreaditslengthwithout
scanningit?
A:Whywouldyouwanttodothat?
Q:HowcanIwriteanassemblylanguagesubroutinecallablefromTurbo
Pascal?
A:Whywouldyouwanttodothat?
Q:HowcanIwriteWindowsappsinassemblylanguage?
A:Whywouldyouwanttodothat?
Yougettheidea.TheanswertotheInfamousQuestionisalwaysthe
same,andiftheweaselseveraskitofyou,snapbackasquicklyas
possible:BecauseIwanttoknowhowitworks.
Thatisacompletelysufficientanswer.It'stheanswerI'veusedevery
singletime,exceptforoneoccasionaconsiderablenumberofyearsago,
whenIputforththatIwantedtowriteabookthattaughtpeoplehowto

programinassemblylanguageastheirfirstexperienceinprogramming.
Q:Goodgrief,whywouldyouwanttodothat?
A:Becauseit'sthebestwaythereistobuildtheskillsrequiredto
understandhowalltherestoftheprogramminguniverseworks.
Beingaprogrammerisonethingaboveallelse:Itisunderstandinghow
thingswork.Learningtobeaprogrammer,furthermore,isalmostentirely
aprocessoflearninghowthingswork.Thiscanbedoneatvarious
levels,dependingonthetoolsyou'reworkingwith.Ifyou'reprogramming
inVisualBasic,youhavetounderstandhowcertainthingswork,but
thosethingsarebyandlargeconfinedtoVisualBasicitself.Agreatdeal
ofmachineryishiddenbythelayerthatVisualBasicplacesbetweenthe
programmerandthecomputer.(ThesameistrueofDelphi,Java,Perl,
andmanyotherveryhigh-levelprogrammingenvironments.)Ifyou're
usingaCcompiler,you'realotclosertothemachine,andyouseealot


moreofthatmachinery-andmust,therefore,understandhowitworksto
beabletouseit.However,quiteabitremainshidden,evenfromthe
hardenedCprogrammer.(ManyCprogrammersfoolthemselvesinto
thinkingtheyknowwaymorethantheyactuallydo-andhavethebad
karmatobeprettydamnedarrogantaboutit.)
If,ontheotherhand,you'reworkinginassemblylanguage,you'reas
closetothemachineasyoucanget.Assemblylanguagehidesnothing,
andwithholdsnopower.Theflipside,ofcourse,isthatnomagicallayer
betweenyouandthemachinewillabsolveanyignoranceandtakecare
ofthingsforyou.Ifyoudon'tunderstandhowsomethingworks,you're
deadinthewater-unlessyouknowenoughtobeabletofigureitouton
yourown.
That'sakeypoint:Mygoalincreatingthisbookisnotentirelytoteach
youassemblylanguageperse.Ifthisbookhasaprimedirectiveatall,it

istoimpartacertaindisciplinedcuriosityaboutthemachine,alongwith
somebasiccontextfromwhichyoucanbegintoexplorethemachineat
itslowestlevels.That,andtheconfidencetogiveityourbestshot.Thisis
difficultstuff,butit'snothingyoucan'tmastergivensomeconcentration,
patience,andthetimeitrequires-which,Icaution,maybeconsiderable.
Intruth,whatI'mreallyteachingyouishowtolearn.


TheMasterPlan
YouneedanIntel-basedcomputer.ForalotofwhatI'llbeexplaining,
literallyanyIntel-basedmachinewilldo-rightbacktotheprimordial8088basedIBMPCfrom1981.However,tobeabletotryalltheexamples,
you'llneedatleasta386.Mostofthebookrelatesto16-bitDOS,which
comeswithWindows95and98,and(inaslightlylimitedform)is
emulatedbyWindowsNT.Towardtheendofthebook,Iexplainhowto
workwithassemblyunderLinux,andforthatyouwilldefinitelyneeda
386ormore-advancedIntelmachine.
Althoughmostpeoplethinkofmasteringassemblylanguageasthe
processoflearningacollectionofmachineinstructions,that'sactuallythe
easypart.Therealchallengeinassemblyislearningthemachine's
memorymodels-sothat'sactuallywhatI'llbeemphasizing.
TherearethreegeneralmemorymodelsfortheIntelprocessorfamily:
16-bitflatmodel(sometimescalledtheTinymodel,orjustthe"COMfile
model"),16-bitsegmentedmodel,and32-bitflatmodel.I'mspendinga
fairamountoftimeon16-bitflatmodel,becauseit'sverymuchlikethe
32-bitflatmodelinminiature.Thesegmentedmodelruledforagood
manyyears(includingthetimewhenIwrotethefirsteditionofthisbook),
butit'sactuallyacompromisethatlivedfarlongerthanitdeservedto.
WhateverfutureIntelcomputingmayhave,itwillhappeninaflatmemory
model.Youneedtoknowaboutsegments-butIhopeyou'llneveractually
havetousethem.

TheCD-ROMforthisbookcontainsanassembler:NASM,theNet-Wide
Assembler.It'sfree,it'seasytolearn,andfullsourcecodeisavailable,
freeofcharge,fromtheInternet.That'stheassemblerI'llbeteaching.If
youcanunderstandNASM,youcanpickupMicrosoft'sMASMwithout
trouble.NASMcangenerateprogramsforboth16-bitDOSand32-bit
Linux,soit'stheidealassemblerformetoteachinthisbook.Although
NASMisincludedontheCD-ROM,youmightchecktheNASMWebsite
toseeifanewerversionisavailable.(Thefirsteditionofthisbook
remainedinprintforeightyears.Youcouldbereadingthesewordsinthe


year2005orlater-bywhichtimemostofthesoftwareIspeakofwillbein
amuchmorehighlyevolvedstate.)TheWeblocationsofallthesoftware
mentionedorusedinthisbookaregiveninAppendixC.
InthefirsteditionofthisbookIpresentedasimpleeditor/environment
calledJED.JEDishistory,gonewithsomeBorlandcodelibrariesthat
werepulledfromthemarket.InitsplaceIpresentNASM-IDE,a
conceptuallysimilarutilitycreatedforNASMbyRobertAndertonofthe
UnitedKingdom.NASM-IDEoperatesonlyunderDOS.Itwon'thelpyou
withLinux.ButinLinuxthereareamultitudeofeditorsavailable,andin
theprocessoflearningLinuxyoucertainlylearnedoneofthem.
Whateveritis,useit.(Iuse,andwillrecommend,EMACS.)IfI'velearned
nothingelseaboutLinux,it'sthatpeoplegetveryattachedtotheirtext
editors.Iwon'taskyoutolearnanotherone.
Thewaytogetthemostfromthisbookistostartatthebeginningand
readitthrough,onechapteratatime,inorder.Evenifyourollyoureyes
andsayyoualreadyknowwhathexadecimalis,readitanyway.It'sa
goodreview-andyouwon'tmissanyofmyjokesandfunnystories.Load
andrunalltheexampleprograms.Tryyourbesttounderstandwhat
everysinglelineineveryprogramdoes.

Thatis,ultimately,whatI'mafter:toshowyouthewaytounderstand
whateveryhowever-distantcornerofyourmachineisdoing,andhowall
itsmanypiecesworktogether.Thisdoesn'tmeanI'llexplaineverycorner
ofitmyself-noonewilllivelongenoughtodothat;computingisn'tsimple
anymore-butifyoudevelopthedisciplineofpatientresearchand
experimentation,youcanprobablyworkitoutforyourself.Ultimately,
that'stheonlywaytolearnit:byyourself.Theguidanceyoufind-in
friends,ontheNet,inbookslikethis-isonlyguidance,andgreaseonthe
axles.Youhavetodecidewho'stobethemaster,youorthemachine,
andmakeitso.Assemblyprogrammersaretheonlyprogrammerswho
cantrulyclaimtobethemasters,andthat'satruthworthmeditatingon.
Ifitmeansanythingatall(optimistandthoroughgoingPelagianthatI
am),Ibelieveinyou.Goforit.
-JeffDuntemann


Scottsdale,Arizona
May2000


Chapter1:AnotherPleasantValleySaturday
UnderstandingWhatComputersReallyDo


It'sAllinthePlan
"Quick,getthekidsup,it'spast7.Nicky'sgotLittleLeagueat9and
Dione'sgotballetat10.Mike,giveMaxhisheartwormpill!(We'reoutof
them,Ma,remember?)Yourfatherpickedagreatweekendtogo
fishing…Here,letmegiveyou10bucksandgogetmorepillsatthe
vet's…MyGod,that'sright,Hankneededgasmoneyandleftmebroke.

There'satellermachineoverbyKmart,andifIgothereIcantakethat
stupidtoiletseatbackandgettherightone.
"IguessI'dbettermakealist…"
It'sanotherPleasantValleySaturday,andthirty-oddmillionsuburban
homemakerssitdownwithapencilandpadatthekitchentabletotryand
makesenseofamorningthatwouldkillandpickleanylesserbeing.In
hermindshethinksofthedependenciesandtracestheroute:
DropNickyatRandPark,gobacktoDempsterandit'sabout10minutes
toGolfMillMall.DoIhavegas?I'dbettercheckfirst-ifnot,stopatDel's
ShellorIwon'tmakeittoMilwaukeeAvenue.Milkthetellermachineat
GolfMill,thencrosstheparkinglottoKmarttoreturnthetoiletseatthat
Hankboughtlastweekendwithoutcheckingwhatshapeitwas.Gotta
remembertothrowthetoiletseatinbackofthevan-writethatatthetop
ofthelist.
Bythenit'llbehalfpast,maybelater.Balletisallthewaydown
GreenwoodinParkRidge.NoleftturnfromMilwaukee-butthere'sthe
sneakpatharoundbehindtheMall.Ihavetoremembernottoturnright
ontoMilwaukeelikeIalwaysdo-jotthatdown.WhileI'minParkRidgeI
cancheckandseeifHank'snewglassesarein-shouldcallbuttheywon't
evenbeopenuntil9:30.Oh,andgroceries-candothatwhileDione
dances.OnthewaybackIcancutovertoOaktonandgetthedog'spills.
Inabout90secondsflatthelistiscomplete:
Throwtoiletseatinvan.


Checkgas-ifempty,stopatDel'sShell.
DropNickyatRandPark.
StopatGolfMilltellermachine.
ReturntoiletseatatKmart.
DropDioneatballet(rememberbackpathtoGreenwood).

SeeifHank'sglassesareatPearleVision-iftheyare,make
doublesuretheyrememberedtheextrascratchcoating.
GetgroceriesatJewel.
PickupDione.
Stopatvet'sforheartwormpills.
Dropoffgroceriesathome.
Ifit'stime,pickupNicky.Ifnot,collapseforafewminutes,then
pickupNicky.
Collapse!
Inwhatweoftencalla"laundrylist"(whetheritinvolveslaundryornot)is
theperfectmetaphorforacomputerprogram.Withoutrealizingit,our
intrepidhomemakerhaswrittenherselfacomputerprogramandthenset
out(actingasthecomputer)toexecuteitandbedonebeforenoon.
Computerprogrammingisnothingmorethanthis:Youtheprogrammer
writealistofstepsandtests.Thecomputerthenperformseachstepand
testinsequence.Whenthelistofstepshasbeenexecuted,thecomputer
stops.
Acomputerprogramisalistofstepsandtests,nothingmore.

StepsandTests
ThinkforamomentaboutwhatIcalla"test"intheprecedinglaundrylist.


Atestisthesortofeither/ordecisionwemakedozensorhundredsof
timesoneventhemostplacidofdays,sometimesnearlywithoutthinking
aboutit.
Ourhomemakerperformedatestwhenshejumpedintothevantoget
startedonheradventure.Shelookedatthegasgauge.Thegasgauge
wouldtellheroneoftwothings:(1)Shehasenoughgas,or(2)no,she
doesn't.Ifshehasenoughgas,shetakesarightandheadsforRand

Park.Ifshedoesn'thaveenoughgas,shetakesaleftdowntothecorner
andfillsthetankatDel'sShell.(Deltakescreditcards.)Then,withafull
tank,shecontinuestheprogrambytakingaU-turnandheadingforRand
Park.
Intheabstract,atestconsistsofthosetwoparts:
First,youtakealookatsomethingthatcangooneoftwoways.
Thenyoudooneoftwothings,dependingonwhatyousawwhen
youtookalook.
Towardtheendoftheprogram,ourhomemakergothome,tookthe
groceriesoutofthevan,andtookalookattheclock.Ifitisn'ttimetoget
NickybackfromLittleLeague,shehasamomenttocollapseonthe
couchinanearlyemptyhouse.IfitistimetogetNicky,there'snorestfor
theragged:ShesprintsforthevanandheadsbacktoRandPark.
(Anyguessesastowhethershereallygetstocollapsewhentheprogram
iscomplete?)

MorethanTwoWays?
Youmightobject,sayingthatmanyormosttestsinvolvemorethantwo
alternatives.Ha-hah,sorry,you'redeadwrong-ineverycase.
Furthermore,you'rewrongwhetheryouthinkyouareornot.
Exceptfortotallyimpulsiveorpsychoticbehavior,everyhuman
decisioncomesdowntothechoicebetweentwoalternatives.
Whatyouhavetodoislookalittlemorecloselyatwhatgoesthrough


yourmindwhenyoumakedecisions.Thenexttimeyoubuzzdownto
MooFooGooforfastChinese,observeyourselfwhileyou'reporingover
themenu.Thechoicemightseem,atfirst,tobeofoneitemoutof26
Cantonesemaincourses.Notso-thechoice,infact,isbetweenchoosing
oneitemandnotchoosingthatoneitem.YoureyesrestonChickenwith

Cashews.Naw,toobland.Thatwasatest.Youslidedowntothenext
item.ChickenwithBlackMushrooms.Hmmm,no,hadthatlastweek.
Thatwasanothertest.Nextitem:KungPaoChicken.Yeah,that'sit!That
wasathirdtest.
Thechoicewasnotamongchickenwithcashews,chickenwithblack
mushrooms,orchickenwithkungpao.Eachdishhaditsmoment,poised
beforethecriticaleyeofyourmind,andyouturnedthumbsuporthumbs
downonit,individually.Eventually,onedishwon,butitwoninthatsame
gameof"toeatornottoeat."
Letmegiveyouanotherexample.Manyoflife'smostcomplicated
decisionscomeaboutduetothefactthat99.99867percentofusarenot
nudists.You'vebeenthere:You'restandingintheclothesclosetinyour
underwear,flippingthroughyourrackofpants.Thetestscomethickand
fast.Thisone?No.Thisone?No.Thisone?No.Thisone?Yeah.You
pickapairofbluepants,say.(It'saMonday,afterall,andbluewould
seemanappropriatecolor.)Thenyoustumbleovertoyoursockdrawer
andtakealook.Whoops,nobluesocks.Thatwasatest.Soyoustumble
backtotheclothescloset,hangyourbluepantsbackonthepantsrack,
andstartover.Thisone?No.Thisone?No.Thisone?Yeah.Thistime
it'sbrownpants,andyoutossthemoveryourarmandheadbacktothe
sockdrawertotakeanotherlook.Nertz,outofbrownsocks,too.Soit's
backtotheclothescloset…
Whatyoumightconsiderasingledecision,orperhapstwodecisions
inextricablytangled(likepickingpantsandsocksofthesamecolor,given
stockonhand),isactuallyaseriesofsmalldecisions,alwaysbinaryin
nature:Pick'emordon'tpick'em.Find'emordon'tfind'em.TheMonday
morningepisodeintheclothesclosetisagoodanalogyofa
programmingstructurecalledaloop:Youkeepdoingaseriesofthings
untilyougetitright,andthenyoustop.(Assumingyou'renotthekindof
nerdwhowearsbluesockswithbrownpants.)Butwhetheryouget

everythingrightalwayscomesdowntoasequenceofsimpleeither/or
decisions.

ComputersThinkLikeUs
Icanalmosthearwhatyou'rethinking:"Sure,it'sacomputerbook,and
he'stryingtogetmetothinklikeacomputer."Notatall.Computersthink
likeus.Wedesignedthem;howelsecouldtheythink?No,whatI'mtrying
todoisgetyoutotakealong,hardlookathowyouthink.Werunon
automaticforsomuchofourlivesthatweliterallydomostofourthinking
withoutreallythinkingaboutit.
Theverybestmodelforthelogicofacomputerprogramistheverysame
logicweusetoplanandmanageourdailyaffairs.Nomatterwhatwedo,
itcomesdowntoamatterofconfrontingtwoalternativesandpicking
one.Whatwemightthinkofasasinglelargeandcomplicateddecisionis
nothingmorethanamessytangleofmanysmallerdecisions.Theskillof
lookingatacomplexdecisionandseeingallthelittledecisionsinits
tummywillserveyouwellinlearninghowtoprogram.Observeyourself
thenexttimeyouhavetodecidesomething.Countupthelittledecisions
thatmakeupthebigone.You'llbesurprised.
And,surprise!You'llbeaprogrammer.


HadThisBeentheRealThing…
Donotbealarmed.Whatyouhavejustexperiencedwasametaphor.It
wasnottherealthing.(Therealthingcomeslater.)
Iusemetaphorsalotinthisbook.Ametaphorisaloosecomparison
drawnbetweensomethingfamiliar(suchasaSaturdaymorninglaundry
list)andsomethingunfamiliar(suchasacomputerprogram).Theideais

toanchortheunfamiliarinthetermsofthefamiliar,sothatwhenIbegin
tossingfactsatyou,you'llhavesomeplacecomfortabletolaythem
down.
Themostimportantthingforyoutodorightnowiskeepanopenmind.If
youknowalittlebitaboutcomputersorprogramming,don'tpicknits.
Yes,thereareimportantdifferencesbetweenahomemakerfollowinga
scribbledlaundrylistandacomputerexecutingaprogram.I'llmention
thosedifferencesallingoodtime.
Fornow,it'sstillChapter1.Taketheseinitialmetaphorsontheirown
terms.Lateron,they'llhelpalot.


DoNotPassGO
"There'sareasonboredandboardarehomonyms,"saidmybestfriend
Artoneevening,aswesat(twosuper-sophisticatedtwelve-year-olds)
playingsomegameinhisbasement.(Hemayhavebeenunhappy
becausehewaslosing.)WasitMilleBornes?OrStratego?Or
Monopoly?Orsomethingelseentirely?Iconfess,Idon'tremember.I
simplyrecallhoppingsomelittlepieceofplasticshapedlikeapregnant
bowlingpinupanddownaseriesofcoloredsquaresthattoldmetodo
dumbthingslikegobacktwospacesorput$100inthepotornukeOuter
Mongolia.
OuterMongolianotwithstanding,therearestrongparallelstobedrawn
betweenthatpeculiarAmericanobsession,theboardgame,and
assembly-languageprogramming.Firstofall,everythingwesaidbefore
stillholds:Boardgames,byandlarge,consistofaprogressionofsteps
andtests.Insomegames,suchasTrivialPursuit,everysteponthe
boardisatest:toseeifyoucananswer,ornotanswer,aquestionona
card.Inotherboardgames,eachlittlesquareontheboardcontains
somesortofinstruction:LoseOneTurn;GoBackTwoSquares;Takea

CardfromCommunityChest;and,ofcourse,GotoJail.
CertainboardgamesmadeforsomelivelyargumentsbetweenArtand
myself(itwasthatorbebored,asitwere)concerningwhatitmeantto
GoForwardorBackwardFiveSteps.Itseemedtomethatyoushould
countthesquareyouwerealreadyon,andArt,traditionalistalways,
thoughtyoushouldstartcountingwiththefirststepinthedirectionyou
hadtogo.Thismadeadifferenceinthegame,ofcourse.(Iconveniently
forgottopressmypointwhendoingsowouldlandmeonsomethinglike
ParkPlacewith15ofArt'shotelsonit…)

TheGameofBigBux
Toavoidgettinginserioustrouble,Ihaveinventedmyownboardgame
tocontinuedowntheroadwiththisparticularmetaphor.Inthesensethat
artmirrorslife,theGameofBigBuxmirrorslifeinSiliconValley,where


moneyseemstobespontaneouslycreated(generallyinsomebodyelse's
pocket)andthethreebigMoneyBlackHolesarefastcars,Californiareal
estate,andmessydivorces.
AportionoftheBigBuxGameBoardisshowninFigure1.1.Thelineof
rectanglesontheleftsideofthepagecontinuesallthewayaroundthe
board.Inthemiddleoftheboardarecubbyholestostoreyourplay
moneyandgamepieces;stacksofcardstobereadoccasionally;and
shortdetourswithsuchnamesasMessyDivorceandStartaBusiness,
whicharebriefsequencesofthesamesortofactionrectanglesasthose
formingthepatharoundtheedgeoftheboard.


Figure1.1:TheGameofBigBux.
Unlikemanyboardgames,youdon'tthrowdicetodeterminehowmany

stepsaroundtheboardyoutake.BigBuxrequiresthatyoumoveone
stepforwardoneachturn,unlessthesquareyoulandoninstructsyouto