| OGT Home | UPF Home | View Cart |
 CITATION 
 PDF VIEWER 
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Full Citation | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 STANDARD VIEW
 MARC VIEW
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Downloads | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Full Text | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
PAGE 1 ProgrammingfromtheGroundUpJonathanBartlettEditedbyDominickBruno,Jr. PAGE 2 ProgrammingfromtheGroundUpbyJonathanBartlettEditedbyDominickBruno,Jr.Copyright2003byJonathanBartlettPermissionisgrantedtocopy,distributeand/ormodifythisdocumentunderthetermsoftheGNUFreeDocumentationLicense,Version1.1oranylaterversionpublishedbytheFreeSoftwareFoundation;withnoInvariantSections,withnoFront-CoverTexts,andwithnoBack-CoverTexts.Acopyofthelicenseisincludedin AppendixH .Inaddition,youaregrantedfullrightstousethecodeexamplesforanypurposewithoutevenhavingtocredittheauthors.Alltrademarksarepropertyoftheirrespectiveowners.Thisbookcanbepurchasedathttp://www.bartlettpublishing.com/Thisbookisnotareferencebook,itisanintroductorybook.Itisthereforenotsuitablebyitselftolearnhowtoprofessionallyprograminx86assemblylanguage,assomedetailshavebeenleftouttomakethelearningprocesssmoother.Thepointofthebookistohelpthestudentunderstandhowassemblylanguageandcomputerprogrammingworks,nottobeareferencetothesubject.Referenceinformationaboutaparticularprocessorcanbeobtainedbycontactingthecompanywhichmakesit.Toreceiveacopyofthisbookinelectronicform,pleasevisitthewebsitehttp://savannah.nongnu.org/projects/pgubook/ThissitecontainstheinstructionsfordownloadingatransparentcopyofthisbookasdenedbytheGNUFreeDocumentationLicense. PAGE 3 TableofContents 1.Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 WelcometoProgramming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 YourTools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.ComputerArchitecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 StructureofComputerMemory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 TheCPU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 SomeTerms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 InterpretingMemory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 DataAccessingMethods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3.YourFirstPrograms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 EnteringintheProgram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 OutlineofanAssemblyLanguageProgram . . . . . . . . . . . . . . . . . . . . . . . . . 22 PlanningtheProgram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 FindingaMaximumValue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 AddressingModes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.AllAboutFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 DealingwithComplexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 HowFunctionsWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Assembly-LanguageFunctionsusingtheCCallingConvention . . . . . . . . . 52 AFunctionExample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 RecursiveFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 5.DealingwithFiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 TheUNIXFileConcept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 Buffersand.bss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 StandardandSpecialFiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 UsingFilesinaProgram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 iii PAGE 4 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 6.ReadingandWritingSimpleRecords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 WritingRecords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 ReadingRecords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 ModifyingtheRecords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 7.DevelopingRobustPrograms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 WhereDoestheTimeGo? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 SomeTipsforDevelopingRobustPrograms . . . . . . . . . . . . . . . . . . . . . . . 118 HandlingErrorsEffectively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 MakingOurProgramMoreRobust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 8.SharingFunctionswithCodeLibraries . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 UsingaSharedLibrary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 HowSharedLibrariesWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 FindingInformationaboutLibraries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 UsefulFunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 BuildingaSharedLibrary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 9.IntermediateMemoryTopics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 HowaComputerViewsMemory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 TheMemoryLayoutofaLinuxProgram . . . . . . . . . . . . . . . . . . . . . . . . . . 149 EveryMemoryAddressisaLie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 GettingMoreMemory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 ASimpleMemoryManager . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 UsingourAllocator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 MoreInformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 iv PAGE 5 10.CountingLikeaComputer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Truth,Falsehood,andBinaryNumbers . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 TheProgramStatusRegister . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195 OtherNumberingSystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 OctalandHexadecimalNumbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 OrderofBytesinaWord . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 ConvertingNumbersforDisplay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 11.High-LevelLanguages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 CompiledandInterpretedLanguages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 YourFirstCProgram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Perl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 Python . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 12.Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 WhentoOptimize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 WheretoOptimize . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 LocalOptimizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 GlobalOptimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230 13.MovingOnfromHere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233 FromtheBottomUp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 FromtheTopDown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 FromtheMiddleOut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 SpecializedTopics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 FurtherResourcesonAssemblyLanguage . . . . . . . . . . . . . . . . . . . . . . . . . 237 v PAGE 6 A.GUIProgramming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 B.Commonx86Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 C.ImportantSystemCalls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 D.TableofASCIICodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 E.CIdiomsinAssemblyLanguage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 F.UsingtheGDBDebugger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289 G.DocumentHistory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 H.GNUFreeDocumentationLicense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 I.PersonalDedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 vi PAGE 7 Chapter1.IntroductionWelcometoProgrammingIloveprogramming.Ienjoythechallengetonotonlymakeaworkingprogram,buttodosowithstyle.Programmingislikepoetry.Itconveysamessage,notonlytothecomputer,buttothosewhomodifyanduseyourprogram.Withaprogram,youbuildyourownworldwithyourownrules.Youcreateyourworldaccordingtoyourconceptionofboththeproblemandthesolution.Masterfulprogrammerscreateworldswithprogramsthatareclearandsuccinct,muchlikeapoemoressay.Oneofthegreatestprogrammers,DonaldKnuth,describesprogrammingnotastellingacomputerhowtodosomething,buttellingapersonhowtheywouldinstructacomputertodosomething.Thepointisthatprogramsaremeanttobereadbypeople,notjustcomputers.Yourprogramswillbemodiedandupdatedbyotherslongafteryoumoveontootherprojects.Thus,programmingisnotasmuchaboutcommunicatingtoacomputerasitiscommunicatingtothosewhocomeafteryou.Aprogrammerisaproblem-solver,apoet,andaninstructorallatonce.Yourgoalistosolvetheproblemathand,doingsowithbalanceandtaste,andteachyoursolutiontofutureprogrammers.Ihopethatthisbookcanteachatleastsomeofthepoetryandmagicthatmakescomputingexciting.Mostintroductorybooksonprogrammingfrustratemetonoend.Attheendofthemyoucanstillask"howdoesthecomputerreallywork?"andnothaveagoodanswer.Theytendtopassovertopicsthataredifculteventhoughtheyareimportant.Iwilltakeyouthroughthedifcultissuesbecausethatistheonlywaytomoveontomasterfulprogramming.Mygoalistotakeyoufromknowingnothingaboutprogrammingtounderstandinghowtothink,write,andlearnlikeaprogrammer.Youwon'tknoweverything,butyouwillhaveabackgroundforhoweverythingtstogether.Attheendofthisbook,youshouldbeabletodothefollowing: 1 PAGE 8 Chapter1.Introduction Understandhowaprogramworksandinteractswithotherprograms Readotherpeople'sprogramsandlearnhowtheywork Learnnewprogramminglanguagesquickly LearnadvancedconceptsincomputersciencequicklyIwillnotteachyoueverything.Computerscienceisamassiveeld,especiallywhenyoucombinethetheorywiththepracticeofcomputerprogramming.However,Iwillattempttogetyoustartedonthefoundationssoyoucaneasilygowhereveryouwantafterwards.Thereissomewhatofachickenandeggprobleminteachingprogramming,especiallyassemblylanguage.Thereisalottolearn-it'salmosttoomuchtolearnalmostatonce,buteachpiecedependsonalltheothers.Therefore,youmustbepatientwithyourselfandthecomputerwhilelearningtoprogram.Ifyoudon'tunderstandsomethingthersttime,rereadit.Ifyoustilldon'tunderstandit,itissometimesbesttotakeitbyfaithandcomebacktoitlater.Oftenaftermoreexposuretoprogrammingtheideaswillmakemoresense.Don'tgetdiscouraged.It'salongclimb,butveryworthwhile.Attheendofeachchapterarethreesetsofreviewexercises.Therstsetismoreorlessregurgitation-theychecktoseeifcanyougivebackwhatyoulearnedinthechapter.Thesecondsetcontainsapplicationquestions-theychecktoseeifyoucanapplywhatyoulearnedtosolveproblems.Thenalsetistoseeifyouarecapableofbroadeningyourhorizons.Someofthesequestionsmaynotbeanswerableuntillaterinthebook,buttheygiveyousomethingstothinkabout.Otherquestionsrequiresomeresearchintooutsidesourcestodiscovertheanswer.Stillothersrequireyoutosimplyanalyzeyouroptionsandexplainabestsolution.Manyofthequestionsdon'thaverightorwronganswers,butthatdoesn'tmeantheyareunimportant.Learningtheissuesinvolvedinprogramming,learninghowtoresearchanswers,andlearninghowtolookaheadareallamajorpartofaprogrammer'swork.Ifyouhaveproblemsthatyoujustcan'tgetpast,thereisamailinglistforthis 2 PAGE 9 Chapter1.Introduction bookwherereaderscandiscussandgethelpwithwhattheyarereading.Theaddressispgubook-readers@nongnu.org.Thismailinglistisopenforanytypeofquestionordiscussionalongthelinesofthisbook.Youcansubscribetothislistbygoingtohttp://mail.nongnu.org/mailman/listinfo/pgubook-readers.YourToolsThisbookteachesassemblylanguageforx86processorsandtheGNU/Linuxoperatingsystem.ThereforewewillbegivingalloftheexamplesusingtheGNU/LinuxstandardGCCtoolset.IfyouarenotfamiliarwithGNU/LinuxandtheGCCtoolset,theywillbedescribedshortly.IfyouarenewtoLinux,youshouldcheckouttheguideavailableathttp://rute.sourceforge.net/1WhatIintendtoshowyouismoreaboutprogrammingingeneralthanusingaspecictoolsetonaspecicplatform,butstandardizingononemakesthetaskmucheasier.ThosenewtoLinuxshouldalsotrytogetinvolvedintheirlocalGNU/LinuxUser'sGroup.User'sGroupmembersareusuallyveryhelpfulfornewpeople,andwillhelpyoufromeverythingfrominstallingLinuxtolearningtouseitmostefciently.AlistingofGNU/LinuxUser'sGroupsisavailableathttp://www.linux.org/groups/AlloftheseprogramshavebeentestedusingRedHatLinux8.0,andshouldworkwithanyotherGNU/Linuxdistribution,too.2Theywillnotworkwithnon-LinuxoperatingsystemssuchasBSDorothersystems.However,alloftheskillslearnedinthisbookshouldbeeasilytransferabletoanyothersystem.IfyoudonothaveaccesstoaGNU/Linuxmachine,youcanlookforahostingproviderwhooffersaLinuxshellaccount,whichisacommand-lineonlyinterface 1. Thisisquitealargedocument.Youcertainlydon'tneedtoknoweverythingtogetstartedwiththisbook.Yousimplyneedtoknowhowtonavigatefromthecommandlineandhowtouseaneditorlikepico,emacs,orviorothers. 2. By"GNU/Linuxdistribution",Imeananx86GNU/Linuxdistribution.GNU/Linuxdis-tributionsforthePowerMacintosh,theAlphaprocessor,orotherprocessorswillnotworkwiththisbook. 3 PAGE 10 Chapter1.Introduction toaLinuxmachine.Therearemanylow-costshellaccountproviders,butyouhavetomakesurethattheymatchtherequirementsabovei.e.-Linuxonx86.SomeoneatyourlocalGNU/LinuxUser'sGroupmaybeabletogiveyouoneaswell.ShellaccountsonlyrequirethatyoualreadyhaveanInternetconnectionandatelnetprogram.IfyouuseWindows,youalreadyhaveatelnetclient-justclickonstart,thenrun,thentypeintelnet.However,itisusuallybettertodownloadPuTTYfromhttp://www.chiart.greenend.co.uk/~sgtatham/putty/becauseWindows'telnethassomeweirdproblems.TherearealotofoptionsfortheMacintosh,too.NiftyTelnetismyfavorite.Ifyoudon'thaveGNU/Linuxandcan'tndashellaccountservice,thenyoucandownloadKnoppixfromhttp://www.knoppix.org/KnoppixisaGNU/LinuxdistributionthatbootsfromCDsothatyoudon'thavetoactuallyinstallit.Onceyouaredoneusingit,youjustrebootandremovetheCDandyouarebacktoyourregularoperatingsystem.SowhatisGNU/Linux?GNU/LinuxisanoperatingsystemmodeledafterUNIX.TheGNUpartcomesfromtheGNUProjecthttp://www.gnu.org/3,whichincludesmostoftheprogramsyouwillrun,includingtheGCCtoolsetthatwewillusetoprogramwith.TheGCCtoolsetcontainsalloftheprogramsnecessarytocreateprogramsinvariouscomputerlanguages.Linuxisthenameofthekernel.Thekernelisthecorepartofanoperatingsystemthatkeepstrackofeverything.Thekernelisbothanfenceandagate.Asagate,itallowsprogramstoaccesshardwareinauniformway.Withoutthekernel,youwouldhavetowriteprogramstodealwitheverydevicemodelevermade.Thekernelhandlesalldevice-specicinteractionssoyoudon'thaveto.Italsohandlesleaccessandinteractionbetweenprocesses.Forexample,whenyoutype,yourtypinggoesthroughseveralprogramsbeforeithitsyoureditor.First,thekerneliswhathandlesyourhardware,soitisthersttoreceivenoticeaboutthekeypress.Thekeyboardsendsinscancodestothekernel,whichthenconvertsthemtotheactualletters,numbers,andsymbolstheyrepresent.Ifyouareusingawindowing 3. TheGNUProjectisaprojectbytheFreeSoftwareFoundationtoproduceacomplete,freeoperatingsystem. 4 PAGE 11 Chapter1.Introduction systemlikeMicrosoftWindowsortheXWindowSystem,thenthewindowingsystemreadsthekeypressfromthekernel,anddeliversittowhateverprogramiscurrentlyinfocusontheuser'sdisplay.Example1-1.HowthecomputerprocesseskeyboardsigalsKeyboard->Kernel->Windowingsystem->ApplicationprogramThekernelalsocontrolstheowofinformationbetweenprograms.Thekernelisaprogram'sgatetotheworldaroundit.Everytimethatdatamovesbetweenprocesses,thekernelcontrolsthemessaging.Inourkeyboardexampleabove,thekernelwouldhavetobeinvolvedforthewindowingsystemtocommunicatethekeypresstotheapplicationprogram.Asafence,thekernelpreventsprogramsfromaccidentallyoverwritingeachother'sdataandfromaccessinglesanddevicesthattheydon'thavepermissionto.Itlimitstheamountofdamageapoorly-writtenprogramcandotootherrunningprograms.Inourcase,thekernelisLinux.Now,thekernelallbyitselfwon'tdoanything.Youcan'tevenbootupacomputerwithjustakernel.Thinkofthekernelasthewaterpipesforahouse.Withoutthepipes,thefaucetswon'twork,butthepipesareprettyuselessiftherearenofaucets.Together,theuserapplicationsfromtheGNUprojectandotherplacesandthekernelLinuxmakeuptheentireoperatingsystem,GNU/Linux.Forthemostpart,thisbookwillbeusingthecomputer'slow-levelassemblylanguage.Thereareessentiallythreekindsoflanguages:MachineLanguage Thisiswhatthecomputeractuallyseesanddealswith.Everycommandthecomputerseesisgivenasanumberorsequenceofnumbers. 5 PAGE 12 Chapter1.Introduction AssemblyLanguage Thisisthesameasmachinelanguage,exceptthecommandnumbershavebeenreplacedbylettersequenceswhichareeasiertomemorize.Othersmallthingsaredonetomakeiteasieraswell.High-LevelLanguage High-levellanguagesaretheretomakeprogrammingeasier.Assemblylanguagerequiresyoutoworkwiththemachineitself.High-levellanguagesallowyoutodescribetheprograminamorenaturallanguage.Asinglecommandinahigh-levellanguageusuallyisequivalenttoseveralcommandsinanassemblylanguage.Inthisbookwewilllearnassemblylanguage,althoughwewillcoverabitofhigh-levellanguages.Hopefullybylearningassemblylanguage,yourunderstandingofhowprogrammingandcomputersworkwillputyouastepahead. 6 PAGE 13 Chapter2.ComputerArchitectureBeforelearninghowtoprogram,youneedtorstunderstandhowacomputerinterpretsprograms.Youdon'tneedadegreeinelectricalengineering,butyouneedtounderstandsomebasics.ModerncomputerarchitectureisbasedoffofanarchitecturecalledtheVonNeumannarchitecture,namedafteritscreator.TheVonNeumannarchitecturedividesthecomputerupintotwomainparts-theCPUforCentralProcessingUnitandthememory.Thisarchitectureisusedinallmoderncomputers,includingpersonalcomputers,supercomputers,mainframes,andevencellphones.StructureofComputerMemoryTounderstandhowthecomputerviewsmemory,imagineyourlocalpostofce.TheyusuallyhavearoomlledwithPOBoxes.Theseboxesaresimilartocomputermemoryinthateacharenumberedsequencesofxed-sizestoragelocations.Forexample,ifyouhave256megabytesofcomputermemory,thatmeansthatyourcomputercontainsroughly256millionxed-sizestoragelocations.Or,touseouranalogy,256millionPOBoxes.Eachlocationhasanumber,andeachlocationhasthesame,xed-lengthsize.ThedifferencebetweenaPOBoxandcomputermemoryisthatyoucanstorealldifferentkindsofthingsinaPOBox,butyoucanonlystoreasinglenumberinacomputermemorystoragelocation. 7 PAGE 14 Chapter2.ComputerArchitecture MemorylocationsarelikePOBoxesYoumaywonderwhyacomputerisorganizedthisway.Itisbecauseitissimpletoimplement.Ifthecomputerwerecomposedofalotofdifferently-sizedlocations,orifyoucouldstoredifferentkindsofdatainthem,itwouldbedifcultandexpensivetoimplement.Thecomputer'smemoryisusedforanumberofdifferentthings.Alloftheresultsofanycalculationsarestoredinmemory.Infact,everythingthatis"stored"isstoredinmemory.Thinkofyourcomputerathome,andimaginewhatallisstoredinyourcomputer'smemory. Thelocationofyourcursoronthescreen Thesizeofeachwindowonthescreen Theshapeofeachletterofeachfontbeingused Thelayoutofallofthecontrolsoneachwindow Thegraphicsforallofthetoolbaricons 8 PAGE 15 Chapter2.ComputerArchitecture Thetextforeacherrormessageanddialogbox Thelistgoesonandon...Inadditiontoallofthis,theVonNeumannarchitecturespeciesthatnotonlycomputerdatashouldliveinmemory,buttheprogramsthatcontrolthecomputer'soperationshouldlivethere,too.Infact,inacomputer,thereisnodifferencebetweenaprogramandaprogram'sdataexcepthowitisusedbythecomputer.Theyarebothstoredandaccessedthesameway.TheCPUSohowdoesthecomputerfunction?Obviously,simplystoringdatadoesn'tdomuchhelp-youneedtobeabletoaccess,manipulate,andmoveit.That'swheretheCPUcomesin.TheCPUreadsininstructionsfrommemoryoneatatimeandexecutesthem.Thisisknownasthefetch-executecycle.TheCPUcontainsthefollowingelementstoaccomplishthis: ProgramCounter InstructionDecoder Databus General-purposeregisters ArithmeticandlogicunitTheprogramcounterisusedtotellthecomputerwheretofetchthenextinstructionfrom.Wementionedearlierthatthereisnodifferencebetweenthewaydataandprogramsarestored,theyarejustinterpreteddifferentlybytheCPU.Theprogramcounterholdsthememoryaddressofthenextinstructiontobeexecuted.TheCPUbeginsbylookingattheprogramcounter,andfetchingwhatevernumberisstoredinmemoryatthelocationspecied.Itisthenpassedontotheinstruction 9 PAGE 16 Chapter2.ComputerArchitecture decoderwhichguresoutwhattheinstructionmeans.Thisincludeswhatprocessneedstotakeplaceaddition,subtraction,multiplication,datamovement,etc.andwhatmemorylocationsaregoingtobeinvolvedinthisprocess.Computerinstructionsusuallyconsistofboththeactualinstructionandthelistofmemorylocationsthatareusedtocarryitout.Nowthecomputerusesthedatabustofetchthememorylocationstobeusedinthecalculation.ThedatabusistheconnectionbetweentheCPUandmemory.Itistheactualwirethatconnectsthem.Ifyoulookatthemotherboardofthecomputer,thewiresthatgooutfromthememoryareyourdatabus.Inadditiontothememoryontheoutsideoftheprocessor,theprocessoritselfhassomespecial,high-speedmemorylocationscalledregisters.Therearetwokindsofregisters-generalregistersandspecial-purposeregisters.General-purposeregistersarewherethemainactionhappens.Addition,subtraction,multiplication,comparisions,andotheroperationsgenerallyusegeneral-purposeregistersforprocessing.However,computershaveveryfewgeneral-purposeregisters.Mostinformationisstoredinmainmemory,broughtintotheregistersforprocessing,andthenputbackintomemorywhentheprocessingiscompleted.special-purposeregistersareregisterswhichhaveveryspecicpurposes.Wewilldiscusstheseaswecometothem.NowthattheCPUhasretrievedallofthedataitneeds,itpassesonthedataandthedecodedinstructiontothearithmeticandlogicunitforfurtherprocessing.Heretheinstructionisactuallyexecuted.Aftertheresultsofthecomputationhavebeencalculated,theresultsarethenplacedonthedatabusandsenttotheappropriatelocationinmemoryorinaregister,asspeciedbytheinstruction.Thisisaverysimpliedexplanation.Processorshaveadvancedquiteabitinrecentyears,andarenowmuchmorecomplex.Althoughthebasicoperationisstillthesame,itiscomplicatedbytheuseofcachehierarchies,superscalarprocessors,pipelining,branchprediction,out-of-orderexecution,microcodetranslation,coprocessors,andotheroptimizations.Don'tworryifyoudon'tknowwhatthosewordsmean,youcanjustusethemasInternetsearchtermsifyouwant 10 PAGE 17 Chapter2.ComputerArchitecture tolearnmoreabouttheCPU.SomeTermsComputermemoryisanumberedsequenceofxed-sizestoragelocations.Thenumberattachedtoeachstoragelocationiscalledit'saddress.Thesizeofasinglestoragelocationiscalledabyte.Onx86processors,abyteisanumberbetween0and255.Youmaybewonderinghowcomputerscandisplayandusetext,graphics,andevenlargenumberswhenalltheycandoisstorenumbersbetween0and255.Firstofall,specializedhardwarelikegraphicscardshavespecialinterpretationsofeachnumber.Whendisplayingtothescreen,thecomputerusesASCIIcodetablestotranslatethenumbersyouaresendingitintoletterstodisplayonthescreen,witheachnumbertranslatingtoexactlyoneletterornumeral.1Forexample,thecapitalletterAisrepresentedbythenumber65.Thenumeral1isrepresentedbythenumber49.So,toprintout"HELLO",youwouldactuallygivethecomputerthesequenceofnumbers72,69,76,76,79.Toprintoutthenumber100,youwouldgivethecomputerthesequenceofnumbers49,48,48.AlistofASCIIcharactersandtheirnumericcodesisfoundin AppendixD .InadditiontousingnumberstorepresentASCIIcharacters,youastheprogrammergettomakethenumbersmeananythingyouwantthemto,aswell.Forexample,ifIamrunningastore,IwoulduseanumbertorepresenteachitemIwasselling.EachnumberwouldbelinkedtoaseriesofothernumberswhichwouldbetheASCIIcodesforwhatIwantedtodisplaywhentheitemswerescannedin.Iwouldhavemorenumbersfortheprice,howmanyIhaveininventory,andsoon. 1. WiththeadventofinternationalcharactersetsandUnicode,thisisnotentirelytrueanymore.However,forthepurposesofkeepingthissimpleforbeginners,wewillusetheassumptionthatonenumbertranslatesdirectlytoonecharacter.Formoreinformation,see AppendixD 11 PAGE 18 Chapter2.ComputerArchitecture Sowhataboutifweneednumberslargerthan255?Wecansimplyuseacombinationofbytestorepresentlargernumbers.Twobytescanbeusedtorepresentanynumberbetween0and65536.Fourbytescanbeusedtorepresentanynumberbetween0and4294967295.Now,itisquitedifculttowriteprogramstostickbytestogethertoincreasethesizeofyournumbers,andrequiresabitofmath.Luckily,thecomputerwilldoitforusfornumbersupto4byteslong.Infact,four-bytenumbersarewhatwewillworkwithbydefault.Wementionedearlierthatinadditiontotheregularmemorythatthecomputerhas,italsohasspecial-purposestoragelocationscalledregisters.Registersarewhatthecomputerusesforcomputation.Thinkofaregisterasaplaceonyourdesk-itholdsthingsyouarecurrentlyworkingon.Youmayhavelotsofinformationtuckedawayinfoldersanddrawers,butthestuffyouareworkingonrightnowisonthedesk.Registerskeepthecontentsofnumbersthatyouarecurrentlymanipulating.Onthecomputersweareusing,registersareeachfourbyteslong.Thesizeofatypicalregisteriscalledacomputer'swordsize.x86processorshavefour-bytewords.Thismeansthatitismostnaturalonthesecomputerstodocomputationsfourbytesatatime.Thisgivesusroughly4billionvalues.Addressesarealsofourbyteswordlong,andthereforealsotintoaregister.x86processorscanaccessupto4294967296bytesifenoughmemoryisinstalled.Noticethatthismeansthatwecanstoreaddressesthesamewaywestoreanyothernumber.Infact,thecomputercan'ttellthedifferencebetweenavaluethatisanaddress,avaluethatisanumber,avaluethatisanASCIIcode,oravaluethatyouhavedecidedtouseforanotherpurpose.AnumberbecomesanASCIIcodewhenyouattempttodisplayit.Anumberbecomesanaddresswhenyoutrytolookupthebyteitpointsto.Takeamomenttothinkaboutthis,becauseitiscrucialtounderstandinghowcomputerprogramswork.Addresseswhicharestoredinmemoryarealsocalledpointers,becauseinsteadofhavingaregularvalueinthem,theypointyoutoadifferentlocationinmemory.Aswe'vementioned,computerinstructionsarealsostoredinmemory.Infact, 12 PAGE 19 Chapter2.ComputerArchitecture theyarestoredexactlythesamewaythatotherdataisstored.Theonlywaythecomputerknowsthatamemorylocationisaninstructionisthataspecial-purposeregistercalledtheinstructionpointerpointstothematonepointoranother.Iftheinstructionpointerpointstoamemoryword,itisloadedasaninstruction.Otherthanthat,thecomputerhasnowayofknowingthedifferencebetweenprogramsandothertypesofdata.2InterpretingMemoryComputersareveryexact.Becausetheyareexact,programmershavetobeequallyexact.Acomputerhasnoideawhatyourprogramissupposedtodo.Therefore,itwillonlydoexactlywhatyoutellittodo.IfyouaccidentallyprintoutaregularnumberinsteadoftheASCIIcodesthatmakeupthenumber'sdigits,thecomputerwillletyou-andyouwillwindupwithjibberishonyourscreenitwilltrytolookupwhatyournumberrepresentsinASCIIandprintthat.Ifyoutellthecomputertostartexecutinginstructionsatalocationcontainingdatainsteadofprograminstructions,whoknowshowthecomputerwillinterpretthat-butitwillcertainlytry.Thecomputerwillexecuteyourinstructionsintheexactorderyouspecify,evenifitdoesn'tmakesense.Thepointis,thecomputerwilldoexactlywhatyoutellit,nomatterhowlittlesenseitmakes.Therefore,asaprogrammer,youneedtoknowexactlyhowyouhaveyourdataarrangedinmemory.Remember,computerscanonlystorenumbers,soletters,pictures,music,webpages,documents,andanythingelsearejustlongsequencesofnumbersinthecomputer,whichparticularprogramsknowhowtointerpret.Forexample,saythatyouwantedtostorecustomerinformationinmemory.Onewaytodosowouldbetosetamaximumsizeforthecustomer'snameandaddress-say50ASCIIcharactersforeach,whichwouldbe50bytesforeach.Then,after 2. Notethatherewearetalkingaboutgeneralcomputertheory.Someprocessorsandop-eratingsystemsactuallymarktheregionsofmemorythatcanbeexecutedwithaspecialmarkerthatindicatesthis. 13 PAGE 20 Chapter2.ComputerArchitecture that,haveanumberforthecustomer'sageandtheircustomerid.Inthiscase,youwouldhaveablockofmemorythatwouldlooklikethis:StartofRecord:Customer'snamebytes-startofrecordCustomer'saddressbytes-startofrecord+50bytesCustomer'sageword-4bytes-startofrecord+100bytesCustomer'sidnumberword-4bytes-startofrecord+104bytesThisway,giventheaddressofacustomerrecord,youknowwheretherestofthedatalies.However,itdoeslimitthecustomer'snameandaddresstoonly50ASCIIcharacterseach.Whatifwedidn'twanttospecifyalimit?Anotherwaytodothiswouldbetohaveinourrecordpointerstothisinformation.Forexample,insteadofthecustomer'sname,wewouldhaveapointertotheirname.Inthiscase,thememorywouldlooklikethis:StartofRecord:Customer'snamepointerword-startofrecordCustomer'saddresspointerword-startofrecord+4Customer'sageword-startofrecord+8Customer'sidnumberword-startofrecord+12Theactualnameandaddresswouldbestoredelsewhereinmemory.Thisway,itiseasytotellwhereeachpartofthedataisfromthestartoftherecord,withoutexplicitlylimittingthesizeofthenameandaddress.Ifthelengthoftheeldswithinourrecordscouldchange,wewouldhavenoideawherethenexteldstarted.Becauserecordswouldbedifferentsizes,itwouldalsobehardtondwherethenextrecordbegan.Therefore,almostallrecordsareofxedlengths.Variable-lengthdataisusuallystoreseparatelyfromtherestoftherecord. 14 PAGE 21 Chapter2.ComputerArchitecture DataAccessingMethodsProcessorshaveanumberofdifferentwaysofaccessingdata,knownasaddressingmodes.Thesimplestmodeisimmediatemode,inwhichthedatatoaccessisembeddedintheinstructionitself.Forexample,ifwewanttoinitializearegisterto0,insteadofgivingthecomputeranaddresstoreadthe0from,wewouldspecifyimmediatemode,andgiveitthenumber0.Intheregisteraddressingmode,theinstructioncontainsaregistertoaccess,ratherthanamemorylocation.Therestofthemodeswilldealwithaddresses.Inthedirectaddressingmode,theinstructioncontainsthememoryaddresstoaccess.Forexample,Icouldsay,pleaseloadthisregisterwiththedataataddress2002.Thecomputerwouldgodirectlytobytenumber2002andcopythecontentsintoourregister.Intheindexedaddressingmode,theinstructioncontainsamemoryaddresstoaccess,andalsospeciesanindexregistertooffsetthataddress.Forexample,wecouldspecifyaddress2002andanindexregister.Iftheindexregistercontainsthenumber4,theactualaddressthedataisloadedfromwouldbe2006.Thisway,ifyouhaveasetofnumbersstartingatlocation2002,youcancyclebetweeneachofthemusinganindexregister.Onx86processors,youcanalsospecifyamultiplierfortheindex.Thisallowsyoutoaccessmemoryabyteatatimeorawordatatimebytes.Ifyouareaccessinganentireword,yourindexwillneedtobemultipliedby4togettheexactlocationofthefourthelementfromyouraddress.Forexample,ifyouwantedtoaccessthefourthbytefromlocation2002,youwouldloadyourindexregisterwith3remember,westartcountingat0andsetthemultiplierto1sinceyouaregoingabyteatatime.Thiswouldgetyoulocation2005.However,ifyouwantedtoaccessthefourthwordfromlocation2002,youwouldloadyourindexregisterwith3andsetthemultiplierto4.Thiswouldloadfromlocation2014-thefourthword.Takethetimetocalculatetheseyourselftomakesureyouunderstandhowitworks.Intheindirectaddressingmode,theinstructioncontainsaregisterthatcontainsapointertowherethedatashouldbeaccessed.Forexample,ifweusedindirect 15 PAGE 22 Chapter2.ComputerArchitecture addressingmodeandspeciedthe%eaxregister,andthe%eaxregistercontainedthevalue4,whatevervaluewasatmemorylocation4wouldbeused.Indirectaddressing,wewouldjustloadthevalue4,butinindirectaddressing,weuse4astheaddresstousetondthedatawewant.Finally,thereisthebasepointeraddressingmode.Thisissimilartoindirectaddressing,butyoualsoincludeanumbercalledtheoffsettoaddtotheregister'svaluebeforeusingitforlookup.Wewillusethismodequiteabitinthisbook.In theSectioncalledInterpretingMemory wediscussedhavingastructureinmemoryholdingcustomerinformation.Let'ssaywewantedtoaccessthecustomer'sage,whichwastheeighthbyteofthedata,andwehadtheaddressofthestartofthestructureinaregister.Wecouldusebasepointeraddressingandspecifytheregisterasthebasepointer,and8asouroffset.Thisisalotlikeindexedaddressing,withthedifferencethattheoffsetisconstantandthepointerisheldinaregister,andinindexedaddressingtheoffsetisinaregisterandthepointerisconstant.Thereareotherformsofaddressing,butthesearethemostimportantones.ReviewKnowtheConcepts Describethefetch-executecycle. Whatisaregister?Howwouldcomputationbemoredifcultwithoutregisters? Howdoyourepresentnumberslargerthan255? Howbigaretheregistersonthemachineswewillbeusing? Howdoesacomputerknowhowtointerpretagivenbyteorsetofbytesofmemory? 16 PAGE 23 Chapter2.ComputerArchitecture Whataretheaddressingmodesandwhataretheyusedfor? Whatdoestheinstructionpointerdo?UsetheConcepts Whatdatawouldyouuseinanemployeerecord?Howwouldyoulayitoutinmemory? IfIhadthepointerthethebeginningoftheemployeerecordabove,andwantedtoaccessaparticularpieceofdatainsideofit,whataddressingmodewouldIuse? Inbasepointeraddressingmode,ifyouhavearegisterholdingthevalue3122,andanoffsetof20,whataddresswouldyoubetryingtoaccess? Inindexedaddressingmode,ifthebaseaddressis6512,theindexregisterhasa5,andthemultiplieris4,whataddresswouldyoubetryingtoaccess? Inindexedaddressingmode,ifthebaseaddressis123472,theindexregisterhasa0,andthemultiplieris4,whataddresswouldyoubetryingtoaccess? Inindexedaddressingmode,ifthebaseaddressis9123478,theindexregisterhasa20,andthemultiplieris1,whataddresswouldyoubetryingtoaccess?GoingFurther Whataretheminimumnumberofaddressingmodesneededforcomputation? Whyincludeaddressingmodesthataren'tstrictlyneeded? Researchandthendescribehowpipeliningoroneoftheothercomplicatingfactorsaffectsthefetch-executecycle. 17 PAGE 24 Chapter2.ComputerArchitecture Researchandthendescribethetradeoffsbetweenxed-lengthinstructionsandvariable-lengthinstructions. 18 PAGE 25 Chapter3.YourFirstProgramsInthischapteryouwilllearntheprocessforwritingandbuildingLinuxassembly-languageprograms.Inaddition,youwilllearnthestructureofassembly-languageprograms,andafewassembly-languagecommands.Asyougothroughthischapter,youmaywanttoreferalsoto AppendixB and AppendixF .Theseprogramsmayoverwhelmyouatrst.However,gothroughthemwithdiligence,readthemandtheirexplanationsasmanytimesasnecessary,andyouwillhaveasolidfoundationofknowledgetobuildon.Pleasetinkeraroundwiththeprogramsasmuchasyoucan.Evenifyourtinkeringdoesnotwork,everyfailurewillhelpyoulearn.EnteringintheProgramOkay,thisrstprogramissimple.Infact,it'snotgoingtodoanythingbutexit!It'sshort,butitshowssomebasicsaboutassemblylanguageandLinuxprogramming.Youneedtoentertheprograminaneditorexactlyaswritten,withthelenameexit.s.Theprogramfollows.Don'tworryaboutnotunderstandingit.Thissectiononlydealswithtypingitinandrunningit.In theSectioncalledOutlineofanAssemblyLanguageProgram wewilldescribehowitworks.#PURPOSE:Simpleprogramthatexitsandreturnsa#statuscodebacktotheLinuxkernel##INPUT:none##OUTPUT:returnsastatuscode.Thiscanbeviewed#bytyping##echo$?# 19 PAGE 26 Chapter3.YourFirstPrograms #afterrunningtheprogram##VARIABLES:#%eaxholdsthesystemcallnumber#%ebxholdsthereturnstatus#.section.data.section.text.globl_start_start:movl$1,%eax#thisisthelinuxkernelcommand#numbersystemcallforexiting#aprogrammovl$0,%ebx#thisisthestatusnumberwewill#returntotheoperatingsystem.#Changethisaroundanditwill#returndifferentthingsto#echo$?int$0x80#thiswakesupthekerneltorun#theexitcommandWhatyouhavetypediniscalledthesourcecode.Sourcecodeisthehuman-readableformofaprogram.Inordertotransformitintoaprogramthatacomputercanrun,weneedtoassembleandlinkit.Therststepistoassembleit.Assemblingistheprocessthattransformswhatyoutypedintoinstructionsforthemachine.Themachineitselfonlyreadssetsofnumbers,buthumanspreferwords.Anassemblylanguageisamorehuman-readableformoftheinstructionsacomputerunderstands.Assembling 20 PAGE 27 Chapter3.YourFirstPrograms transformsthehuman-readableleintoamachine-readableone.Toassemblytheprogramtypeinthecommandasexit.s-oexit.oasisthecommandwhichrunstheassembler,exit.sisthesourcele,and-oexit.otellstheassembletoputit'soutputintheleexit.o.exit.oisanobjectle.Anobjectleiscodethatisinthemachine'slanguage,buthasnotbeencompletelyputtogether.Inmostlargeprograms,youwillhaveseveralsourceles,andyouwillconverteachoneintoanobjectle.Thelinkeristheprogramthatisresponsibleforputtingtheobjectlestogetherandaddinginformationtoitsothatthekernelknowshowtoloadandrunit.Inourcase,weonlyhaveoneobjectle,sothelinkerisonlyaddingtheinformationtoenableittorun.Tolinkthele,enterthecommandldexit.o-oexitldisthecommandtorunthelinker,exit.oistheobjectlewewanttolink,and-oexitinstructsthelinkertooutputthenewprogramintoalecalledexit.1Ifanyofthesecommandsreportederrors,youhaveeithermistypedyourprogramorthecommand.Aftercorrectingtheprogram,youhavetore-runallthecommands.Youmustalwaysre-assembleandre-linkprogramsafteryoumodifythesourceleforthechangestooccurintheprogram.Youcanrunexitbytypinginthecommand./exitThe./isusedtotellthecomputerthattheprogramisn'tinoneofthenormalprogramdirectories,butisthecurrentdirectoryinstead2.You'llnoticewhenyoutypethiscommand,theonlythingthathappensisthatyou'llgotothenextline. 1. IfyouarenewtoLinuxandUNIX,youmaynotbeawarethatlesdon'thavetohaveextensions.Infact,whileWindowsusesthe.exeextensiontosignifyanexecutableprogram,UNIXexecutablesusuallyhavenoextension. 2. .referstothecurrentdirectoryinLinuxandUNIXsystems. 21 PAGE 28 Chapter3.YourFirstPrograms That'sbecausethisprogramdoesnothingbutexit.However,immediatelyafteryouruntheprogram,ifyoutypeinecho$?Itwillsay0.WhatishappeningisthateveryprogramwhenitexitsgivesLinuxanexitstatuscode,whichtellsitifeverythingwentallright.Ifeverythingwasokay,itreturns0.UNIXprogramsreturnnumbersotherthanzerotoindicatefailureorothererrors,warnings,orstatuses.Theprogrammerdetermineswhateachnumbermeans.Youcanviewthiscodebytypinginecho$?.Inthefollowingsectionwewilllookatwhateachpartofthecodedoes.OutlineofanAssemblyLanguageProgramTakealookattheprogramwejustentered.Atthebeginningtherearelotsoflinesthatbeginwithhashes#.Thesearecomments.Commentsarenottranslatedbytheassembler.Theyareusedonlyfortheprogrammertotalktoanyonewholooksatthecodeinthefuture.Mostprogramsyouwritewillbemodiedbyothers.Getintothehabitofwritingcommentsinyourcodethatwillhelpthemunderstandbothwhytheprogramexistsandhowitworks.Alwaysincludethefollowinginyourcomments: Thepurposeofthecode Anoverviewoftheprocessinginvolved Anythingstrangeyourprogramdoesandwhyitdoesit3Afterthecomments,thenextlinesays 3. You'llndthatmanyprogramsendupdoingthingsstrangeways.Usuallythereisarea-sonforthat,but,unfortunately,programmersneverdocumentsuchthingsintheircomments.So,futureprogrammerseitherhavetolearnthereasonthehardwaybymodifyingthecodeandwatchingitbreak,orjustleavingitalonewhetheritisstillneededornot.Youshouldalwaysdocumentanystrangebehavioryourprogramperforms.Unfortunately,guringoutwhatisstrangeandwhatisstraightforwardcomesmostlywithexperience. 22 PAGE 29 Chapter3.YourFirstPrograms .section.dataAnythingstartingwithaperiodisn'tdirectlytranslatedintoamachineinstruction.Instead,it'saninstructiontotheassembleritself.Thesearecalledassemblerdirectivesorpseudo-operationsbecausetheyarehandledbytheassemblerandarenotactuallyrunbythecomputer.The.sectioncommandbreaksyourprogramupintosections.Thiscommandstartsthedatasection,whereyoulistanymemorystorageyouwillneedfordata.Ourprogramdoesn'tuseany,sowedon'tneedthesection.It'sjusthereforcompleteness.Almosteveryprogramyouwriteinthefuturewillhavedata.Rightafterthisyouhave.section.textwhichstartsthetextsection.Thetextsectionofaprogramiswheretheprograminstructionslive.Thenextinstructionis.globl_startThisinstructstheassemblerthat_startisimportanttoremember._startisasymbol,whichmeansthatitisgoingtobereplacedbysomethingelseeitherduringassemblyorlinking.Symbolsaregenerallyusedtomarklocationsofprogramsordata,soyoucanrefertothembynameinsteadofbytheirlocationnumber.Imagineifyouhadtorefertoeverymemorylocationbyit'saddress.Firstofall,itwouldbeveryconfusingbecauseyouwouldhavetomemorizeorlookupthenumericmemoryaddressofeverypieceofcodeordata.Inaddition,everytimeyouhadtoinsertapieceofdataorcodeyouwouldhavetochangealltheaddressesinyourprogram!Symbolsareusedsothattheassemblerandlinkercantakecareofkeepingtrackofaddresses,andyoucanconcentrateonwritingyourprogram. 23 PAGE 30 Chapter3.YourFirstPrograms .globlmeansthattheassemblershouldn'tdiscardthissymbolafterassembly,becausethelinkerwillneedit._startisaspecialsymbolthatalwaysneedstobemarkedwith.globlbecauseitmarksthelocationofthestartoftheprogram.Withoutmarkingthislocationinthisway,whenthecomputerloadsyourprogramitwon'tknowwheretobeginrunningyourprogram.Thenextline_start:denesthevalueofthe_startlabel.Alabelisasymbolfollowedbyacolon.Labelsdeneasymbol'svalue.Whentheassemblerisassemblingtheprogram,ithastoassigneachdatavalueandinstructionanaddress.Labelstelltheassemblertomakethesymbol'svaluebewhereverthenextinstructionordataelementwillbe.Thisway,iftheactualphysicallocationofthedataorinstructionchanges,youdon'thavetorewriteanyreferencestoit-thesymbolautomaticallygetsthenewvalue.Nowwegetintoactualcomputerinstructions.Therstsuchinstructionisthis:movl$1,%eaxWhentheprogramruns,thisinstructiontransfersthenumber1intothe%eaxregister.Inassemblylanguage,manyinstructionshaveoperands.movlhastwooperands-thesourceandthedestination.Inthiscase,thesourceistheliteralnumber1,andthedestinationisthe%eaxregister.Operandscanbenumbers,memorylocationreferences,orregisters.Differentinstructionsallowdifferenttypesofoperands.See AppendixB formoreinformationonwhichinstructionstakewhichkindsofoperands.Onmostinstructionswhichhavetwooperands,therstoneisthesourceoperandandthesecondoneisthedestination.Notethatinthesecases,thesourceoperandisnotmodiedatall.Otherinstructionsofthistypeare,forexample,addl,subl,andimull.Theseadd/subtract/multiplythesourceoperandfrom/to/bythedestinationoperandandandsavetheresultinthedestinationoperand.Other 24 PAGE 31 Chapter3.YourFirstPrograms instructionsmayhaveanoperandhardcodedin.idivl,forexample,requiresthatthedividendbein%eax,and%edxbezero,andthequotientisthentransferredto%eaxandtheremainderto%edx.However,thedivisorcanbeanyregisterormemorylocation.Onx86processors,thereareseveralgeneral-purposeregisters4allofwhichcanbeusedwithmovl: %eax %ebx %ecx %edx %edi %esiInadditiontothesegeneral-purposeregisters,therearealsoseveralspecial-purposeregisters,including: %ebp %esp %eip %eflagsWe'lldiscusstheselater,justbeawarethattheyexist.5Someoftheseregisters, 4. Notethatonx86processors,eventhegeneral-purposeregistershavesomespecialpur-poses,orusedtobeforeitwent32-bit.However,thesearegeneral-purposeregistersformostinstructions.Eachofthemhasatleastoneinstructionwhereitisusedinaspecialway.However,formostofthem,thoseinstructionsaren'tcoveredinthisbook. 5. Youmaybewondering,whydoalloftheseregistersbeginwiththelettere?Thereasonisthatearlygenerationsofx86processorswere16bitsratherthan32bits.Therefore,theregisterswereonlyhalfthelengththeyarenow.Inlatergenerationsofx86processors,thesizeoftheregistersdoubled.Theykepttheoldnamestorefertothersthalfoftheregister, 25 PAGE 32 Chapter3.YourFirstPrograms like%eipand%eflagscanonlybeaccessedthroughspecialinstructions.Theotherscanbeaccessedusingthesameinstructionsasgeneral-purposeregisters,buttheyhavespecialmeanings,specialuses,oraresimplyfasterwhenusedinaspecicway.So,themovlinstructionmovesthenumber1into%eax.Thedollar-signinfrontoftheoneindicatesthatwewanttouseimmediatemodeaddressingreferbackto theSectioncalledDataAccessingMethodsinChapter2 .Withoutthedollar-signitwoulddodirectaddressing,loadingwhatevernumberisataddress1.Wewanttheactualnumber1loadedin,sowehavetouseimmediatemode.Thereasonwearemovingthenumber1into%eaxisbecausewearepreparingtocalltheLinuxKernel.Thenumber1isthenumberoftheexitsystemcall.Wewilldiscusssystemcallsinmoredepthsoon,butbasicallytheyarerequestsfortheoperatingsystem'shelp.Normalprogramscan'tdoeverything.Manyoperationssuchascallingotherprograms,dealingwithles,andexitinghavetobehandledbytheoperatingsystemthroughsystemcalls.Whenyoumakeasystemcall,whichwewilldoshortly,thesystemcallnumberhastobeloadedinto%eaxforacompletelistingofsystemcallsandtheirnumbers,see AppendixC .Dependingonthesystemcall,otherregistersmayhavetohavevaluesinthemaswell.Notethatsystemcallsisnottheonlyuseoreventhemainuseofregisters.Itisjusttheonewearedealingwithinthisrstprogram.Laterprogramswilluseregistersforregularcomputation.Theoperatingsystem,however,usuallyneedsmoreinformationthanjustwhichcalltomake.Forexample,whendealingwithles,theoperatingsystemneedstoknowwhichleyouaredealingwith,whatdatayouwanttowrite,andotherdetails.Theextradetails,calledparametersarestoredinotherregisters.Inthecaseoftheexitsystemcall,theoperatingsystemrequiresastatuscodebeloaded andaddedanetorefertotheextendedversionsoftheregister.Usuallyyouwillonlyusetheextendedversions.Newermodelsalsooffera64-bitmode,whichdoublesthesizeoftheseregistersyetagainandusesanrprextoindicatethelargerregistersi.e.%raxisthe64-bitversionof%eax.However,theseprocessorsarenotwidelyused,andarenotcoveredinthisbook. 26 PAGE 33 Chapter3.YourFirstPrograms in%ebx.Thisvalueisthenreturnedtothesystem.Thisisthevalueyouretrievedwhenyoutypedecho$?.So,weload%ebxwith0bytypingthefollowing:movl$0,%ebxNow,loadingregisterswiththesenumbersdoesn'tdoanythingitself.Registersareusedforallsortsofthingsbesidessystemcalls.Theyarewhereallprogramlogicsuchasaddition,subtraction,andcomparisonstakeplace.Linuxsimplyrequiresthatcertainregistersbeloadedwithcertainparametervaluesbeforemakingasystemcall.%eaxisalwaysrequiredtobeloadedwiththesystemcallnumber.Fortheotherregisters,however,eachsystemcallhasdifferentrequirements.Intheexitsystemcall,%ebxisrequiredtobeloadedwiththeexitstatus.Wewilldiscussdifferentsystemcallsastheyareneeded.Foralistofcommonsystemcallsandwhatisrequiredtobeineachregister,see AppendixC Thenextinstructionisthe"magic"one.Itlookslikethis:int$0x80Theintstandsforinterrupt.The0x80istheinterruptnumbertouse.6Aninterruptinterruptsthenormalprogramow,andtransferscontrolfromourprogramtoLinuxsothatitwilldoasystemcall.7.YoucanthinkofitaslikesignalingBatmanorLarry-Boy8,ifyouprefer.Youneedsomethingdone,yousendthesignal,andthenhecomestotherescue.Youdon'tcarehowhedoeshiswork-it'smoreorlessmagic-andwhenhe'sdoneyou'rebackincontrol.Inthiscase,allwe'redoingisaskingLinuxtoterminatetheprogram,inwhichcasewe 6. Youmaybewonderingwhyit's0x80insteadofjust80.Thereasonisthatthenumberiswritteninhexadecimal.Inhexadecimal,asingledigitcanhold16valuesinsteadofthenormal10.Thisisdonebyutilizingthelettersathroughfinadditiontotheregulardigits.arepresents10,brepresents11,andsoon.0x10representsthenumber16,andsoon.Thiswillbediscussedmoreindepthlater,butjustbeawarethatnumbersstartingwith0xareinhexadecimal.TackingonanHattheendisalsosometimesusedinstead,butwewon'tdothatinthisbook.Formoreinformationaboutthis,see Chapter10 7. Actually,theinterrupttransferscontroltowhoeversetupaninterrupthandlerfortheinterruptnumber.InthecaseofLinux,allofthemaresettobehandledbytheLinuxkernel. 8. Ifyoudon'twatchVeggieTales,youshould.StartwithDaveandtheGiantPickle. 27 PAGE 34 Chapter3.YourFirstPrograms won'tbebackincontrol.Ifwedidn'tsignaltheinterrupt,thennosystemcallwouldhavebeenperformed.QuickSystemCallReview:Torecap-OperatingSystemfeaturesareaccessedthroughsystemcalls.Theseareinvokedbysettinguptheregistersinaspecialwayandissuingtheinstructionint$0x80.Linuxknowswhichsystemcallwewanttoaccessbywhatwestoredinthe%eaxregister.Eachsystemcallhasotherrequirementsastowhatneedstobestoredintheotherregisters.Systemcallnumber1istheexitsystemcall,whichrequiresthestatuscodetobeplacedin%ebx.Nowthatyou'veassembled,linked,run,andexaminedtheprogram,youshouldmakesomebasicedits.Dothingslikechangethenumberthatisloadedinto%ebx,andwatchitcomeoutattheendwithecho$?.Don'tforgettoassembleandlinkitagainbeforerunningit.Addsomecomments.Don'tworry,theworsethingthatwouldhappenisthattheprogramwon'tassembleorlink,orwillfreezeyourscreen.That'sjustpartoflearning!PlanningtheProgramInournextprogramwewilltrytondthemaximumofalistofnumbers.Computersareverydetail-oriented,soinordertowritetheprogramwewillhavetohaveplannedoutanumberofdetails.Thesedetailsinclude: Wherewilltheoriginallistofnumbersbestored? Whatprocedurewillweneedtofollowtondthemaximumnumber? Howmuchstoragedoweneedtocarryoutthatprocedure? Willallofthestoragetintoregisters,ordoweneedtousesomememoryaswell? 28 PAGE 35 Chapter3.YourFirstPrograms Youmightnotthinkthatsomethingassimpleasndingthemaximumnumberfromalistwouldtakemuchplanning.Youcanusuallytellpeopletondthemaximumnumber,andtheycandosowithlittletrouble.However,ourmindsareusedtoputtingtogethercomplextasksautomatically.Computersneedtobeinstructedthroughtheprocess.Inaddition,wecanusuallyholdanynumberofthingsinourmindwithoutmuchtrouble.Weusuallydon'tevenrealizewearedoingit.Forexample,ifyouscanalistofnumbersforthemaximum,youwillprobablykeepinmindboththehighestnumberyou'veseensofar,andwhereyouareinthelist.Whileyourminddoesthisautomatically,withcomputersyouhavetoexplicitlysetupstorageforholdingthecurrentpositiononthelistandthecurrentmaximumnumber.Youalsohaveotherproblemssuchashowtoknowwhentostop.Whenreadingapieceofpaper,youcanstopwhenyourunoutofnumbers.However,thecomputeronlycontainsnumbers,soithasnoideawhenithasreachedthelastofyournumbers.Incomputers,youhavetoplaneverystepoftheway.So,let'sdoalittleplanning.Firstofall,justforreference,let'snametheaddresswherethelistofnumbersstartsasdata_items.Let'ssaythatthelastnumberinthelistwillbeazero,soweknowwheretostop.Wealsoneedavaluetoholdthecurrentpositioninthelist,avaluetoholdthecurrentlistelementbeingexamined,andthecurrenthighestvalueonthelist.Let'sassigneachofthesearegister: %ediwillholdthecurrentpositioninthelist. %ebxwillholdthecurrenthighestvalueinthelist. %eaxwillholdthecurrentelementbeingexamined.Whenwebegintheprogramandlookattherstiteminthelist,sincewehaven'tseenanyotheritems,thatitemwillautomaticallybethecurrentlargestelementinthelist.Also,wewillsetthecurrentpositioninthelisttobezero-therstelement.Fromthen,wewillfollowthefollowingsteps: 29 PAGE 36 Chapter3.YourFirstPrograms 1. Checkthecurrentlistelement%eaxtoseeifit'szerotheterminatingelement. 2. Ifitiszero,exit. 3. Increasethecurrentposition%edi. 4. Loadthenextvalueinthelistintothecurrentvalueregister%eax.Whataddressingmodemightweusehere?Why? 5. Comparethecurrentvalue%eaxwiththecurrenthighestvalue%ebx. 6. Ifthecurrentvalueisgreaterthanthecurrenthighestvalue,replacethecurrenthighestvaluewiththecurrentvalue. 7. Repeat.Thatistheprocedure.ManytimesinthatprocedureImadeuseoftheword"if".Theseplacesarewheredecisionsaretobemade.Yousee,thecomputerdoesn'tfollowtheexactsamesequenceofinstructionseverytime.Dependingonwhich"if"sarecorrect,thecomputermayfollowadifferentsetofinstructions.Thesecondtimethrough,itmightnothavethehighestvalue.Inthatcase,itwillskipstep6,butcomebacktostep7.Ineverycaseexceptthelastone,itwillskipstep2.Inmorecomplicatedprograms,theskippingaroundincreasesdramatically.These"if"sareaclassofinstructionscalledowcontrolinstructions,becausetheytellthecomputewhichstepstofollowandwhichpathstotake.Inthepreviousprogram,wedidnothaveanyowcontrolinstructions,astherewasonlyonepossiblepathtotake-exit.Thisprogramismuchmoredynamicinthatitisdirectedbydata.Dependingonwhatdataitreceives,itwillfollowdifferentinstructionpaths.Inthisprogram,thiswillbeaccomplishedbytwodifferentinstructions,theconditionaljumpandtheunconditionaljump.Theconditionaljumpchangespathsbasedontheresultsofapreviouscomparisonorcalculation.Theunconditionaljumpjustgoesdirectlytoadifferentpathnomatterwhat.Theunconditionaljumpmayseemuseless,butitisverynecessarysincealloftheinstructionswillbelaid 30 PAGE 37 Chapter3.YourFirstPrograms outonaline.Ifapathneedstoconvergebacktothemainpath,itwillhavetodothisbyanunconditionaljump.Wewillseemoreofbothofthesejumpsinthenextsection.Anotheruseofowcontrolisinimplementingloops.Aloopisapieceofprogramcodethatismeanttoberepeated.Inourexample,therstpartoftheprogramsettingthecurrentpositionto0andloadingthecurrenthighestvaluewiththecurrentvaluewasonlydoneonce,soitwasn'taloop.However,thenextpartisrepeatedoverandoveragainforeverynumberinthelist.Itisonlyleftwhenwehavecometothelastelement,indicatedbyazero.Thisiscalledaloopbecauseitoccursoverandoveragain.Itisimplementedbydoingunconditionaljumpstothebeginningoftheloopattheendoftheloop,whichcausesittostartover.However,youhavetoalwaysremembertohaveaconditionaljumptoexittheloopsomewhere,ortheloopwillcontinueforever!Thisconditioniscalledaninniteloop.Ifweaccidentallyleftoutstep1,2,or3,theloopandourprogramwouldneverend.Inthenextsection,wewillimplementthisprogramthatwehaveplanned.Programplanningsoundscomplicated-anditis,tosomedegree.Whenyourststartprogramming,it'softenhardtoconvertournormalthoughtprocessintoaprocedurethatthecomputercanunderstand.Weoftenforgetthenumberof"temporarystoragelocations"thatourmindsareusingtoprocessproblems.Asyoureadandwriteprograms,however,thiswilleventuallybecomeverynaturaltoyou.Justhavepatience.FindingaMaximumValueEnterthefollowingprogramasmaximum.s:#PURPOSE:Thisprogramfindsthemaximumnumberofa#setofdataitems.# 31 PAGE 38 Chapter3.YourFirstPrograms #VARIABLES:Theregistershavethefollowinguses:##%edi-Holdstheindexofthedataitembeingexamined#%ebx-Largestdataitemfound#%eax-Currentdataitem##Thefollowingmemorylocationsareused:##data_items-containstheitemdata.A0isused#toterminatethedata#.section.datadata_items:#Thesearethedataitems.long3,67,34,222,45,75,54,34,44,33,22,11,66,0.section.text.globl_start_start:movl$0,%edi#move0intotheindexregistermovldata_items,%edi,4,%eax#loadthefirstbyteofdatamovl%eax,%ebx#sincethisisthefirstitem,%eaxis#thebiggeststart_loop:#startloopcmpl$0,%eax#checktoseeifwe'vehittheendjeloop_exitincl%edi#loadnextvaluemovldata_items,%edi,4,%eaxcmpl%ebx,%eax#comparevaluesjlestart_loop#jumptoloopbeginningifthenew#oneisn'tbiggermovl%eax,%ebx#movethevalueasthelargest 32 PAGE 39 Chapter3.YourFirstPrograms jmpstart_loop#jumptoloopbeginningloop_exit:#%ebxisthestatuscodefortheexitsystemcall#anditalreadyhasthemaximumnumbermovl$1,%eax#1istheexitsyscallint$0x80Now,assembleandlinkitwiththesecommands:asmaximum.s-omaximum.oldmaximum.o-omaximumNowrunit,andcheckit'sstatus../maximumecho$?You'llnoticeitreturnsthevalue222.Let'stakealookattheprogramandwhatitdoes.Ifyoulookinthecomments,you'llseethattheprogramndsthemaximumofasetofnumbersaren'tcommentswonderful!.Youmayalsonoticethatinthisprogramweactuallyhavesomethinginthedatasection.Theselinesarethedatasection:data_items:#Thesearethedataitems.long3,67,34,222,45,75,54,34,44,33,22,11,66,0Letslookatthis.data_itemsisalabelthatreferstothelocationthatfollowsit.Then,thereisadirectivethatstartswith.long.Thatcausestheassemblertoreservememoryforthelistofnumbersthatfollowit.data_itemsreferstothelocationoftherstone.Becausedata_itemsisalabel,anytimeinourprogramwhereweneedtorefertothisaddresswecanusethedata_itemssymbol,andtheassemblerwillsubstituteitwiththeaddresswherethenumbersstartduringassembly.Forexample,theinstructionmovldata_items,%eaxwouldmove 33 PAGE 40 Chapter3.YourFirstPrograms thevalue3into%eax.Thereareseveraldifferenttypesofmemorylocationsotherthan.longthatcanbereserved.Themainonesareasfollows:.byte Bytestakeuponestoragelocationforeachnumber.Theyarelimitedtonumbersbetween0and255..int Intswhichdifferfromtheintinstructiontakeuptwostoragelocationsforeachnumber.Thesearelimittedtonumbersbetween0and65535.9.long Longstakeupfourstoragelocations.Thisisthesameamountofspacetheregistersuse,whichiswhytheyareusedinthisprogram.Theycanholdnumbersbetween0and4294967295..ascii The.asciidirectiveistoenterincharactersintomemory.Characterseachtakeuponestoragelocationtheyareconvertedintobytesinternally.So,ifyougavethedirective.ascii"Hellothere",theassemblerwouldreserve12storagelocationsbytes.TherstbytecontainsthenumericcodeforH,thesecondbytecontainsthenumericcodefore,andsoforth.Thelastcharacterisrepresentedby,anditistheterminatingcharacteritwillneverdisplay,itjusttellsotherpartsoftheprogramthatthat'stheendofthecharacters.Lettersandnumbersthatstartwithabackslashrepresentcharactersthatarenottypeableonthekeyboardoreasilyviewableonthescreen.Forexample,nreferstothe"newline"characterwhichcausesthe 9. NotethatnonumbersinassemblylanguageoranyothercomputerlanguageI'veseenhavecommasembeddedinthem.So,alwayswritenumberslike65535,andneverlike65,535. 34 PAGE 41 Chapter3.YourFirstPrograms computertostartoutputonthenextlineandtreferstothe"tab"character.Allofthelettersinan.asciidirectiveshouldbeinquotes.Inourexample,theassemblerreserves14.longs,onerightafteranother.Sinceeachlongtakesup4bytes,thatmeansthatthewholelisttakesup56bytes.Thesearethenumberswewillbesearchingthroughtondthemaximum.data_itemsisusedbytheassemblertorefertotheaddressoftherstofthesevalues.Takenotethatthelastdataiteminthelistisazero.Idecidedtouseazerototellmyprogramthatithashittheendofthelist.Icouldhavedonethisotherways.Icouldhavehadthesizeofthelisthard-codedintotheprogram.Also,Icouldhaveputthelengthofthelistastherstitem,orinaseparatelocation.Ialsocouldhavemadeasymbolwhichmarkedthelastlocationofthelistitems.NomatterhowIdoit,Imusthavesomemethodofdeterminingtheendofthelist.Thecomputerknowsnothing-itcanonlydowhatitstold.It'snotgoingtostopprocessingunlessIgiveitsomesortofsignal.Otherwiseitwouldcontinueprocessingpasttheendofthelistintothedatathatfollowsit,andeventolocationswherewehaven'tputanydata.Noticethatwedon'thavea.globldeclarationfordata_items.Thisisbecauseweonlyrefertotheselocationswithintheprogram.Nootherleorprogramneedstoknowwheretheyarelocated.Thisisincontrasttothe_startsymbol,whichLinuxneedstoknowwhereitissothatitknowswheretobegintheprogram'sexecution.It'snotanerrortowrite.globldata_items,it'sjustnotnecessary.Anyway,playaroundwiththislineandaddyourownnumbers.Eventhoughtheyare.long,theprogramwillproducestrangeresultsifanynumberisgreaterthan255,becausethat'sthelargestallowedexitstatus.Alsonoticethatifyoumovethe0toearlierinthelist,therestgetignored.Rememberthatanytimeyouchangethesourcele,youhavetore-assembleandre-linkyourprogram.Dothisnowandseetheresults.Allright,we'veplayedwiththedataalittlebit.Nowlet'slookatthecode.Inthecommentsyouwillnoticethatwe'vemarkedsomevariablesthatweplantouse.Avariableisadedicatedstoragelocationusedforaspecicpurpose,usually 35 PAGE 42 Chapter3.YourFirstPrograms givenadistinctnamebytheprogrammer.Wetalkedabouttheseintheprevioussection,butdidn'tgivethemaname.Inthisprogram,wehaveseveralvariables: avariableforthecurrentmaximumnumberfound avariableforwhichnumberofthelistwearecurrentlyexamining,calledtheindex avariableholdingthecurrentnumberbeingexaminedInthiscase,wehavefewenoughvariablesthatwecanholdthemallinregisters.Inlargerprograms,youhavetoputtheminmemory,andthenmovethemtoregisterswhenyouarereadytousethem.Wewilldiscusshowtodothatlater.Whenpeoplestartoutprogramming,theyusuallyunderestimatethenumberofvariablestheywillneed.Peoplearenotusedtohavingtothinkthrougheverydetailofaprocess,andthereforeleaveoutneededvariablesintheirrstprogrammingattempts.Inthisprogram,weareusing%ebxasthelocationofthelargestitemwe'vefound.%ediisusedastheindextothecurrentdataitemwe'relookingat.Now,let'stalkaboutwhatanindexis.Whenwereadtheinformationfromdata_items,wewillstartwiththerstonedataitemnumber0,thengotothesecondonedataitemnumber1,thenthethirddataitemnumber2,andsoon.Thedataitemnumberistheindexofdata_items.You'llnoticethattherstinstructionwegivetothecomputeris:movl$0,%ediSinceweareusing%ediasourindex,andwewanttostartlookingattherstitem,weload%ediwith0.Now,thenextinstructionistricky,butcrucialtowhatwe'redoing.Itsays:movldata_items,%edi,4,%eaxNowtounderstandthisline,youneedtokeepseveralthingsinmind: 36 PAGE 43 Chapter3.YourFirstPrograms data_itemsisthelocationnumberofthestartofournumberlist. Eachnumberisstoredacross4storagelocationsbecausewedeclareditusing.long %ediisholding0atthispointSo,basicallywhatthislinedoesissay,"startatthebeginningofdata_items,andtaketherstitemnumberbecause%ediis0,andrememberthateachnumbertakesupfourstoragelocations."Thenitstoresthatnumberin%eax.Thisishowyouwriteindexedaddressingmodeinstructionsinassemblylanguage.Theinstructioninageneralformisthis:movlBEGINNINGADDRESS,%INDEXREGISTER,WORDSIZEInourcasedata_itemswasourbeginningaddress,%ediwasourindexregister,and4wasourwordsize.Thistopicisdiscussedfurtherin theSectioncalledAddressingModes .Ifyoulookatthenumbersindata_items,youwillseethatthenumber3isnowin%eax.If%ediwassetto1,thenumber67wouldbein%eax,andifitwassetto2,thenumber34wouldbein%eax,andsoforth.Verystrangethingswouldhappenifweusedanumberotherthan4asthesizeofourstoragelocations.10Thewayyouwritethisisveryawkward,butifyouknowwhateachpiecedoes,it'snottoodifcult.Formoreinformationaboutthis,see theSectioncalledAddressingModes Let'slookatthenextline:movl%eax,%ebx 10. Theinstructiondoesn'treallyuse4forthesizeofthestoragelocations,althoughlookingatitthatwayworksforourpurposesnow.It'sactuallywhat'scalledamultiplier.basically,thewayitworksisthatyoustartatthelocationspeciedbydata_items,thenyouadd%edi*4storagelocations,andretrievethenumberthere.Usually,youusethesizeofthenumbersasyourmultiplier,butinsomecircumstancesyou'llwanttodootherthings. 37 PAGE 44 Chapter3.YourFirstPrograms Wehavetherstitemtolookatstoredin%eax.Sinceitistherstitem,weknowit'sthebiggestonewe'velookedat.Westoreitin%ebx,sincethat'swherewearekeepingthelargestnumberfound.Also,eventhoughmovlstandsformove,itactuallycopiesthevalue,so%eaxand%ebxbothcontainthestartingvalue.11Nowwemoveintoaloop.Aloopisasegmentofyourprogramthatmightrunmorethanonce.Wehavemarkedthestartinglocationoftheloopinthesymbolstart_loop.Thereasonwearedoingaloopisbecausewedon'tknowhowmanydataitemswehavetoprocess,buttheprocedurewillbethesamenomatterhowmanythereare.Wedon'twanttohavetorewriteourprogramforeverylistlengthpossible.Infact,wedon'tevenwanttohavetowriteoutcodeforacomparisonforeverylistitem.Therefore,wehaveasinglesectionofcodealoopthatweexecuteoverandoveragainforeveryelementindata_items.Intheprevioussection,weoutlinedwhatthisloopneededtodo.Let'sreview: Checktoseeifthecurrentvaluebeinglookedatiszero.Ifso,thatmeansweareattheendofourdataandshouldexittheloop. Wehavetoloadthenextvalueofourlist. Wehavetoseeifthenextvalueisbiggerthanourcurrentbiggestvalue. Ifitis,wehavetocopyittothelocationweareholdingthelargestvaluein. Nowweneedtogobacktothebeginningoftheloop.Okay,sonowletsgotothecode.Wehavethebeginningoftheloopmarkedwithstart_loop.Thatissoweknowwheretogobacktoattheendofourloop.Thenwehavetheseinstructions:cmpl$0,%eaxjeend_loop 11. Also,thelinmovlstandsformovelongsincewearemovingavaluethattakesupfourstoragelocations. 38 PAGE 45 Chapter3.YourFirstPrograms Thecmplinstructioncomparesthetwovalues.Here,wearecomparingthenumber0tothenumberstoredin%eaxThiscompareinstructionalsoaffectsaregisternotmentionedhere,the%eflagsregister.Thisisalsoknownasthestatusregister,andhasmanyuseswhichwewilldiscusslater.Justbeawarethattheresultofthecomparisonisstoredinthestatusregister.Thenextlineisaowcontrolinstructionwhichsaystojumptotheend_looplocationifthevaluesthatwerejustcomparedareequalthat'swhattheeofjemeans.Itusesthestatusregistertoholdthevalueofthelastcomparison.Weusedje,buttherearemanyjumpstatementsthatyoucanuse:je Jumpifthevalueswereequaljg Jumpifthesecondvaluewasgreaterthantherstvalue12jge Jumpifthesecondvaluewasgreaterthanorequaltotherstvaluejl Jumpifthesecondvaluewaslessthantherstvaluejle Jumpifthesecondvaluewaslessthanorequaltotherstvaluejmp Jumpnomatterwhat.Thisdoesnotneedtobepreceededbyacomparison. 12. noticethatthecomparisonistoseeifthesecondvalueisgreaterthantherst.Iwouldhavethoughtittheotherwayaround.Youwillndalotofthingslikethiswhenlearningprogramming.Itoccursbecausedifferentthingsmakesensetodifferentpeople.Anyway,you'lljusthavetomemorizesuchthingsandgoon. 39 PAGE 46 Chapter3.YourFirstPrograms Thecompletelistisdocumentedin AppendixB .Inthiscase,wearejumpingif%eaxholdsthevalueofzero.Ifso,wearedoneandwegotoloop_exit.13Ifthelastloadedelementwasnotzero,wegoontothenextinstructions:incl%edimovldata_items,%edi,4,%eaxIfyourememberfromourpreviousdiscussion,%edicontainstheindextoourlistofvaluesindata_items.inclincrementsthevalueof%edibyone.Thenthemovlisjustliketheonewedidbeforehand.However,sincewealreadyincremented%edi,%eaxisgettingthenextvaluefromthelist.Now%eaxhasthenextvaluetobetested.So,let'stestit!cmpl%ebx,%eaxjlestart_loopHerewecompareourcurrentvalue,storedin%eaxtoourbiggestvaluesofar,storedin%ebx.Ifthecurrentvalueislessorequaltoourbiggestvaluesofar,wedon'tcareaboutit,sowejustjumpbacktothebeginningoftheloop.Otherwise,weneedtorecordthatvalueasthelargestone:movl%eax,%ebxjmpstart_loopwhichmovesthecurrentvalueinto%ebx,whichweareusingtostorethecurrentlargestvalue,andstartstheloopoveragain.Okay,sotheloopexecutesuntilitreachesa0,whenitjumpstoloop_exit.ThispartoftheprogramcallstheLinuxkerneltoexit.Ifyourememberfromthelast 13. Thenamesofthesesymbolscanbeanythingyouwantthemtobe,aslongastheyonlycontainlettersandtheunderscorecharacter_.Theonlyonethatisforcedis_start,andpossiblyothersthatyoudeclarewith.globl.However,ifitsasymbolyoudeneandonlyyouuse,feelfreetocallitanythingyouwantthatisadequatelydescriptiverememberthatotherswillhavetomodifyyourcodelater,andwillhavetogureoutwhatyoursymbolsmean. 40 PAGE 47 Chapter3.YourFirstPrograms program,whenyoucalltheoperatingsystemrememberit'slikesignalingBatman,youstorethesystemcallnumberin%eaxfortheexitcall,andstoretheothervaluesintheotherregisters.Theexitcallrequiresthatweputourexitstatusin%ebxWealreadyhavetheexitstatustheresinceweareusing%ebxasourlargestnumber,soallwehavetodoisload%eaxwiththenumberoneandcallthekerneltoexit.Likethis:movl$1,%eaxint0x80Okay,thatwasalotofworkandexplanation,especiallyforsuchasmallprogram.Buthey,you'relearningalot!Now,readthroughthewholeprogramagain,payingspecialattentiontothecomments.Makesurethatyouunderstandwhatisgoingonateachline.Ifyoudon'tunderstandaline,gobackthroughthissectionandgureoutwhatthelinemeans.Youmightalsograbapieceofpaper,andgothroughtheprogramstep-by-step,recordingeverychangetoeveryregister,soyoucanseemoreclearlywhatisgoingon.AddressingModesIn theSectioncalledDataAccessingMethodsinChapter2 welearnedthedifferenttypesofaddressingmodesavailableforuseinassemblylanguage.Thissectionwilldealwithhowthoseaddressingmodesarerepresentedinassemblylanguageinstructions.Thegeneralformofmemoryaddressreferencesisthis:ADDRESS_OR_OFFSET%BASE_OR_OFFSET,%INDEX,MULTIPLIERAlloftheeldsareoptional.Tocalculatetheaddress,simplyperformthefollowingcalculation: 41 PAGE 48 Chapter3.YourFirstPrograms FINALADDRESS=ADDRESS_OR_OFFSET+%BASE_OR_OFFSET+MULTIPLIER*%INDEXADDRESS_OR_OFFSETandMULTIPLIERmustbothbeconstants,whiletheothertwomustberegisters.Ifanyofthepiecesisleftout,itisjustsubstitutedwithzerointheequation.Alloftheaddressingmodesmentionedin theSectioncalledDataAccessingMethodsinChapter2 exceptimmediate-modecanberepresentedinthisfashion.directaddressingmode ThisisdonebyonlyusingtheADDRESS_OR_OFFSETportion.Example:movlADDRESS,%eaxThisloads%eaxwiththevalueatmemoryaddressADDRESS.indexedaddressingmode ThisisdonebyusingtheADDRESS_OR_OFFSETandthe%INDEXportion.Youcanuseanygeneral-purposeregisterastheindexregister.Youcanalsohaveaconstantmultiplierof1,2,or4fortheindexregister,tomakeiteasiertoindexbybytes,double-bytes,andwords.Forexample,let'ssaythatwehadastringofbytesasstring_startandwantedtoaccessthethirdoneanindexof2sincewestartcountingtheindexatzero,and%ecxheldthevalue2.Ifyouwantedtoloaditinto%eaxyoucoulddothefollowing:movlstring_start,%ecx,1,%eaxThisstartsatstring_start,andadds1*%ecxtothataddress,andloadsthevalueinto%eax.indirectaddressingmode Indirectaddressingmodeloadsavaluefromtheaddressindicatedbyaregister.Forexample,if%eaxheldanaddress,wecouldmovethevalueat 42 PAGE 49 Chapter3.YourFirstPrograms thataddressto%ebxbydoingthefollowing:movl%eax,%ebxbasepointeraddressingmode Base-pointeraddressingissimilartoindirectaddressing,exceptthatitaddsaconstantvaluetotheaddressintheregister.Forexample,ifyouhavearecordwheretheagevalueis4bytesintotherecord,andyouhavetheaddressoftherecordin%eax,youcanretrievetheageinto%ebxbyissuingthefollowinginstruction:movl4%eax,%ebximmediatemode Immediatemodeisverysimple.Itdoesnotfollowthegeneralformwehavebeenusing.Immediatemodeisusedtoloaddirectvaluesintoregistersormemorylocations.Forexample,ifyouwantedtoloadthenumber12into%eax,youwouldsimplydothefollowing:movl$12,%eaxNoticethattoindicateimmediatemode,weusedadollarsigninfrontofthenumber.Ifwedidnot,itwouldbedirectaddressingmode,inwhichcasethevaluelocatedatmemorylocation12wouldbeloadedinto%eaxratherthanthenumber12itself.registeraddressingmode Registermodesimplymovesdatainoroutofaregister.Inallofourexamples,registeraddressingmodewasusedfortheotheroperand.Theseaddressingmodesareveryimportant,aseverymemoryaccesswilluseoneofthese.Everymodeexceptimmediatemodecanbeusedaseitherthesourceordestinationoperand.Immediatemodecanonlybeasourceoperand. 43 PAGE 50 Chapter3.YourFirstPrograms Inadditiontothesemodes,therearealsodifferentinstructionsfordifferentsizesofvaluestomove.Forexample,wehavebeenusingmovltomovedataawordatatime.inmanycases,youwillonlywanttomovedataabyteatatime.Thisisaccomplishedbytheinstructionmovb.However,sincetheregisterswehavediscussedareword-sizedandnotbyte-sized,youcannotusethefullregister.Instead,youhavetouseaportionoftheregister.Takeforinstance%eax.Ifyouonlywantedtoworkwithtwobytesatatime,youcouldjustuse%ax.%axistheleast-signicanthalfi.e.-thelastpartofthenumberofthe%eaxregister,andisusefulwhendealingwithtwo-bytequantities.%axisfurtherdividedupinto%aland%ah.%alistheleast-signicantbyteof%ax,and%ahisthemostsignicantbyte.14Loadingavalueinto%eaxwillwipeoutwhateverwasin%aland%ahandalso%ax,since%axismadeupofthem.Similarly,loadingavalueintoeither%alor%ahwillcorruptanyvaluethatwasformerlyin%eax.Basically,it'swisetoonlyusearegisterforeitherabyteoraword,butneverbothatthesametime. 14. Whenwetalkaboutthemostorleastsignicantbyte,itmaybealittleconfusing.Let'stakethenumber5432.Inthatnumber,54isthemostsignicanthalfofthatnumberand32istheleastsignicanthalf.Youcan'tquitedivideitlikethatforregisters,sincetheyoperateonbase2ratherthanbase10numbers,butthat'sthebasicidea.Formoreinformationonthistopic,see Chapter10 44 PAGE 51 Chapter3.YourFirstPrograms Layoutofthe%eaxregisterForamorecomprehensivelistofinstructions,see AppendixB .ReviewKnowtheConcepts Whatdoesifmeanifalineintheprogramstartswiththe'#'character? Whatisthedifferencebetweenanassemblylanguageleandanobjectcodele? Whatdoesthelinkerdo? Howdoyouchecktheresultstatuscodeofthelastprogramyouran? Whatisthedifferencebetweenmovl$1,%eaxandmovl1,%eax? 45 PAGE 52 Chapter3.YourFirstPrograms Whichregisterholdsthesystemcallnumber? Whatareindexesusedfor? Whydoindexesusuallystartat0? IfIissuedthecommandmovldata_items,%edi,4,%eaxanddata_itemswasaddress3634and%ediheldthevalue13,whataddresswouldyoubeusingtomoveinto%eax? Listthegeneral-purposeregisters. Whatisthedifferencebetweenmovlandmovb? Whatisowcontrol? Whatdoesaconditionaljumpdo? Whatthingsdoyouhavetoplanforwhenwritingaprogram? Gothrougheveryinstructionandlistwhataddressingmodeisbeingusedforeachoperand.UsetheConcepts Modifytherstprogramtoreturnthevalue3. Modifythemaximumprogramtondtheminimuminstead. Modifythemaximumprogramtousethenumber255toendthelistratherthanthenumber0 Modifythemaximumprogramtouseanendingaddressratherthanthenumber0toknowwhentostop. Modifythemaximumprogramtousealengthcountratherthanthenumber0toknowwhentostop. 46 PAGE 53 Chapter3.YourFirstPrograms Whatwouldtheinstructionmovl_start,%eaxdo?Bespecic,basedonyourknowledgeofbothaddressingmodesandthemeaningof_start.Howwouldthisdifferfromtheinstructionmovl$_start,%eax?GoingFurther Modifytherstprogramtoleaveofftheintinstructionline.Assemble,link,andexecutethenewprogram.Whaterrormessagedoyouget.Whydoyouthinkthismightbe? Sofar,wehavediscussedthreeapproachestondingtheendofthelist-usingaspecialnumber,usingtheendingaddress,andusingthelengthcount.Whichapproachdoyouthinkisbest?Why?Whichapproachwouldyouuseifyouknewthatthelistwassorted?Why? 47 PAGE 54 Chapter3.YourFirstPrograms 48 PAGE 55 Chapter4.AllAboutFunctionsDealingwithComplexityIn Chapter3 ,theprogramswewroteonlyconsistedofonesectionofcode.However,ifwewroterealprogramslikethat,itwouldbeimpossibletomaintainthem.Itwouldbereallydifculttogetmultiplepeopleworkingontheproject,asanychangeinonepartmightadverselyaffectanotherpartthatanotherdeveloperisworkingon.Toassistprogrammersinworkingtogetheringroups,itisnecessarytobreakprogramsapartintoseparatepieces,whichcommunicatewitheachotherthroughwell-denedinterfaces.Thisway,eachpiececanbedevelopedandtestedindependentlyoftheothers,makingiteasierformultipleprogrammerstoworkontheproject.Programmersusefunctionstobreaktheirprogramsintopieceswhichcanbeindependentlydevelopedandtested.Functionsareunitsofcodethatdoadenedpieceofworkonspeciedtypesofdata.Forexample,inawordprocessorprogram,Imayhaveafunctioncalledhandle_typed_characterwhichisactivatedwheneverausertypesinakey.Thedatathefunctionuseswouldprobablybethekeypressitselfandthedocumenttheusercurrentlyhasopen.Thefunctionwouldthenmodifythedocumentaccordingtothekeypressitwastoldabout.Thedataitemsafunctionisgiventoprocessarecalledit'sparameters.Inthewordprocessingexample,thekeywhichwaspressedandthedocumentwouldbeconsideredparameterstothehandle_typed_charactersfunction.Theparameterlistandtheprocessingexpectationsofafunctionwhatitisexpectedtodowiththeparametersarecalledthefunction'sinterface.Muchcaregoesintodesigningfunctioninterfaces,becauseiftheyarecalledfrommanyplaceswithinaproject,itisdifculttochangethemifnecessary.Atypicalprogramiscomposedofhundredsorthousandsoffunctions,eachwitha 49 PAGE 56 Chapter4.AllAboutFunctions small,well-denedtasktoperform.However,ultimatelytherearethingsthatyoucannotwritefunctionsforwhichmustbeprovidedbythesystem.Thosearecalledprimitivefunctionsorjustprimitives-theyarethebasicswhicheverythingelseisbuiltoffof.Forexample,imagineaprogramthatdrawsagraphicaluserinterface.Therehastobeafunctiontocreatethemenus.Thatfunctionprobablycallsotherfunctionstowritetext,towriteicons,topaintthebackground,calculatewherethemousepointeris,etc.However,ultimately,theywillreachasetofprimitivesprovidedbytheoperatingsystemtodobasiclineorpointdrawing.Programmingcaneitherbeviewedasbreakingalargeprogramdownintosmallerpiecesuntilyougettotheprimitivefunctions,orincrementallybuildingfunctionsontopofprimitivesuntilyougetthelargepictureinfocus.Inassemblylanguage,theprimitivesareusuallythesamethingasthesystemcalls,eventhoughsystemcallsaren'ttruefunctionsaswewilltalkaboutinthischapter.HowFunctionsWorkFunctionsarecomposedofseveraldifferentpieces:functionname Afunction'snameisasymbolthatrepresentstheaddresswherethefunction'scodestarts.Inassemblylanguage,thesymbolisdenedbytypingthethefunction'snameasalabelbeforethefunction'scode.Thisisjustlikelabelsyouhaveusedforjumping.functionparameters Afunction'sparametersarethedataitemsthatareexplicitlygiventothefunctionforprocessing.Forexample,inmathematics,thereisasinefunction.Ifyouweretoaskacomputertondthesineof2,sinewouldbethefunction'sname,and2wouldbetheparameter.Somefunctionshave 50 PAGE 57 Chapter4.AllAboutFunctions manyparameters,othershavenone.1localvariables Localvariablesaredatastoragethatafunctionuseswhileprocessingthatisthrownawaywhenitreturns.It'skindoflikeascratchpadofpaper.Functionsgetanewpieceofpapereverytimetheyareactivated,andtheyhavetothrowitawaywhentheyarenishedprocessing.Localvariablesofafunctionarenotaccessibletoanyotherfunctionwithinaprogram.staticvariables Staticvariablesaredatastoragethatafunctionuseswhileprocessingthatisnotthrownawayafterwards,butisreusedforeverytimethefunction'scodeisactivated.Thisdataisnotaccessibletoanyotherpartoftheprogram.Staticvariablesaregenerallynotusedunlessabsolutelynecessary,astheycancauseproblemslateron.globalvariables Globalvariablesaredatastoragethatafunctionusesforprocessingwhicharemanagedoutsidethefunction.Forexample,asimpletexteditormayputtheentirecontentsoftheleitisworkingoninaglobalvariablesoitdoesn'thavetobepassedtoeveryfunctionthatoperatesonit.2Congurationvaluesarealsooftenstoredinglobalvariables.returnaddress Thereturnaddressisan"invisible"parameterinthatitisn'tdirectlyusedduringthefunction.Thereturnaddressisaparameterwhichtellsthefunction 1. Functionparameterscanalsobeusedtoholdpointerstodatathatthefunctionwantstosendbacktotheprogram. 2. Thisisgenerallyconsideredbadpractice.Imagineifaprogramiswrittenthisway,andinthenextversiontheydecidedtoallowasingleinstanceoftheprogrameditmultipleles.Eachfunctionwouldthenhavetobemodiedsothatthelethatwasbeingmanipulatedwouldbepassedasaparameter.Ifyouhadsimplypasseditasaparametertobeginwith,mostofyourfunctionscouldhavesurvivedyourupgradeunchanged. 51 PAGE 58 Chapter4.AllAboutFunctions wheretoresumeexecutingafterthefunctioniscompleted.Thisisneededbecausefunctionscanbecalledtodoprocessingfrommanydifferentpartsofyourprogram,andthefunctionneedstobeabletogetbacktowhereveritwascalledfrom.Inmostprogramminglanguages,thisparameterispassedautomaticallywhenthefunctioniscalled.Inassemblylanguage,thecallinstructionhandlespassingthereturnaddressforyou,andrethandlesusingthataddresstoreturnbacktowhereyoucalledthefunctionfrom.returnvalue Thereturnvalueisthemainmethodoftransferringdatabacktothemainprogram.Mostprogramminglanguagesonlyallowasinglereturnvalueforafunction.Thesepiecesarepresentinmostprogramminglanguages.Howyouspecifyeachpieceisdifferentineachone,however.Thewaythatthevariablesarestoredandtheparametersandreturnvaluesaretransferredbythecomputervariesfromlanguagetolanguageaswell.Thisvarianceisknownasalanguage'scallingconvention,becauseitdescribeshowfunctionsexpecttogetandreceivedatawhentheyarecalled.3Assemblylanguagecanuseanycallingconventionitwantsto.Youcanevenmakeoneupyourself.However,ifyouwanttointeroperatewithfunctionswritteninotherlanguages,youhavetoobeytheircallingconventions.WewillusethecallingconventionoftheCprogramminglanguageforourexamplesbecauseitisthemostwidelyused,andbecauseitisthestandardforLinuxplatforms. 3. Aconventionisawayofdoingthingsthatisstandardized,butnotforciblyso.Forex-ample,itisaconventionforpeopletoshakehandswhentheymeet.IfIrefusetoshakehandswithyou,youmaythinkIdon'tlikeyou.Followingconventionsisimportantbe-causeitmakesiteasierforotherstounderstandwhatyouaredoing,andmakesiteasierforprogramswrittenbymultipleindependentauthorstoworktogether. 52 PAGE 59 Chapter4.AllAboutFunctions Assembly-LanguageFunctionsusingtheCCallingConventionYoucannotwriteassembly-languagefunctionswithoutunderstandinghowthecomputer'sstackworks.Eachcomputerprogramthatrunsusesaregionofmemorycalledthestacktoenablefunctionstoworkproperly.Thinkofastackasapileofpapersonyourdeskwhichcanbeaddedtoindenitely.Yougenerallykeepthethingsthatyouareworkingontowardthetop,andyoutakethingsoffasyouarenishedworkingwiththem.Yourcomputerhasastack,too.Thecomputer'sstacklivesattheverytopaddressesofmemory.Youcanpushvaluesontothetopofthestackthroughaninstructioncalledpushl,whichpusheseitheraregisterormemoryvalueontothetopofthestack.Well,wesayit'sthetop,butthe"top"ofthestackisactuallythebottomofthestack'smemory.Althoughthisisconfusing,thereasonforitisthatwhenwethinkofastackofanything-dishes,papers,etc.-wethinkofaddingandremovingtothetopofit.However,inmemorythestackstartsatthetopofmemoryandgrowsdownwardduetoarchitecturalconsiderations.Therefore,whenwerefertothe"topofthestack"rememberit'satthebottomofthestack'smemory.Youcanalsopopvaluesoffthetopusinganinstructioncalledpopl.Thisremovesthetopvaluefromthestackandplacesitintoaregisterormemorylocationofyourchoosing..Whenwepushavalueontothestack,thetopofthestackmovestoaccomodatetheadditionalvalue.Wecanactuallycontinuallypushvaluesontothestackanditwillkeepgrowingfurtherandfurtherdowninmemoryuntilwehitourcodeordata.Sohowdoweknowwherethecurrent"top"ofthestackis?Thestackregister,%esp,alwayscontainsapointertothecurrenttopofthestack,whereveritis.Everytimewepushsomethingontothestackwithpushl,%espgetssubtractedby4sothatitpointstothenewtopofthestackremember,eachwordisfourbyteslong,andthestackgrowsdownward.Ifwewanttoremovesomethingfromthestack,wesimplyusethepoplinstruction,whichadds4to%espandputstheprevioustopvalueinwhateverregisteryouspecied.pushlandpopleachtake 53 PAGE 60 Chapter4.AllAboutFunctions oneoperand-theregistertopushontothestackforpushl,orreceivethedatathatispoppedoffthestackforpopl.Ifwesimplywanttoaccessthevalueonthetopofthestackwithoutremovingit,wecansimplyusethe%espregisterinindirectaddressingmode.Forexample,thefollowingcodemoveswhateverisatthetopofthestackinto%eax:movl%esp,%eaxIfweweretojustdothis:movl%esp,%eaxthen%eaxwouldjustholdthepointertothetopofthestackratherthanthevalueatthetop.Putting%espinparenthesiscausesthecomputertogotoindirectaddressingmode,andthereforewegetthevaluepointedtoby%esp.Ifwewanttoaccessthevaluerightbelowthetopofthestack,wecansimplyissuethisinstruction:movl4%esp,%eaxThisinstructionusesthebasepointeraddressingmodesee theSectioncalledDataAccessingMethodsinChapter2 whichsimplyadds4to%espbeforelookingupthevaluebeingpointedto.IntheClanguagecallingconvention,thestackisthekeyelementforimplementingafunction'slocalvariables,parameters,andreturnaddress.Beforeexecutingafunction,aprogrampushesalloftheparametersforthefunctionontothestackinthereverseorderthattheyaredocumented.Thentheprogramissuesacallinstructionindicatingwhichfunctionitwishestostart.Thecallinstructiondoestwothings.Firstitpushestheaddressofthenextinstruction,whichisthereturnaddress,ontothestack.Thenitmodiestheinstructionpointer%eiptopointtothestartofthefunction.So,atthetimethefunctionstarts,thestacklookslikethisthe"top"ofthestackisatthebottomon 54 PAGE 61 Chapter4.AllAboutFunctions thisexample:Parameter#N...Parameter2Parameter1ReturnAddress<---%espEachoftheparametersofthefunctionhavebeenpushedontothestack,andnallythereturnaddressisthere.Nowthefunctionitselfhassomeworktodo.Therstthingitdoesissavethecurrentbasepointerregister,%ebp,bydoingpushl%ebp.Thebasepointerisaspecialregisterusedforaccessingfunctionparametersandlocalvariables.Next,itcopiesthestackpointerto%ebpbydoingmovl%esp,%ebp.Thisallowsyoutobeabletoaccessthefunctionparametersasxedindexesfromthebasepointer.Youmaythinkthatyoucanusethestackpointerforthis.However,duringyourprogramyoumaydootherthingswiththestacksuchaspushingargumentstootherfunctions.Copyingthestackpointerintothebasepointeratthebeginningofafunctionallowsyoutoalwaysknowwhereyourparametersareandaswewillsee,localvariablestoo,evenwhileyoumaybepushingthingsonandoffthestack.%ebpwillalwaysbewherethestackpointerwasatthebeginningofthefunction,soitismoreorlessaconstantreferencetothestackframethestackframeconsistsofallofthestackvariablesusedwithinafunction,includingparameters,localvariables,andthereturnaddress.Atthispoint,thestacklookslikethis:Parameter#N<---N*4+4%ebp...Parameter2<---12%ebpParameter1<---8%ebpReturnAddress<---4%ebpOld%ebp<---%espand%ebp 55 PAGE 62 Chapter4.AllAboutFunctions Asyoucansee,eachparametercanbeaccessedusingbasepointeraddressingmodeusingthe%ebpregister.Next,thefunctionreservesspaceonthestackforanylocalvariablesitneeds.Thisisdonebysimplymovingthestackpointeroutoftheway.Let'ssaythatwearegoingtoneedtwowordsofmemorytorunafunction.Wecansimplymovethestackpointerdowntwowordstoreservethespace.Thisisdonelikethis:subl$8,%espThissubtracts8from%espremember,awordisfourbyteslong.4Thisway,wecanusethestackforvariablestoragewithoutworringaboutclobberingthemwithpushesthatwemaymakeforfunctioncalls.Also,sinceitisallocatedonthestackframeforthisfunctioncall,thevariablewillonlybealiveduringthisfunction.Whenwereturn,thestackframewillgoaway,andsowillthesevariables.That'swhytheyarecalledlocal-theyonlyexistwhilethisfunctionisbeingcalled.Nowwehavetwowordsforlocalstorage.Ourstacknowlookslikethis:Parameter#N<---N*4+4%ebp...Parameter2<---12%ebpParameter1<---8%ebpReturnAddress<---4%ebpOld%ebp<---%ebpLocalVariable1<----4%ebpLocalVariable2<----8%ebpand%espSowecannowaccessallofthedataweneedforthisfunctionbyusingbasepointeraddressingusingdifferentoffsetsfrom%ebp.%ebpwasmadespecicallyforthispurpose,whichiswhyitiscalledthebasepointer.Youcanuseotherregistersinbasepointeraddressingmode,butthex86architecturemakesusingthe%ebpregisteralotfaster. 4. Justareminder-thedollarsigninfrontoftheeightindicatesimmediatemodeaddress-ing,meaningthatweloadthenumber8into%espratherthanthevalueataddress8. 56 PAGE 63 Chapter4.AllAboutFunctions Globalvariablesandstaticvariablesareaccessedjustlikewehavebeenaccessingmemoryinpreviouschapters.Theonlydifferencebetweentheglobalandstaticvariablesisthatstaticvariablesareonlyusedbyonefunction,whileglobalvariablesareusedbymanyfunctions.Assemblylanguagetreatsthemexactlythesame,althoughmostotherlanguagesdistinguishthem.Whenafunctionisdoneexecuting,itdoesthreethings: 1. Itstoresit'sreturnvaluein%eax. 2. Itresetsthestacktowhatitwaswhenitwascalleditgetsridofthecurrentstackframeandputsthestackframeofthecallingcodebackintoeffect. 3. Itreturnscontrolbacktowhereveritwascalledfrom.Thisisdoneusingtheretinstruction,whichpopswhatevervalueisatthetopofthestack,andsetstheinstructionpointer,%eip,tothatvalue.So,beforeafunctionreturnscontroltothecodethatcalledit,itmustrestorethepreviousstackframe.Notealsothatwithoutdoingthis,retwouldn'twork,becauseinourcurrentstackframe,thereturnaddressisnotatthetopofthestack.Therefore,beforewereturn,wehavetoresetthestackpointer%espandbasepointer%ebptowhattheywerewhenthefunctionbegan.Thereforetoreturnfromthefunctionyouhavetodothefollowing:movl%ebp,%esppopl%ebpretAtthispoint,youshouldconsideralllocalvariablestobedisposedof.Thereasonisthatafteryoumovethestackpointerback,futurestackpusheswilllikelyoverwriteeverythingyouputthere.Therefore,youshouldneversavetheaddressofalocalvariablepastthelifeofthefunctionitwascreatedin,orelseitwillbeoverwrittenafterthelifeofit'sstackframeends. 57 PAGE 64 Chapter4.AllAboutFunctions Controlhasnowbeenhandedbacktothecallingcode,whichcannowexamine%eaxforthereturnvalue.Thecallingcodealsoneedstopopoffalloftheparametersitpushedontothestackinordertogetthestackpointerbackwhereitwasyoucanalsosimplyadd4*numberofparamtersto%espusingtheaddlinstruction,ifyoudon'tneedthevaluesoftheparametersanymore.5 DestructionofRegistersWhenyoucallafunction,youshouldassumethateverythingcurrentlyinyourregisterswillbewipedout.Theonlyregisterthatisguaranteedtobeleftwiththevalueitstartedwithis%ebp.%eaxisguaranteedtobeoverwritten,andtheotherslikelyare.Ifthereareregistersyouwanttosavebeforecallingafunction,youneedtosavethembypushingthemonthestackbeforepushingthefunction'sparamters.Youcanthenpopthembackoffinreverseorderafterpoppingofftheparameters.Evenifyouknowafunctiondoesnotoverwritearegisteryoushouldsaveit,becausefutureversionsofthatfunctionmay.Otherlanguages'callingconventionsmaybedifferent.Forexample,othercallingconventionsmayplacetheburdenonthefunctiontosaveanyregistersituses.Besuretochecktomakesurethecallingconventionsofyourlanguagesarecompatiblebeforetryingtomixlanguages.Orinthecaseofassemblylanguage,besureyouknowhowtocalltheotherlanguage'sfunctions.them. ExtendedSpecication:DetailsoftheClanguagecallingconventionalsoknownastheABI,orApplicationBinaryInterfaceisavailableonline.Wehaveoversimpliedandleftoutseveralimportantpiecestomakethissimplerfornewprogrammers.Forfulldetails,youshouldcheckoutthedocumentsavailableathttp://www.linuxbase.org/spec/refspecs/Specically,youshould 5. Thisisnotalwaysstrictlyneededunlessyouaresavingregistersonthestackbeforeafunctioncall.Thebasepointerkeepsthestackframeinareasonablyconsistentstate.However,itisstillagoodidea,andisabsolutelynecessaryifyouaretemporarilysavingregistersonthestack.. 58 PAGE 65 Chapter4.AllAboutFunctions lookfortheSystemVApplicationBinaryInterface-Intel386ArchitectureProcessorSupplement.AFunctionExampleLet'stakealookathowafunctioncallworksinarealprogram.Thefunctionwearegoingtowriteisthepowerfunction.Wewillgivethepowerfunctiontwoparameters-thenumberandthepowerwewanttoraiseitto.Forexample,ifwegaveittheparamters2and3,itwouldraise2tothepowerof3,or2*2*2,giving8.Inordertomakethisprogramsimple,wewillonlyallownumbers1andgreater.Thefollowingisthecodeforthecompleteprogram.Asusual,anexplanationfollows.Namethelepower.s.#PURPOSE:Programtoillustratehowfunctionswork#Thisprogramwillcomputethevalueof#2^3+5^2##Everythinginthemainprogramisstoredinregisters,#sothedatasectiondoesn'thaveanything..section.data.section.text.globl_start_start:pushl$3#pushsecondargumentpushl$2#pushfirstargumentcallpower#callthefunctionaddl$8,%esp#movethestackpointerback 59 PAGE 66 Chapter4.AllAboutFunctions pushl%eax#savethefirstanswerbefore#callingthenextfunctionpushl$2#pushsecondargumentpushl$5#pushfirstargumentcallpower#callthefunctionaddl$8,%esp#movethestackpointerbackpopl%ebx#Thesecondanswerisalready#in%eax.Wesavedthe#firstanswerontothestack,#sonowwecanjustpopit#outinto%ebxaddl%eax,%ebx#addthemtogether#theresultisin%ebxmovl$1,%eax#exit%ebxisreturnedint$0x80#PURPOSE:Thisfunctionisusedtocompute#thevalueofanumberraisedto#apower.##INPUT:Firstargument-thebasenumber#Secondargument-thepowerto#raiseitto##OUTPUT:Willgivetheresultasareturnvalue##NOTES:Thepowermustbe1orgreater##VARIABLES:#%ebx-holdsthebasenumber#%ecx-holdsthepower 60 PAGE 67 Chapter4.AllAboutFunctions ##-4%ebp-holdsthecurrentresult##%eaxisusedfortemporarystorage#.typepower,@functionpower:pushl%ebp#saveoldbasepointermovl%esp,%ebp#makestackpointerthebasepointersubl$4,%esp#getroomforourlocalstoragemovl8%ebp,%ebx#putfirstargumentin%eaxmovl12%ebp,%ecx#putsecondargumentin%ecxmovl%ebx,-4%ebp#storecurrentresultpower_loop_start:cmpl$1,%ecx#ifthepoweris1,wearedonejeend_powermovl-4%ebp,%eax#movethecurrentresultinto%eaximull%ebx,%eax#multiplythecurrentresultby#thebasenumbermovl%eax,-4%ebp#storethecurrentresultdecl%ecx#decreasethepowerjmppower_loop_start#runforthenextpowerend_power:movl-4%ebp,%eax#returnvaluegoesin%eaxmovl%ebp,%esp#restorethestackpointerpopl%ebp#restorethebasepointerret 61 PAGE 68 Chapter4.AllAboutFunctions Typeintheprogram,assembleit,andrunit.Trycallingpowerfordifferentvalues,butrememberthattheresulthastobelessthan256whenitispassedbacktotheoperatingsystem.Alsotrysubtractingtheresultsofthetwocomputations.Tryaddingathirdcalltothepowerfunction,andaddit'sresultbackin.Themainprogramcodeisprettysimple.Youpushtheargumentsontothestack,callthefunction,andthenmovethestackpointerback.Theresultisstoredin%eax.Notethatbetweenthetwocallstopower,wesavetherstvalueontothestack.Thisisbecausetheonlyregisterthatisguaranteedtobesavedis%ebp.Thereforewepushthevalueontothestack,andpopthevaluebackoffafterthesecondfunctioncalliscomplete.Let'slookathowthefunctionitselfiswritten.Noticethatbeforethefunction,thereisdocumentationastowhatthefunctiondoes,whatit'sargumentsare,andwhatitgivesasareturnvalue.Thisisusefulforprogrammerswhousethisfunction.Thisisthefunction'sinterface.Thisletstheprogrammerknowwhatvaluesareneededonthestack,andwhatwillbein%eaxattheend.Wethenhavethefollowingline:.typepower,@functionThistellsthelinkerthatthesymbolpowershouldbetreatedasafunction.Sincethisprogramisonlyinonele,itwouldworkjustthesamewiththisleftout.However,itisgoodpractice.Afterthat,wedenethevalueofthepowerlabel:power:Asmentionedpreviously,thisdenesthesymbolpowertobetheaddresswheretheinstructionsfollowingthelabelbegin.Thisishowcallpowerworks.Ittransferscontroltothisspotoftheprogram.Thedifferencebetweencallandjmpisthatcallalsopushesthereturnaddressontothestacksothatthefunctioncanreturn,whilethejmpdoesnot. 62 PAGE 69 Chapter4.AllAboutFunctions Next,wehaveourinstructionstosetupourfunction:pushl%ebpmovl%esp,%ebpsubl$4,%espAtthispoint,ourstacklookslikethis:BaseNumber<---12%ebpPower<---8%ebpReturnAddress<---4%ebpOld%ebp<---%ebpCurrentresult<----4%ebpand%espAlthoughwecouldusearegisterfortemporarystorage,thisprogramusesalocalvariableinordertoshowhowtosetitup.Oftentimestherejustaren'tenoughregisterstostoreeverything,soyouhavetoofoadthemintolocalvariables.Othertimes,yourfunctionwillneedtocallanotherfunctionandsenditapointertosomeofyourdata.Youcan'thaveapointertoaregister,soyouhavetostoreitinalocalvariableinordertosendapointertoit.Basically,whattheprogramdoesisstartwiththebasenumber,andstoreitbothasthemultiplierstoredin%ebxandthecurrentvaluestoredin-4%ebp.Italsohasthepowerstoredin%ecxItthencontinuallymultipliesthecurrentvaluebythemultiplier,decreasesthepower,andleavestheloopifthepowerin%ecxgetsdownto1.Bynow,youshouldbeabletogothroughtheprogramwithouthelp.Theonlythingsyoushouldneedtoknowisthatimulldoesintegermultiplicationandstorestheresultinthesecondoperand,anddecldecreasesthegivenregisterby1.Formoreinformationontheseandotherinstructions,see AppendixB Agoodprojecttotrynowistoextendtheprogramsoitwillreturnthevalueofanumberifthepoweris0hint,anythingraisedtothezeropoweris1.Keeptrying.Ifitdoesn'tworkatrst,trygoingthroughyourprogrambyhandwitha 63 PAGE 70 Chapter4.AllAboutFunctions scrapofpaper,keepingtrackofwhere%ebpand%esparepointing,whatisonthestack,andwhatthevaluesareineachregister.RecursiveFunctionsThenextprogramwillstretchyourbrainsevenmore.Theprogramwillcomputethefactorialofanumber.Afactorialistheproductofanumberandallthenumbersbetweenitandone.Forexample,thefactorialof7is7*6*5*4*3*2*1,andthefactorialof4is4*3*2*1.Now,onethingyoumightnoticeisthatthefactorialofanumberisthesameastheproductofanumberandthefactorialjustbelowit.Forexample,thefactorialof4is4timesthefactorialof3.Thefactorialof3is3timesthefactorialof2.2is2timesthefactorialof1.Thefactorialof1is1.Thistypeofdenitioniscalledarecursivedenition.Thatmeans,thedenitionofthefactorialfunctionincludesthefactorialfuntionitself.However,sinceallfunctionsneedtoend,arecursivedenitionmustincludeabasecase.Thebasecaseisthepointwhererecursionwillstop.Withoutabasecase,thefunctionwouldgoonforevercallingitselfuntiliteventuallyranoutofstackspace.Inthecaseofthefactorial,thebasecaseisthenumber1.Whenwehitthenumber1,wedon'trunthefactorialagain,wejustsaythatthefactorialof1is1.So,let'srunthroughwhatwewantthecodetolooklikeforourfactorialfunction: 1. Examinethenumber 2. Isthenumber1? 3. Ifso,theanswerisone 4. Otherwise,theansweristhenumbertimesthefactorialofthenumberminusoneThiswouldbeproblematicifwedidn'thavelocalvariables.Inotherprograms,storingvaluesinglobalvariablesworkedne.However,globalvariablesonlyprovideonecopyofeachvariable.Inthisprogram,wewillhavemultiplecopies 64 PAGE 71 Chapter4.AllAboutFunctions ofthefunctionrunningatthesametime,allofthemneedingtheirowncopiesofthedata!6Sincelocalvariablesexistonthestackframe,andeachfunctioncallgetsitsownstackframe,weareokay.Let'slookatthecodetoseehowthisworks:#PURPOSE-Givenanumber,thisprogramcomputesthe#factorial.Forexample,thefactorialof#3is3*2*1,or6.Thefactorialof#4is4*3*2*1,or24,andsoon.##Thisprogramshowshowtocallafunctionrecursively..section.data#Thisprogramhasnoglobaldata.section.text.globl_start.globlfactorial#thisisunneededunlesswewanttoshare#thisfunctionamongotherprograms_start:pushl$4#Thefactorialtakesoneargument-the#numberwewantafactorialof.So,it#getspushedcallfactorial#runthefactorialfunctionaddl$4,%esp#Scrubstheparameterthatwaspushedon#thestackmovl%eax,%ebx#factorialreturnstheanswerin%eax,but#wewantitin%ebxtosenditasourexit#status 6. By"runningatthesametime"Iamtalkingaboutthefactthatonewillnothavenishedbeforeanewoneisactivated.Iamnotimplyingthattheirinstructionsarerunningatthesametime. 65 PAGE 72 Chapter4.AllAboutFunctions movl$1,%eax#callthekernel'sexitfunctionint$0x80#Thisistheactualfunctiondefinition.typefactorial,@functionfactorial:pushl%ebp#standardfunctionstuff-wehaveto#restore%ebptoitspriorstatebefore#returning,sowehavetopushitmovl%esp,%ebp#Thisisbecausewedon'twanttomodify#thestackpointer,soweuse%ebp.movl8%ebp,%eax#Thismovesthefirstargumentto%eax#4%ebpholdsthereturnaddress,and#8%ebpholdsthefirstparametercmpl$1,%eax#Ifthenumberis1,thatisourbase#case,andwesimplyreturnis#alreadyin%eaxasthereturnvaluejeend_factorialdecl%eax#otherwise,decreasethevaluepushl%eax#pushitforourcalltofactorialcallfactorial#callfactorialmovl8%ebp,%ebx#%eaxhasthereturnvalue,sowe#reloadourparameterinto%ebximull%ebx,%eax#multiplythatbytheresultofthe#lastcalltofactorialin%eax#theanswerisstoredin%eax,which#isgoodsincethat'swherereturn#valuesgo.end_factorial:movl%ebp,%esp#standardfunctionreturnstuff-wepopl%ebp#havetorestore%ebpand%esptowhere#theywerebeforethefunctionstartedret#returntothefunctionthispopsthe 66 PAGE 73 Chapter4.AllAboutFunctions #returnvalue,tooAssemble,link,andrunitwiththesecommands:asfactorial.s-ofactorial.oldfactorial.o-ofactorial./factorialecho$?Thisshouldgiveyouthevalue24.24isthefactorialof4,youcantestitoutyourselfwithacalculator:4*3*2*1=24.I'mguessingyoudidn'tunderstandthewholecodelisting.Let'sgothroughitalineatatimetoseewhatishappening._start:pushl$4callfactorialOkay,thisprogramisintendedtocomputethefactorialofthenumber4.Whenprogrammingfunctions,youaresupposedtoputtheparametersofthefunctiononthetopofthestackrightbeforeyoucallit.Remember,afunction'sparametersarethedatathatyouwantthefunctiontoworkwith.Inthiscase,thefactorialfunctiontakes1parameter-thenumberyouwantthefactorialof.Thepushlinstructionputsthegivenvalueatthetopofthestack.Thecallinstructionthenmakesthefunctioncall.Nextwehavetheselines:addl$4,%espmovl%eax,%ebxmovl$1,%eax 67 PAGE 74 Chapter4.AllAboutFunctions int$0x80Thistakesplaceafterfactorialhasnishedandcomputedthefactorialof4forus.Nowwehavetocleanupthestack.Theaddlinstructionmovesthestackpointerbacktowhereitwasbeforewepushedthe$4ontothestack.Youshouldalwayscleanupyourstackparametersafterafunctioncallreturns.Thenextinstructionmoves%eaxto%ebx.What'sin%eax?Itisfactorial'sreturnvalue.Inourcase,itisthevalueofthefactorialfunction.With4asourparameter,24shouldbeourreturnvalue.Remember,returnvaluesarealwaysstoredin%eax.Wewanttoreturnthisvalueasthestatuscodetotheoperatingsystem.However,Linuxrequiresthattheprogram'sexitstatusbestoredin%ebx,not%eax,sowehavetomoveit.Thenwedothestandardexitsystemcall.Thenicethingaboutfunctioncallsisthat: Otherprogrammersdon'thavetoknowanythingaboutthemexceptit'sargumentstousethem. Theyprovidestandardizedbuildingblocksfromwhichyoucanformaprogram. Theycanbecalledmultipletimesandfrommultiplelocationsandtheyalwaysknowhowtogetbacktowheretheyweresincecallpushesthereturnaddressontothestack.Thesearethemainadvantagesoffunctions.Largerprogramsalsousefunctionstobreakdowncomplexpiecesofcodeintosmaller,simplerones.Infact,almostallofprogrammingiswritingandcallingfunctions.Let'snowtakealookathowthefactorialfunctionitselfisimplemented.Beforethefunctionstarts,wehavethisdirective:.typefactorial,@functionfactorial: 68 PAGE 75 Chapter4.AllAboutFunctions The.typedirectivetellsthelinkerthatfactorialisafunction.Thisisn'treallyneededunlesswewereusingfactorialinotherprograms.Wehaveincludeditforcompleteness.Thelinethatsaysfactorial:givesthesymbolfactorialthestoragelocationofthenextinstruction.That'showcallknewwheretogowhenwesaidcallfactorial.Therstrealinstructionsofthefunctionare:pushl%ebpmovl%esp,%ebpAsshowninthepreviousprogram,thiscreatesthestackframeforthisfunction.Thesetwolineswillbethewayyoushouldstarteveryfunction.Thenextinstructionisthis:movl8%ebp,%eaxThisusesbasepointeraddressingtomovetherstparameterofthefunctioninto%eax.Remember,%ebphastheold%ebp,4%ebphasthereturnaddress,and8%ebpisthelocationoftherstparametertothefunction.Ifyouthinkback,thiswillbethevalue4ontherstcall,sincethatwaswhatwepushedonthestackbeforecallingthefunctionwithpushl$4.parameterinto%eax.Asthisfunctioncallsitself,itwillhaveothervalues,too.Next,wechecktoseeifwe'vehitourbasecaseaparameterof1.Ifso,wejumptotheinstructionatthelabelend_factorial,whereitwillbereturned.Et'salreadyin%eaxwhichwementionedearlieriswhereyouputreturnvalues.Thatisaccomplishedbytheselines:cmpl$1,%eaxjeend_factorialIfit'snotourbasecase,whatdidwesaywewoulddo?Wewouldcallthefactorialfunctionagainwithourparameterminusone.So,rstwedecrease%eaxbyone: 69 PAGE 76 Chapter4.AllAboutFunctions decl%eaxdeclstandsfordecrement.Itsubtracts1fromthegivenregisterormemorylocation%eaxinourcase.inclistheinverse-itadds1.Afterdecrementing%eaxwepushitontothestacksinceit'sgoingtobetheparameterofthenextfunctioncall.Andthenwecallfactorialagain!pushl%eaxcallfactorialOkay,nowwe'vecalledfactorial.Onethingtorememberisthatafterafunctioncall,wecanneverknowwhattheregistersareexcept%espand%ebp.Soeventhoughwehadthevaluewewerecalledwithin%eax,it'snotthereanymore.Therefore,weneedpullitoffthestackfromthesameplacewegotitthersttimeat8%ebp.So,wedothis:movl8%ebp,%ebxNow,wewanttomultiplythatnumberwiththeresultofthefactorialfunction.Ifyourememberourpreviousdiscussion,theresultoffunctionsareleftin%eax.So,weneedtomultiply%ebxwith%eax.Thisisdonewiththisinstruction:imull%ebx,%eaxThisalsostorestheresultin%eax,whichisexactlywherewewantthereturnvalueforthefunctiontobe!Sincethereturnvalueisinplacewejustneedtoleavethefunction.Ifyouremember,atthestartofthefunctionwepushed%ebp,andmoved%espinto%ebptocreatethecurrentstackframe.Nowwereversetheoperationtodestroythecurrentstackframeandreactivatethelastone:end_factorial:movl%ebp,%esppopl%ebpNowwe'realreadytoreturn,soweissuethefollowingcommand 70 PAGE 77 Chapter4.AllAboutFunctions retThispopsthetopvalueoffofthestack,andthenjumpstoit.Ifyourememberourdiscussionaboutcall,wesaidthatcallrstpushedtheaddressofthenextinstructionontothestackbeforeitjumpedtothebeginningofthefunction.So,herewepopitbackoffsowecanreturnthere.Thefunctionisdone,andwehaveouranswer!Likeourpreviousprogram,youshouldlookovertheprogramagain,andmakesureyouknowwhateverythingdoes.Lookbackthroughthissectionandtheprevioussectionsfortheexplanationofanythingyoudon'tunderstand.Then,takeapieceofpaper,andgothroughtheprogramstep-by-step,keepingtrackofwhatthevaluesoftheregistersareateachstep,andwhatvaluesareonthestack.Doingthisshoulddeepenyourunderstandingofwhatisgoingon.ReviewKnowtheConcepts Whatareprimitives? Whatarecallingconventions? Whatisthestack? Howdopushlandpoplaffectthestack?Whatspecial-purposeregisterdotheyaffect? Whatarelocalvariablesandwhataretheyusedfor? Whyarelocalvariablessonecessaryinrecursivefunctions? Whatare%ebpand%espusedfor? Whatisastackframe? 71 PAGE 78 Chapter4.AllAboutFunctions UsetheConcepts Writeafunctioncalledsquarewhichreceivesoneargumentandreturnsthesquareofthatargument. Writeaprogramtotestyoursquarefunction. Convertthemaximumprogramgivenin theSectioncalledFindingaMaximumValueinChapter3 sothatitisafunctionwhichtakesapointertoseveralvaluesandreturnstheirmaximum.Writeaprogramthatcallsmaximumwith3differentlists,andreturnstheresultofthelastoneastheprogram'sexitstatuscode. Explaintheproblemsthatwouldarisewithoutastandardcallingconvention.GoingFurther Doyouthinkit'sbetterforasystemtohavealargesetofprimitivesorasmallone,assumingthatthelargersetcanbewrittenintermsofthesmallerone? Thefactorialfunctioncanbewrittennon-recursively.Doso. Findanapplicationonthecomputeryouuseregularly.Trytolocateaspecicfeature,andpracticebreakingthatfeatureoutintofunctions.Denethefunctioninterfacesbetweenthatfeatureandtherestoftheprogram. Comeupwithyourowncallingconvention.Rewritetheprogramsinthischapterusingit.Anexampleofadifferentcallingconventionwouldbetopassparamtersinregistersratherthanthestack,topasstheminadifferentorder,toreturnvaluesinotherregistersormemorylocations.Whateveryoupick,beconsistentandapplyitthroughoutthewholeprogram. Canyoubuildacallingconventionwithoutusingthestack?Whatlimitationsmightithave? 72 PAGE 79 Chapter4.AllAboutFunctions Whattestcasesshouldweuseinourexampleprogramtochecktoseeifitisworkingproperly? 73 PAGE 80 Chapter4.AllAboutFunctions 74 PAGE 81 Chapter5.DealingwithFilesAlotofcomputerprogrammingdealswithles.Afterall,whenwerebootourcomputers,theonlythingthatremainsfromprevioussessionsarethethingsthathavebeenputondisk.Datawhichisstoredinlesiscalledpersistentdata,becauseitpersistsinlesthatremainonthediskevenwhentheprogramisn'trunning..TheUNIXFileConceptEachoperatingsystemhasit'sownwayofdealingwithles.However,theUNIXmethod,whichisusedonLinux,isthesimplestandmostuniversal.UNIXles,nomatterwhatprogramcreatedthem,canallbeaccessedasasequentialstreamofbytes.Whenyouaccessale,youstartbyopeningitbyname.Theoperatingsystemthengivesyouanumber,calledaledescriptor,whichyouusetorefertotheleuntilyouarethroughwithit.Youcanthenreadandwritetotheleusingitsledescriptor.Whenyouaredonereadingandwriting,youthenclosethele,whichthenmakestheledescriptoruseless.Inourprogramswewilldealwithlesinthefollowingways: 1. TellLinuxthenameoftheletoopen,andinwhatmodeyouwantitopenedread,write,bothreadandwrite,createitifitdoesn'texist,etc..Thisishandledwiththeopensystemcall,whichtakesalename,anumberrepresentingthemode,andapermissionsetasitsparameters.%eaxwillholdthesystemcallnumber,whichis5.Theaddressoftherstcharacterofthelenameshouldbestoredin%ebx.Theread/writeintentions,representedasanumber,shouldbestoredin%ecx.Fornow,use0forlesyouwanttoreadfrom,and03101forlesyouwanttowritetoyoumustincludetheleadingzero.1Finally,thepermissionsetshouldbestoredasanumberin%edx.If 1. Thiswillbeexplainedinmoredetailin theSectioncalledTruth,Falsehood,andBinaryNumbersinChapter10 75 PAGE 82 Chapter5.DealingwithFiles youareunfamiliarwithUNIXpermissions,justuse0666forthepermissionsagain,youmustincludetheleadingzero. 2. Linuxwillthenreturntoyoualedescriptorin%eax.Remember,thisisanumberthatyouusetorefertothislethroughoutyourprogram. 3. Nextyouwilloperateontheledoingreadsand/orwrites,eachtimegivingLinuxtheledescriptoryouwanttouse.readissystemcall3,andtocallityouneedtohavetheledescriptorin%ebx,theaddressofabufferforstoringthedatathatisreadin%ecx,andthesizeofthebufferin%edx.Bufferswillbeexplainedin theSectioncalledBuffersand.bss .readwillreturnwitheitherthenumberofcharactersreadfromthele,oranerrorcode.Errorcodescanbedistinguishedbecausetheyarealwaysnegativenumbersmoreinformationonnegativenumberscanbefoundin Chapter10 .writeissystemcall4,anditrequiresthesameparametersasthereadsystemcall,exceptthatthebuffershouldalreadybelledwiththedatatowriteout.Thewritesystemcallwillgivebackthenumberofbyteswrittenin%eaxoranerrorcode. 4. Whenyouarethroughwithyourles,youcanthentellLinuxtoclosethem.Afterwards,yourledescriptorisnolongervalid.Thisisdoneusingclose,systemcall6.Theonlyparametertocloseistheledescriptor,whichisplacedin%ebxBuffersand.bssIntheprevioussectionwementionedbufferswithoutexplainingwhattheywere.Abufferisacontinuousblockofbytesusedforbulkdatatransfer.Whenyourequesttoreadale,theoperatingsystemneedstohaveaplacetostorethedataitreads.Thatplaceiscalledabuffer.Usuallybuffersareonlyusedtostoredatatemporarily,anditisthenreadfromthebuffersandconvertedtoaformthatiseasierfortheprogramstohandle.Ourprogramswon'tbecomplicatedenoughtoneedthatdone.Foranexample,let'ssaythatyouwanttoreadinasinglelineoftextfromalebutyoudonotknowhowlongthatlineis.Youwouldthensimply 76 PAGE 83 Chapter5.DealingwithFiles readalargenumberofbytes/charactersfromtheleintoabuffer,lookfortheend-of-linecharacter,andcopyallofthecharacterstothatend-of-linecharactertoanotherlocation.Ifyoudidn'tndandend-of-linecharacter,youwouldallocateanotherbufferandcontinuereading.Youwouldprobablywindupwithsomecharactersleftoverinyourbufferinthiscase,whichyouwoulduseasthestartingpointwhenyounextneeddatafromthele.2Anotherthingtonoteisthatbuffersareaxedsize,setbytheprogrammer.So,ifyouwanttoreadindata500bytesatatime,yousendthereadsystemcalltheaddressofa500-byteunusedlocation,andsenditthenumber500soitknowshowbigitis.Youcanmakeitsmallerorbigger,dependingonyourapplication'sneeds.Tocreateabuffer,youneedtoeitherreservestaticordynamicstorage.Staticstorageiswhatwehavetalkedaboutsofar,storagelocationsdeclaredusing.longor.bytedirectives.Dynamicstoragewillbediscussedin theSectioncalledGettingMoreMemoryinChapter9 .Thereareproblems,though,withdeclaringbuffersusing.byte.First,itistedioustotype.Youwouldhavetotype500numbersafterthe.bytedeclaration,andtheywouldn'tbeusedforanythingbuttotakeupspace.Second,itusesupspaceintheexecutable.Intheexampleswe'veusedsofar,itdoesn'tuseuptoomuch,butthatcanchangeinlargerprograms.Ifyouwant500bytesyouhavetotypein500numbersanditwastes500bytesintheexecutable.Thereisasolutiontobothofthese.Sofar,wehavediscussedtwoprogramsections,the.textandthe.datasections.Thereisanothersectioncalledthe.bss.Thissectionislikethedatasection,exceptthatitdoesn'ttakeupspaceintheexecutable.Thissectioncanreservestorage,butitcan'tinitializeit.Inthe.datasection,youcouldreservestorageandsetittoaninitialvalue.Inthe.bsssection,youcan'tsetaninitialvalue.Thisisusefulforbuffersbecausewedon'tneedtoinitializethemanyway,wejustneedtoreservestorage.Inordertodothis,wedothefollowingcommands:.section.bss 2. Whilethissoundscomplicated,mostofthetimeinprogrammingyouwillnotneedtodealdirectlywithbuffersandledescriptors.In Chapter8 youwilllearnhowtouseexistingcodepresentinLinuxtohandlemostofthecomplicationsofleinput/outputforyou. 77 PAGE 84 Chapter5.DealingwithFiles .lcommmy_buffer,500Thisdirective,.lcomm,willcreateasymbol,my_buffer,thatreferstoa500-bytestoragelocationthatwecanuseasabuffer.Wecanthendothefollowing,assumingwehaveopenedaleforreadingandhaveplacedtheledescriptorin%ebx:movl$my_buffer,%ecxmovl500,%edxmovl3,%eaxint$0x80Thiswillreadupto500bytesintoourbuffer.Inthisexample,Iplacedadollarsigninfrontofmy_buffer.Rememberthatthereasonforthisisthatwithoutthedollarsign,my_bufferistreatedasamemorylocation,andisaccessedindirectaddressingmode.Thedollarsignswitchesittoimmediatemodeaddressing,whichactuallyloadsthenumberrepresentedbymy_bufferi.e.-theaddressofthestartofourbuffer.whichistheaddressofmy_bufferitselfinto%ecx.StandardandSpecialFilesYoumightthinkthatprogramsstartwithoutanylesopenbydefault.Thisisnottrue.Linuxprogramsusuallyhaveatleastthreeopenledescriptorswhentheybegin.Theyare:STDIN Thisisthestandardinput.Itisaread-onlyle,andusuallyrepresentsyourkeyboard.3Thisisalwaysledescriptor0. 3. Aswementionedearlier,inLinux,almosteverythingisa"le".Yourkeyboardinputisconsideredale,andsoisyourscreendisplay. 78 PAGE 85 Chapter5.DealingwithFiles STDOUT Thisisthestandardoutput.Itisawrite-onlyle,andusuallyrepresentsyourscreendisplay.Thisisalwaysledescriptor1.STDERR Thisisyourstandarderror.Itisawrite-onlyle,andusuallyrepresentsyourscreendisplay.MostregularprocessingoutputgoestoSTDOUT,butanyerrormessagesthatcomeupintheprocessgotoSTDERR.Thisway,ifyouwantto,youcansplitthemupintoseparateplaces.Thisisalwaysledescriptor2.Anyofthese"les"canberedirectedfromortoarealle,ratherthanascreenorakeyboard.Thisisoutsidethescopeofthisbook,butanygoodbookontheUNIXcommand-linewilldescribeitindetail.Theprogramitselfdoesnotevenneedtobeawareofthisindirection-itcanjustusethestandardledescriptorsasusual.Noticethatmanyofthelesyouwritetoaren'tlesatall.UNIX-basedoperatingsystemstreatallinput/outputsystemsasles.Networkconnectionsaretreatedasles,yourserialportistreatedlikeale,evenyouraudiodevicesaretreatedasles.Communicationbetweenprocessesisusuallydonethroughspeciallescalledpipes.Someoftheseleshavedifferentmethodsofopeningandcreatingthemthanregularlesi.e.-theydon'tusetheopensystemcall,buttheycanallbereadfromandwrittentousingthestandardreadandwritesystemcalls.UsingFilesinaProgramWearegoingtowriteasimpleprogramtoillustratetheseconcepts.Theprogramwilltaketwoles,andreadfromone,convertallofitslower-caseletterstoupper-case,andwritetotheotherle.Beforewedoso,let'sthinkaboutwhatweneedtodotogetthejobdone: Haveafunctionthattakesablockofmemoryandconvertsittoupper-case.Thisfunctionwouldneedanaddressofablockofmemoryanditssizeas 79 PAGE 86 Chapter5.DealingwithFiles parameters. Haveasectionofcodethatrepeatedlyreadsintoabuffer,callsourconversionfunctiononthebuffer,andthenwritesthebufferbackouttotheotherle. Begintheprogrambyopeningthenecessaryles.NoticethatI'vespeciedthingsinreverseorderthattheywillbedone.That'sausefultrickinwritingcomplexprograms-rstdecidethemeatofwhatisbeingdone.Inthiscase,it'sconvertingblocksofcharacterstoupper-case.Then,youthinkaboutwhatallneedstobesetupandprocessedtogetthattohappen.Inthiscase,youhavetoopenles,andcontinuallyreadandwriteblockstodisk.Oneofthekeysofprogrammingiscontinuallybreakingdownproblemsintosmallerandsmallerchunksuntilit'ssmallenoughthatyoucaneasilysolvetheproblem.Thenyoucanbuildthesechunksbackupuntilyouhaveaworkingprogram.4Youmayhavebeenthinkingthatyouwillneverrememberallofthesenumbersbeingthrownatyou-thesystemcallnumbers,theinterruptnumber,etc.Inthisprogramwewillalsointroduceanewdirective,.equwhichshouldhelpout..equallowsyoutoassignnamestonumbers.Forexample,ifyoudid.equLINUX_SYSCALL,0x80,anytimeafterthatyouwroteLINUX_SYSCALL,theassemblerwouldsubstitue0x80forthat.Sonow,youcanwriteint$LINUX_SYSCALLwhichismucheasiertoread,andmucheasiertoremember.Codingiscomplex,buttherearealotofthingswecandolikethistomakeiteasier.Hereistheprogram.Notethatwehavemorelabelsthanweactuallyuseforjumps,becausesomeofthemarejustthereforclarity.Trytotracethroughtheprogramandseewhathappensinvariouscases.Anin-depthexplanationoftheprogramwillfollow.#PURPOSE:Thisprogramconvertsaninputfile 4. MaureenSprankle'sProblemSolvingandProgrammingConceptsisanexcellentbookontheproblem-solvingprocessappliedtocomputerprogramming. 80 PAGE 87 Chapter5.DealingwithFiles #toanoutputfilewithallletters#convertedtouppercase.##PROCESSING:1Opentheinputfile#2Opentheoutputfile#4Whilewe'renotattheendoftheinputfile#areadpartoffileintoourmemorybuffer#bgothrougheachbyteofmemory#ifthebyteisalower-caseletter,#convertittouppercase#cwritethememorybuffertooutputfile.section.data#######CONSTANTS#########systemcallnumbers.equSYS_OPEN,5.equSYS_WRITE,4.equSYS_READ,3.equSYS_CLOSE,6.equSYS_EXIT,1#optionsforopenlookat#/usr/include/asm/fcntl.hfor#variousvalues.Youcancombinethem#byaddingthemorORingthem#Thisisdiscussedatgreaterlength#in"CountingLikeaComputer".equO_RDONLY,0.equO_CREAT_WRONLY_TRUNC,03101#standardfiledescriptors.equSTDIN,0.equSTDOUT,1 81 PAGE 88 Chapter5.DealingwithFiles .equSTDERR,2#systemcallinterrupt.equLINUX_SYSCALL,0x80.equEND_OF_FILE,0#Thisisthereturnvalue#ofreadwhichmeanswe've#hittheendofthefile.equNUMBER_ARGUMENTS,2.section.bss#Buffer-thisiswherethedataisloadedinto#fromthedatafileandwrittenfrom#intotheoutputfile.Thisshould#neverexceed16,000forvarious#reasons..equBUFFER_SIZE,500.lcommBUFFER_DATA,BUFFER_SIZE.section.text#STACKPOSITIONS.equST_SIZE_RESERVE,8.equST_FD_IN,-4.equST_FD_OUT,-8.equST_ARGC,0#Numberofarguments.equST_ARGV_0,4#Nameofprogram.equST_ARGV_1,8#Inputfilename.equST_ARGV_2,12#Outputfilename.globl_start_start:###INITIALIZEPROGRAM####savethestackpointer 82 PAGE 89 Chapter5.DealingwithFiles movl%esp,%ebp#Allocatespaceforourfiledescriptors#onthestacksubl$ST_SIZE_RESERVE,%espopen_files:open_fd_in:###OPENINPUTFILE####opensyscallmovl$SYS_OPEN,%eax#inputfilenameinto%ebxmovlST_ARGV_1%ebp,%ebx#read-onlyflagmovl$O_RDONLY,%ecx#thisdoesn'treallymatterforreadingmovl$0666,%edx#callLinuxint$LINUX_SYSCALLstore_fd_in:#savethegivenfiledescriptormovl%eax,ST_FD_IN%ebpopen_fd_out:###OPENOUTPUTFILE####openthefilemovl$SYS_OPEN,%eax#outputfilenameinto%ebxmovlST_ARGV_2%ebp,%ebx#flagsforwritingtothefilemovl$O_CREAT_WRONLY_TRUNC,%ecx#modefornewfileifit'screatedmovl$0666,%edx#callLinux 83 PAGE 90 Chapter5.DealingwithFiles int$LINUX_SYSCALLstore_fd_out:#storethefiledescriptorheremovl%eax,ST_FD_OUT%ebp###BEGINMAINLOOP###read_loop_begin:###READINABLOCKFROMTHEINPUTFILE###movl$SYS_READ,%eax#gettheinputfiledescriptormovlST_FD_IN%ebp,%ebx#thelocationtoreadintomovl$BUFFER_DATA,%ecx#thesizeofthebuffermovl$BUFFER_SIZE,%edx#Sizeofbufferreadisreturnedin%eaxint$LINUX_SYSCALL###EXITIFWE'VEREACHEDTHEEND####checkforendoffilemarkercmpl$END_OF_FILE,%eax#iffoundoronerror,gototheendjleend_loopcontinue_read_loop:###CONVERTTHEBLOCKTOUPPERCASE###pushl$BUFFER_DATA#locationofbufferpushl%eax#sizeofthebuffercallconvert_to_upperpopl%eax#getthesizebackaddl$4,%esp#restore%esp###WRITETHEBLOCKOUTTOTHEOUTPUTFILE### 84 PAGE 91 Chapter5.DealingwithFiles #sizeofthebuffermovl%eax,%edxmovl$SYS_WRITE,%eax#filetousemovlST_FD_OUT%ebp,%ebx#locationofthebuffermovl$BUFFER_DATA,%ecxint$LINUX_SYSCALL###CONTINUETHELOOP###jmpread_loop_beginend_loop:###CLOSETHEFILES####NOTE-wedon'tneedtodoerrorchecking#onthese,becauseerrorconditions#don'tsignifyanythingspecialheremovl$SYS_CLOSE,%eaxmovlST_FD_OUT%ebp,%ebxint$LINUX_SYSCALLmovl$SYS_CLOSE,%eaxmovlST_FD_IN%ebp,%ebxint$LINUX_SYSCALL###EXIT###movl$SYS_EXIT,%eaxmovl$0,%ebxint$LINUX_SYSCALL#PURPOSE:Thisfunctionactuallydoesthe#conversiontouppercaseforablock##INPUT:Thefirstparameteristhelocation 85 PAGE 92 Chapter5.DealingwithFiles #oftheblockofmemorytoconvert#Thesecondparameteristhelengthof#thatbuffer##OUTPUT:Thisfunctionoverwritesthecurrent#bufferwiththeupper-casifiedversion.##VARIABLES:#%eax-beginningofbuffer#%ebx-lengthofbuffer#%edi-currentbufferoffset#%cl-currentbytebeingexamined#firstpartof%ecx####CONSTANTS###Thelowerboundaryofoursearch.equLOWERCASE_A,'a'#Theupperboundaryofoursearch.equLOWERCASE_Z,'z'#Conversionbetweenupperandlowercase.equUPPER_CONVERSION,'A'-'a'###STACKSTUFF###.equST_BUFFER_LEN,8#Lengthofbuffer.equST_BUFFER,12#actualbufferconvert_to_upper:pushl%ebpmovl%esp,%ebp###SETUPVARIABLES###movlST_BUFFER%ebp,%eaxmovlST_BUFFER_LEN%ebp,%ebxmovl$0,%edi 86 PAGE 93 Chapter5.DealingwithFiles #ifabufferwithzerolengthwasgiven#tous,justleavecmpl$0,%ebxjeend_convert_loopconvert_loop:#getthecurrentbytemovb%eax,%edi,1,%cl#gotothenextbyteunlessitisbetween#'a'and'z'cmpb$LOWERCASE_A,%cljlnext_bytecmpb$LOWERCASE_Z,%cljgnext_byte#otherwiseconvertthebytetouppercaseaddb$UPPER_CONVERSION,%cl#andstoreitbackmovb%cl,%eax,%edi,1next_byte:incl%edi#nextbytecmpl%edi,%ebx#continueunless#we'vereachedthe#endjneconvert_loopend_convert_loop:#noreturnvalue,justleavemovl%ebp,%esppopl%ebpretTypeinthisprogramastoupper.s,andthenenterinthefollowingcommands: 87 PAGE 94 Chapter5.DealingwithFiles astoupper.s-otoupper.oldtoupper.o-otoupperThisbuildsaprogramcalledtoupper,whichconvertsallofthelowercasecharactersinaletouppercase.Forexample,toconverttheletoupper.stouppercase,typeinthefollowingcommand:./touppertoupper.stoupper.uppercaseYouwillnowndintheletoupper.uppercaseanuppercaseversionofyouroriginalle.Let'sexaminehowtheprogramworks.TherstsectionoftheprogramismarkedCONSTANTS.Inprogramming,aconstantisavaluethatisassignedwhenaprogramassemblesorcompiles,andisneverchanged.Imakeahabitofplacingallofmyconstantstogetheratthebeginningoftheprogram.It'sonlynecessarytodeclarethembeforeyouusethem,butputtingthemallatthebeginningmakesthemeasytond.Makingthemallupper-casemakesitobviousinyourprogramwhichvaluesareconstantsandwheretondthem.5Inassemblylanguage,wedeclareconstantswiththe.equdirectiveasmentionedbefore.Here,wesimplygivenamestoallofthestandardnumberswe'veusedsofar,likesystemcallnumbers,thesyscallinterruptnumber,andleopenoptions.ThenextsectionismarkedBUFFERS.Weonlyuseonebufferinthisprogram,whichwecallBUFFER_DATA.Wealsodeneaconstant,BUFFER_SIZE,whichholdsthesizeofthebuffer.Ifwealwaysrefertothisconstantratherthantypingoutthenumber500wheneverweneedtousethesizeofthebuffer,ifitlaterchanges,weonlyneedtomodifythisvalue,ratherthanhavingtogothroughtheentireprogramandchangingallofthevaluesindividually.Insteadofgoingonthethe_startsectionoftheprogram,gototheendwherewedenetheconvert_to_upperfunction.Thisisthepartthatactuallydoesthe 5. Thisisfairlystandardpracticeamongprogrammersinalllanguages. 88 PAGE 95 Chapter5.DealingwithFiles conversion.ThissectionbeginswithalistofconstantsthatwewilluseThereasontheseareputhereratherthanatthetopisthattheyonlydealwiththisonefunction.Wehavethesedenitions:.equLOWERCASE_A,'a'.equLOWERCASE_Z,'z'.equUPPER_CONVERSION,'A'-'a'Thersttwosimplydenethelettersthataretheboundariesofwhatwearesearchingfor.Rememberthatinthecomputer,lettersarerepresentedasnumbers.Therefore,wecanuseLOWERCASE_Aincomparisons,additions,subtractions,oranythingelsewecanusenumbersin.Also,noticewedenetheconstantUPPER_CONVERSION.Sincelettersarerepresentedasnumbers,wecansubtractthem.Subtractinganupper-caseletterfromthesamelower-caselettergivesushowmuchweneedtoaddtoalower-caselettertomakeituppercase.Ifthatdoesn'tmakesense,lookattheASCIIcodetablesthemselvessee AppendixD .You'llnoticethatthenumberforthecharacterAis65andthecharacterais97.Theconversionfactoristhen-32.Foranylowercaseletterifyouadd-32,youwillgetit'scapitalequivalent.Afterthis,wehavesomeconstantslabelledSTACKPOSITIONS.Rememberthatfunctionparametersarepushedontothestackbeforefunctioncalls.TheseconstantsprexedwithSTforclaritydenewhereinthestackweshouldexpecttondeachpieceofdata.Thereturnaddressisatposition4+%esp,thelengthofthebufferisatposition8+%esp,andtheaddressofthebufferisatposition12+%esp.Usingsymbolsforthesenumbersinsteadofthenumbersthemselvesmakesiteasiertoseewhatdataisbeingusedandmoved.Nextcomesthelabelconvert_to_upper.Thisistheentrypointofthefunction.Thersttwolinesareourstandardfunctionlinestosavethestackpointer.ThenexttwolinesmovlST_BUFFER%ebp,%eax 89 PAGE 96 Chapter5.DealingwithFiles movlST_BUFFER_LEN%ebp,%ebxmovethefunctionparametersintotheappropriateregistersforuse.Then,weloadzerointo%edi.Whatwearegoingtodoisiteratethrougheachbyteofthebufferbyloadingfromthelocation%eax+%edi,incrementing%edi,andrepeatinguntil%ediisequaltothebufferlengthstoredin%ebx.Thelinescmpl$0,%ebxjeend_convert_looparejustasanitychecktomakesurethatnoonegaveusabufferofzerosize.Iftheydid,wejustcleanupandleave.Guardingagainstpotentialuserandprogrammingerrorsisanimportanttaskofaprogrammer.Youcanalwaysspecifythatyourfunctionshouldnottakeabufferofzerosize,butit'sevenbettertohavethefunctioncheckandhaveareliableexitplanifithappens.Nowwestartourloop.First,itmovesabyteinto%cl.Thecodeforthisismovb%eax,%edi,1,%clItisusinganindexedindirectaddressingmode.Itsaystostartat%eaxandgo%edilocationsforward,witheachlocationbeing1bytebig.Ittakesthevaluefoundthere,andputitin%cl.Afterthisitcheckstoseeifthatvalueisintherangeoflower-caseatolower-casez.Tochecktherange,itsimplycheckstoseeiftheletterissmallerthana.Ifitis,itcan'tbealower-caseletter.Likewise,ifitislargerthanz,itcan'tbealower-caseletter.So,ineachofthesecases,itsimplymoveson.Ifitisintheproperrange,itthenaddstheuppercaseconversion,andstoresitbackintothebuffer.Eitherway,itthengoestothenextvaluebyincrementing%cl;.Nextitcheckstoseeifweareattheendofthebuffer.Ifwearenotattheend,wejumpbacktothebeginningofthelooptheconvert_looplabel.Ifweareattheend,itsimplycontinuesontotheendofthefunction.Becausewearemodifyingthebufferdirectly,wedon'tneedtoreturnanythingtothecallingprogram-thechangesare 90 PAGE 97 Chapter5.DealingwithFiles alreadyinthebuffer.Thelabelend_convert_loopisnotneeded,butit'stheresoit'seasytoseewherethepartsoftheprogramare.Nowweknowhowtheconversionprocessworks.Nowweneedtogureouthowtogetthedatainandoutoftheles.Beforereadingandwritingtheleswemustopenthem.TheUNIXopensystemcalliswhathandlesthis.Ittakesthefollowingparameters: %eaxcontainsthesystemcallnumberasusual-5inthiscase. %ebxcontainsapointertoastringthatisthenameoftheletoopen.Thestringmustbeterminatedwiththenullcharacter. %ecxcontainstheoptionsusedforopeningthele.ThesetellLinuxhowtoopenthele.Theycanindicatethingssuchasopenforreading,openforwriting,openforreadingandwriting,createifitdoesn'texist,deletetheleifitalreadyexists,etc.Wewillnotgointohowtocreatethenumbersfortheoptionsuntil theSectioncalledTruth,Falsehood,andBinaryNumbersinChapter10 .Fornow,justtrustthenumberswecomeupwith. %edxcontainsthepermissionsthatareusedtoopenthele.Thisisusedincasethelehastobecreatedrst,soLinuxknowswhatpermissionstocreatethelewith.Theseareexpressedinoctal,justlikeregularUNIXpermissions.6Aftermakingthesystemcall,theledescriptorofthenewly-openedleisstoredin%eax.So,whatlesareweopening?Inthisexample,wewillbeopeningthelesspeciedonthecommand-line.Fortunately,command-lineparametersarealreadystoredbyLinuxinaneasy-to-accesslocation,andarealreadynull-terminated.WhenaLinuxprogrambegins,allpointerstocommand-lineargumentsarestoredonthestack.Thenumberofargumentsisstoredat8%esp,thenameoftheprogramisstoredat12%esp,andtheargumentsarestoredfrom16%espon. 6. Ifyouaren'tfamiliarwithUNIXpermissions,justput$0666here.Don'tforgettheleadingzero,asitmeansthatthenumberisanoctalnumber. 91 PAGE 98 Chapter5.DealingwithFiles IntheCProgramminglanguage,thisisreferredtoastheargvarray,sowewillrefertoitthatwayinourprogram.Therstthingourprogramdoesissavethecurrentstackpositionin%ebpandthenreservesomespaceonthestacktostoretheledescriptors.Afterthis,itstartsopeningles.Therstletheprogramopensistheinputle,whichistherstcommand-lineargument.Wedothisbysettingupthesystemcall.Weputthelenameinto%ebx,theread-onlymodenumberinto%ecx,thedefaultmodeof$0666into%edx,andthesystemcallnumberinto%eaxAfterthesystemcall,theleisopenandtheledescriptorisstoredin%eax.7Theledescriptoristhentransferredtoit'sappropriateplaceonthestack.Thesameisthendonefortheoutputle,exceptthatitiscreatedwithawrite-only,create-if-doesn't-exist,truncate-if-does-existmode.Itsledescriptorisstoredaswell.Nowwegettothemainpart-theread/writeloop.Basically,wewillreadxed-sizechunksofdatafromtheinputle,callourconversionfunctiononit,andwriteitbacktotheoutputle.Althoughwearereadingxed-sizechunks,thesizeofthechunksdon'tmatterforthisprogram-wearejustoperatingonstraightsequencesofcharacters.Wecouldreaditinwithaslittleoraslargeofchunksaswewant,anditstillwouldworkproperly.Therstpartoftheloopistoreadthedata.Thisusesthereadsystemcall.Thiscalljusttakesaledescriptortoreadfrom,abuffertowriteinto,andthesizeofthebufferi.e.-themaximumnumberofbytesthatcouldbewritten.Thesystemcallreturnsthenumberofbytesactuallyread,orend-of-lethenumber0. 7. Noticethatwedon'tdoanyerrorcheckingonthis.Thatisdonejusttokeeptheprogramsimple.Innormalprograms,everysystemcallshouldnormallybecheckedforsuccessorfailure.Infailurecases,%eaxwillholdanerrorcodeinsteadofareturnvalue.Errorcodesarenegative,sotheycanbedetectedbycomparing%eaxtozeroandjumpingifitislessthanzero. 92 PAGE 99 Chapter5.DealingwithFiles Afterreadingablock,wecheck%eaxforanend-of-lemarker.Iffound,itexitstheloop.Otherwisewekeepongoing.Afterthedataisread,theconvert_to_upperfunctioniscalledwiththebufferwejustreadinandthenumberofcharactersreadintheprevioussystemcall.Afterthisfunctionexecutes,thebuffershouldbecapitalizedandreadytowriteout.Theregistersarethenrestoredwithwhattheyhadbefore.Finally,weissueawritesystemcall,whichisexactlylikethereadsystemcall,exceptthatitmovesthedatafromthebufferouttothele.Nowwejustgobacktothebeginningoftheloop.Aftertheloopexitsremember,itexitsif,afteraread,itdetectstheendofthele,itsimplyclosesitsledescriptorsandexits.Theclosesystemcalljusttakestheledescriptortoclosein%ebx.Theprogramisthennished!ReviewKnowtheConcepts Describethelifecycleofaledescriptor. Whatarethestandardledescriptorsandwhataretheyusedfor? Whatisabuffer? Whatisthedifferencebetweenthe.datasectionandthe.bsssection? Whatarethesystemcallsrelatedtoreadingandwritingles? 93 PAGE 100 Chapter5.DealingwithFiles UsetheConcepts ModifythetoupperprogramsothatitreadsfromSTDINandwritestoSTDOUTinsteadofusingthelesonthecommand-line. Changethesizeofthebuffer. Rewritetheprogramsothatitusesstorageinthe.bsssectionratherthanthestacktostoretheledescriptors. Writeaprogramthatwillcreatealecalledheynow.txtandwritethewords"Heydiddlediddle!"intoit.GoingFurther Whatdifferencedoesthesizeofthebuffermake? Whaterrorresultscanbereturnedbyeachofthesesystemcalls? Maketheprogramabletoeitheroperateoncommand-lineargumentsoruseSTDINorSTDOUTbasedonthenumberofcommand-lineargumentsspeciedbyARGC. Modifytheprogramsothatitcheckstheresultsofeachsystemcall,andprintsoutanerrormessagetoSTDOUTwhenitoccurs. 94 PAGE 101 Chapter6.ReadingandWritingSimpleRecordsAsmentionedin Chapter5 ,manyapplicationsdealwithdatathatispersistent-meaningthatthedataliveslongerthantheprogrambybeingstoredondiskinfles.Youcanshutdowntheprogramandopenitbackup,andyouarebackwhereyoustarted.Now,therearetwobasickindsofpersistentdata-structuredandunstructured.Unstructureddataislikewhatwedealtwithinthetoupperprogram.Itjustdealtwithtextlesthatwereenteredbyaperson.Thecontentsofthelesweren'tusablebyaprogrambecauseaprogramcan'tinterpretwhattheuseristryingtosayinrandomtext.Structureddata,ontheotherhand,iswhatcomputersexcelathandling.Structureddataisdatathatisdividedupintoeldsandrecords.Forthemostpart,theeldsandrecordsarexed-length.Becausethedataisdividedintoxed-lengthrecordsandxed-formatelds,thecomputercaninterpretthedata.Structureddatacancontainvariable-lengthelds,butatthatpointyouareusuallybetteroffwithadatabase.1Thischapterdealswithreadingandwritingsimplexed-lengthrecords.Let'ssaywewantedtostoresomebasicinformationaboutpeopleweknow.Wecouldimaginethefollowingexamplexed-lengthrecordaboutpeople: Firstname-40bytes Lastname-40bytes Address-240bytes Age-4bytes 1. Adatabaseisaprogramwhichhandlespersistentstructureddataforyou.Youdon'thavetowritetheprogramstoreadandwritethedatatodisk,todolookups,oreventodobasicprocessing.Itisaveryhigh-levelinterfacetostructureddatawhich,althoughitaddssomeoverheadandadditionalcomplexity,isveryusefulforcomplexdataprocessingtasks.Referencesforlearninghowdatabasesworkarelistedin Chapter13 95 PAGE 102 Chapter6.ReadingandWritingSimpleRecords Inthis,everythingischaracterdataexceptfortheage,whichissimplyanumericeld,usingastandard4-bytewordwecouldjustuseasinglebyteforthis,butkeepingitatawordmakesiteasiertoprocess.Inprogramming,youoftenhavecertaindenitionsthatyouwilluseoverandoveragainwithintheprogram,orperhapswithinseveralprograms.Itisgoodtoseparatetheseoutintolesthataresimplyincludedintotheassemblylanguagelesasneeded.Forexample,inournextprogramswewillneedtoaccessthedifferentpartsoftherecordabove.Thismeansweneedtoknowtheoffsetsofeacheldfromthebeginningoftherecordinordertoaccessthemusingbasepointeraddressing.Thefollowingconstantsdescribetheoffsetstotheabovestructure.Puttheminalenamedrecord-def.s:.equRECORD_FIRSTNAME,0.equRECORD_LASTNAME,40.equRECORD_ADDRESS,80.equRECORD_AGE,320.equRECORD_SIZE,324Inaddition,thereareseveralconstantsthatwehavebeendeningoverandoverinourprograms,anditisusefultoputtheminale,sothatwedon'thavetokeepenteringthem.Putthefollowingconstantsinalecalledlinux.s:#CommonLinuxDefinitions#SystemCallNumbers.equSYS_EXIT,1.equSYS_READ,3.equSYS_WRITE,4.equSYS_OPEN,5.equSYS_CLOSE,6.equSYS_BRK,45 96 PAGE 103 Chapter6.ReadingandWritingSimpleRecords #SystemCallInterruptNumber.equLINUX_SYSCALL,0x80#StandardFileDescriptors.equSTDIN,0.equSTDOUT,1.equSTDERR,2#CommonStatusCodes.equEND_OF_FILE,0Wewillwritethreeprogramsinthischapterusingthestructuredenedinrecord-def.s.Therstprogramwillbuildalecontainingseveralrecordsasdenedabove.Thesecondprogramwilldisplaytherecordsinthele.Thethirdprogramwilladd1yeartotheageofeveryrecord.Inadditiontothestandardconstantswewillbeusingthroughouttheprograms,therearealsotwofunctionsthatwewillbeusinginseveraloftheprograms-onewhichreadsarecordandonewhichwritesarecord.Whatparametersdothesefunctionsneedinordertooperate?Webasicallyneed: Thelocationofabufferthatwecanreadarecordinto TheledescriptorthatwewanttoreadfromorwritetoLet'slookatourreadingfunctionrst:.include"record-def.s".include"linux.s"#PURPOSE:Thisfunctionreadsarecordfromthefile 97 PAGE 104 Chapter6.ReadingandWritingSimpleRecords #descriptor##INPUT:Thefiledescriptorandabuffer##OUTPUT:Thisfunctionwritesthedatatothebuffer#andreturnsastatuscode.##STACKLOCALVARIABLES.equST_READ_BUFFER,8.equST_FILEDES,12.section.text.globlread_record.typeread_record,@functionread_record:pushl%ebpmovl%esp,%ebppushl%ebxmovlST_FILEDES%ebp,%ebxmovlST_READ_BUFFER%ebp,%ecxmovl$RECORD_SIZE,%edxmovl$SYS_READ,%eaxint$LINUX_SYSCALL#NOTE-%eaxhasthereturnvalue,whichwewill#givebacktoourcallingprogrampopl%ebxmovl%ebp,%esppopl%ebpretIt'saprettysimplyfunction.Itjustreadsdatathesizeofourstructureintoanappropriatelysizedbufferfromthegivenledescriptor.Thewritingoneissimilar: 98 PAGE 105 Chapter6.ReadingandWritingSimpleRecords .include"linux.s".include"record-def.s"#PURPOSE:Thisfunctionwritesarecordto#thegivenfiledescriptor##INPUT:Thefiledescriptorandabuffer##OUTPUT:Thisfunctionproducesastatuscode##STACKLOCALVARIABLES.equST_WRITE_BUFFER,8.equST_FILEDES,12.section.text.globlwrite_record.typewrite_record,@functionwrite_record:pushl%ebpmovl%esp,%ebppushl%ebxmovl$SYS_WRITE,%eaxmovlST_FILEDES%ebp,%ebxmovlST_WRITE_BUFFER%ebp,%ecxmovl$RECORD_SIZE,%edxint$LINUX_SYSCALL#NOTE-%eaxhasthereturnvalue,whichwewill#givebacktoourcallingprogrampopl%ebxmovl%ebp,%esppopl%ebpret 99 PAGE 106 Chapter6.ReadingandWritingSimpleRecords Nowthatwehaveourbasicdenitionsdown,wearereadytowriteourprograms.WritingRecordsThisprogramwillsimplywritesomehardcodedrecordstodisk.Itwill: Openthele Writethreerecords ClosetheleTypethefollowingcodeintoalecalledwrite-records.s:.include"linux.s".include"record-def.s".section.data#Constantdataoftherecordswewanttowrite#Eachtextdataitemispaddedtotheproper#lengthwithnulli.e.0bytes.#.reptisusedtopadeachitem..repttells#theassemblertorepeatthesectionbetween#.reptand.endrthenumberoftimesspecified.#Thisisusedinthisprogramtoaddextranull#charactersattheendofeachfieldtofill#ituprecord1:.ascii"Fredrick".rept31#Paddingto40bytes.byte0.endr.ascii"Bartlett" 100 PAGE 107 Chapter6.ReadingandWritingSimpleRecords .rept31#Paddingto40bytes.byte0.endr.ascii"4242SPrairienTulsa,OK55555".rept209#Paddingto240bytes.byte0.endr.long45record2:.ascii"Marilyn".rept32#Paddingto40bytes.byte0.endr.ascii"Taylor".rept33#Paddingto40bytes.byte0.endr.ascii"2224SJohannanStnChicago,IL12345".rept203#Paddingto240bytes.byte0.endr.long29record3:.ascii"Derrick".rept32#Paddingto40bytes.byte0.endr 101 PAGE 108 Chapter6.ReadingandWritingSimpleRecords .ascii"McIntire".rept31#Paddingto40bytes.byte0.endr.ascii"500WOaklandnSanDiego,CA54321".rept206#Paddingto240bytes.byte0.endr.long36#Thisisthenameofthefilewewillwritetofile_name:.ascii"test.dat".equST_FILE_DESCRIPTOR,-4.globl_start_start:#Copythestackpointerto%ebpmovl%esp,%ebp#Allocatespacetoholdthefiledescriptorsubl$4,%esp#Openthefilemovl$SYS_OPEN,%eaxmovl$file_name,%ebxmovl$0101,%ecx#Thissaystocreateifit#doesn'texist,andopenfor#writingmovl$0666,%edxint$LINUX_SYSCALL#Storethefiledescriptorawaymovl%eax,ST_FILE_DESCRIPTOR%ebp 102 PAGE 109 Chapter6.ReadingandWritingSimpleRecords #WritethefirstrecordpushlST_FILE_DESCRIPTOR%ebppushl$record1callwrite_recordaddl$8,%esp#WritethesecondrecordpushlST_FILE_DESCRIPTOR%ebppushl$record2callwrite_recordaddl$8,%esp#WritethethirdrecordpushlST_FILE_DESCRIPTOR%ebppushl$record3callwrite_recordaddl$8,%esp#Closethefiledescriptormovl$SYS_CLOSE,%eaxmovlST_FILE_DESCRIPTOR%ebp,%ebxint$LINUX_SYSCALL#Exittheprogrammovl$SYS_EXIT,%eaxmovl$0,%ebxint$LINUX_SYSCALLThisisafairlysimpleprogram.Itmerelyconsistsofdeningthedatawewanttowriteinthe.datasection,andthencallingtherightsystemcallsandfunctioncallstoaccomplishit.Forarefresherofallofthesystemcallsused,see AppendixC 103 PAGE 110 Chapter6.ReadingandWritingSimpleRecords Youmayhavenoticedthelines:.include"linux.s".include"record-def.s"Thesestatementscausethegivenlestobasicallybepastedrightthereinthecode.Youdon'tneedtodothiswithfunctions,becausethelinkercantakecareofcombiningfunctionsexportedwith.globl.However,constantsdenedinanotherledoneedtobeimportedinthisway.Also,youmayhavenoticedtheuseofanewassemblerdirective,.rept.Thisdirectiverepeatsthecontentsofthelebetweenthe.reptandthe.endrdirectivesthenumberoftimesspeciedafter.rept.Thisisusuallyusedthewayweusedit-topadvaluesinthe.datasection.Inourcase,weareaddingnullcharacterstotheendofeachelduntiltheyaretheirdenedlengths.Tobuildtheapplication,runthecommands:aswrite-records.s-owrite-record.oaswrite-record.s-owrite-record.oldwrite-record.owrite-records.o-owrite-recordsHereweareassemblingtwolesseparately,andthencombiningthemtogetherusingthelinker.Toruntheprogram,justtypethefollowing:./write-recordsThiswillcausealecalledtest.dattobecreatedcontainingtherecords.However,sincetheycontainnon-printablecharactersthenullcharacter,specically,theymaynotbeviewablebyatexteditor.Thereforeweneedthenextprogramtoreadthemforus. 104 PAGE 111 Chapter6.ReadingandWritingSimpleRecords ReadingRecordsNowwewillconsidertheprocessofreadingrecords.Inthisprogram,wewillreadeachrecordanddisplaytherstnamelistedwitheachrecord.Sinceeachperson'snameisadifferentlength,wewillneedafunctiontocountthenumberofcharacterswewanttowrite.Sincewepadeacheldwithnullcharacters,wecansimplycountcharactersuntilwereachanullcharacter.2Notethatthismeansourrecordsmustcontainatleastonenullcharactereach.Hereisthecode.Putitinalecalledcount-chars.s:#PURPOSE:Countthecharactersuntilanullbyteisreached.##INPUT:Theaddressofthecharacterstring##OUTPUT:Returnsthecountin%eax##PROCESS:#Registersused:#%ecx-charactercount#%al-currentcharacter#%edx-currentcharacteraddress.typecount_chars,@function.globlcount_chars#Thisiswhereouroneparameterisonthestack.equST_STRING_START_ADDRESS,8count_chars:pushl%ebpmovl%esp,%ebp#Counterstartsatzeromovl$0,%ecx 2. IfyouhaveusedC,thisiswhatthestrlenfunctiondoes. 105 PAGE 112 Chapter6.ReadingandWritingSimpleRecords #StartingaddressofdatamovlST_STRING_START_ADDRESS%ebp,%edxcount_loop_begin:#Grabthecurrentcharactermovb%edx,%al#Isitnull?cmpb$0,%al#Ifyes,we'redonejecount_loop_end#Otherwise,incrementthecounterandthepointerincl%ecxincl%edx#Gobacktothebeginningoftheloopjmpcount_loop_begincount_loop_end:#We'redone.Movethecountinto%eax#andreturn.movl%ecx,%eaxpopl%ebpretAsyoucansee,it'safairlystraightforwardfunction.Itsimplyloopsthroughthebytes,countingasitgoes,untilithitsanullcharacter.Thenitreturnsthecount.Ourrecord-readingprogramwillbefairlystraightforward,too.Itwilldothefollowing: Openthele Attempttoreadarecord 106 PAGE 113 Chapter6.ReadingandWritingSimpleRecords Ifweareattheendofthele,exit Otherwise,countthecharactersoftherstname WritetherstnametoSTDOUT WriteanewlinetoSTDOUT GobacktoreadanotherrecordTowritethis,weneedonemoresimplefunction-afunctiontowriteoutanewlinetoSTDOUT.Putthefollowingcodeintowrite-newline.s:.include"linux.s".globlwrite_newline.typewrite_newline,@function.section.datanewline:.ascii"n".section.text.equST_FILEDES,8write_newline:pushl%ebpmovl%esp,%ebpmovl$SYS_WRITE,%eaxmovlST_FILEDES%ebp,%ebxmovl$newline,%ecxmovl$1,%edxint$LINUX_SYSCALLmovl%ebp,%esppopl%ebpret 107 PAGE 114 Chapter6.ReadingandWritingSimpleRecords Nowwearereadytowritethemainprogram.Hereisthecodetoread-records.s:.include"linux.s".include"record-def.s".section.datafile_name:.ascii"test.dat".section.bss.lcommrecord_buffer,RECORD_SIZE.section.text#Mainprogram.globl_start_start:#Thesearethelocationsonthestackwhere#wewillstoretheinputandoutputdescriptors#FYI-wecouldhaveusedmemoryaddressesin#a.datasectioninstead.equST_INPUT_DESCRIPTOR,-4.equST_OUTPUT_DESCRIPTOR,-8#Copythestackpointerto%ebpmovl%esp,%ebp#Allocatespacetoholdthefiledescriptorssubl$8,%esp#Openthefilemovl$SYS_OPEN,%eaxmovl$file_name,%ebxmovl$0,%ecx#Thissaystoopenread-only 108 PAGE 115 Chapter6.ReadingandWritingSimpleRecords movl$0666,%edxint$LINUX_SYSCALL#Savefiledescriptormovl%eax,ST_INPUT_DESCRIPTOR%ebp#Eventhoughit'saconstant,weare#savingtheoutputfiledescriptorin#alocalvariablesothatifwelater#decidethatitisn'talwaysgoingto#beSTDOUT,wecanchangeiteasily.movl$STDOUT,ST_OUTPUT_DESCRIPTOR%ebprecord_read_loop:pushlST_INPUT_DESCRIPTOR%ebppushl$record_buffercallread_recordaddl$8,%esp#Returnsthenumberofbytesread.#Ifitisn'tthesamenumberwe#requested,thenit'seitheran#end-of-file,oranerror,sowe're#quittingcmpl$RECORD_SIZE,%eaxjnefinished_reading#Otherwise,printoutthefirstname#butfirst,wemustknowit'ssizepushl$RECORD_FIRSTNAME+record_buffercallcount_charsaddl$4,%esp 109 PAGE 116 Chapter6.ReadingandWritingSimpleRecords movl%eax,%edxmovlST_OUTPUT_DESCRIPTOR%ebp,%ebxmovl$SYS_WRITE,%eaxmovl$RECORD_FIRSTNAME+record_buffer,%ecxint$LINUX_SYSCALLpushlST_OUTPUT_DESCRIPTOR%ebpcallwrite_newlineaddl$4,%espjmprecord_read_loopfinished_reading:movl$SYS_EXIT,%eaxmovl$0,%ebxint$LINUX_SYSCALLTobuildthisprogram,weneedtoassembleallofthepartsandlinkthemtogether:asread-record.s-oread-record.oascount-chars.s-ocount-chars.oaswrite-newline.s-owrite-newline.oasread-records.s-oread-records.oldread-record.ocount-chars.owrite-newline.oread-records.o-oread-recordsThebackslashintherstlinesimplymeansthatthecommandcontinuesonthenextline.Youcanrunyourprogrambydoing./read-records.Asyoucansee,thisprogramopenstheleandthenrunsaloopofreading,checkingfortheendofle,andwritingtherstname.Theoneconstructthatmightbenewisthelinethatsays:pushl$RECORD_FIRSTNAME+record_buffer 110 PAGE 117 Chapter6.ReadingandWritingSimpleRecords Itlookslikewearecombiningandaddinstructionwithapushinstruction,butwearenot.Yousee,bothRECORD_FIRSTNAMEandrecord_bufferareconstants.Therstisadirectconstant,createdthroughtheuseofa.equdirective,whilethelatterisdenedautomaticallybytheassemblerthroughitsuseasalabelit'svaluebeingtheaddressthatthedatathatfollowsitwillstartat.Sincetheyarebothconstantsthattheassemblerknows,itisabletoaddthemtogetherwhileitisassemblingyourprogram,sothewholeinstructionisasingleimmediate-modepushofasingleconstant.TheRECORD_FIRSTNAMEconstantisthenumberofbytesafterthebeginningofarecordbeforewehittherstname.record_bufferisthenameofourbufferforholdingrecords.Addingthemtogethergetsustheaddressoftherstnamememberoftherecordstoredinrecord_buffer.ModifyingtheRecordsInthissection,wewillwriteaprogramthat: Opensaninputandoutputle Readsrecordsfromtheinput Incrementstheage WritesthenewrecordtotheoutputleLikemostprogramswe'veencounteredrecently,thisprogramisprettystraightforward.3.include"linux.s".include"record-def.s" 3. Youwillndthatafterlearningthemechanicsofprogramming,mostprogramsareprettystraightforwardonceyouknowexactlywhatitisyouwanttodo.Mostofthemini-tializedata,dosomeprocessinginaloop,andthencleaneverythingup. 111 PAGE 118 Chapter6.ReadingandWritingSimpleRecords .section.datainput_file_name:.ascii"test.dat"output_file_name:.ascii"testout.dat".section.bss.lcommrecord_buffer,RECORD_SIZE#Stackoffsetsoflocalvariables.equST_INPUT_DESCRIPTOR,-4.equST_OUTPUT_DESCRIPTOR,-8.section.text.globl_start_start:#Copystackpointerandmakeroomforlocalvariablesmovl%esp,%ebpsubl$8,%esp#Openfileforreadingmovl$SYS_OPEN,%eaxmovl$input_file_name,%ebxmovl$0,%ecxmovl$0666,%edxint$LINUX_SYSCALLmovl%eax,ST_INPUT_DESCRIPTOR%ebp#Openfileforwritingmovl$SYS_OPEN,%eaxmovl$output_file_name,%ebxmovl$0101,%ecx 112 PAGE 119 Chapter6.ReadingandWritingSimpleRecords movl$0666,%edxint$LINUX_SYSCALLmovl%eax,ST_OUTPUT_DESCRIPTOR%ebploop_begin:pushlST_INPUT_DESCRIPTOR%ebppushl$record_buffercallread_recordaddl$8,%esp#Returnsthenumberofbytesread.#Ifitisn'tthesamenumberwe#requested,thenit'seitheran#end-of-file,oranerror,sowe're#quittingcmpl$RECORD_SIZE,%eaxjneloop_end#Incrementtheageinclrecord_buffer+RECORD_AGE#WritetherecordoutpushlST_OUTPUT_DESCRIPTOR%ebppushl$record_buffercallwrite_recordaddl$8,%espjmploop_beginloop_end:movl$SYS_EXIT,%eaxmovl$0,%ebxint$LINUX_SYSCALL 113 PAGE 120 Chapter6.ReadingandWritingSimpleRecords Youcantypeitinasadd-year.s.Tobuildit,typethefollowing4:asadd-year.s-oadd-year.oldadd-year.oread-record.owrite-record.o-oadd-yearToruntheprogram,justtypeinthefollowing5:./add-yearThiswilladdayeartoeveryrecordlistedintest.datandwritethenewrecordstotheletestout.dat.Asyoucansee,writingxed-lengthrecordsisprettysimple.Youonlyhavetoreadinblocksofdatatoabuffer,processthem,andwritethembackout.Unfortunately,thisprogramdoesn'twritethenewagesouttothescreensoyoucanverifyyourprogram'seffectiveness.Thisisbecausewewon'tgettodisplayingnumbersuntil Chapter8 and Chapter10 .Afterreadingthoseyoumaywanttocomebackandrewritethisprogramtodisplaythenumericdatathatwearemodifying.ReviewKnowtheConcepts Whatisarecord? Whatistheadvantageofxed-lengthrecordsovervariable-lengthrecords? Howdoyouincludeconstantsinmultipleassemblysourceles? 4. Thisassumesthatyouhavealreadybuilttheobjectlesread-record.oandwrite-record.ointhepreviousexamples.Ifnot,youwillhavetodoso. 5. Thisisassumingyoucreatedtheleinapreviousrunofwrite-records.Ifnot,youneedtorunwrite-recordsrstbeforerunningthisprogram. 114 PAGE 121 Chapter6.ReadingandWritingSimpleRecords Whymightyouwanttosplitupaprojectintomultiplesourceles? Whatdoestheinstructioninclrecord_buffer+RECORD_AGEdo?Whataddressingmodeisitusing?Howmanyoperandsdoestheinclinstructionshaveinthiscase?Whichpartsarebeinghandledbytheassemblerandwhichpartsarebeinghandledwhentheprogramisrun?UsetheConcepts Addanotherdatamembertothepersonstructuredenedinthischapter,andrewritethereadingandwritingfunctionsandprogramstotakethemintoaccount.Remembertoreassembleandrelinkyourlesbeforerunningyourprograms. Createaprogramthatusesalooptowrite30identicalrecordstoale. Createaprogramtondthelargestageintheleandreturnthatageasthestatuscodeoftheprogram. Createaprogramtondthesmallestageintheleandreturnthatageasthestatuscodeoftheprogram.GoingFurther Rewritetheprogramsinthischaptertousecommand-lineargumentstospecifythelesnames. Researchthelseeksystemcall.Rewritetheadd-yearprogramtoopenthesourceleforbothreadingandwritinguse$2fortheread/writemode,andwritethemodiedrecordsbacktothesameletheywerereadfrom. Researchthevariouserrorcodesthatcanbereturnedbythesystemcallsmadeintheseprograms.Pickonetorewrite,andaddcodethatchecks%eaxforerror 115 PAGE 122 Chapter6.ReadingandWritingSimpleRecords conditions,and,ifoneisfound,writesamessageaboutittoSTDERRandexit. Writeaprogramthatwilladdasinglerecordtothelebyreadingthedatafromthekeyboard.Remember,youwillhavetomakesurethatthedatahasatleastonenullcharacterattheend,andyouneedtohaveawayfortheusertoindicatetheyaredonetyping.Becausewehavenotgottenintocharacterstonumbersconversion,youwillnotbeabletoreadtheageinfromthekeyboard,soyou'llhavetohaveadefaultage. Writeafunctioncalledcompare-stringsthatwillcomparetwostringsupto5characters.Thenwriteaprogramthatallowstheusertoenter5characters,andhavetheprogramreturnallrecordswhoserstnamestartswiththose5characters. 116 PAGE 123 Chapter7.DevelopingRobustProgramsThischapterdealswithdevelopingprogramsthatarerobust.Robustprogramsareabletohandleerrorconditionsgracefully.Theyareprogramsthatdonotcrashnomatterwhattheuserdoes.Buildingrobustprogramsisessentialtothepracticeofprogramming.Writingrobustprogramstakesdisciplineandwork-itusuallyentailsndingeverypossibleproblemthatcanoccur,andcomingupwithanactionplanforyourprogramtotake.WhereDoestheTimeGo?Programmersschedulepoorly.Inalmosteveryprogrammingproject,programmerswilltaketwo,four,oreveneighttimesaslongtodevelopaprogramorfunctionthantheyoriginallyestimated.Therearemanyreasonsforthisproblem,including: Programmersdon'talwaysscheduletimeformeetingsorothernon-codingactivitiesthatmakeupeveryday. Programmersoftenunderestimatefeedbacktimeshowlongittakestopasschangerequestsandapprovalsbackandforthforprojects. Programmersdon'talwaysunderstandthefullscopeofwhattheyareproducing. Programmersoftenhavetoestimateascheduleonatotallydifferentkindofprojectthantheyareusedto,andthusareunabletoscheduleaccurately. Programmersoftenunderestimatetheamountoftimeittakestogetaprogramfullyrobust.Thelastitemistheoneweareinterestedinhere.Ittakesalotoftimeandefforttodeveloprobustprograms.Moresothanpeopleusuallyguess,including 117 PAGE 124 Chapter7.DevelopingRobustPrograms experiencedprogrammers.Programmersgetsofocusedonsimplysolvingtheproblemathandthattheyfailtolookatthepossiblesideissues.Inthetoupperprogram,wedonothaveanycourseofactioniftheletheuserselectsdoesnotexist.Theprogramwillgoaheadandtrytoworkanyway.Itdoesn'treportanyerrormessagesotheuserwon'tevenknowthattheytypedinthenamewrong.Let'ssaythatthedestinationleisonanetworkdrive,andthenetworktemporarilyfails.Theoperatingsystemisreturningastatuscodetousin%eax,butwearen'tcheckingit.Therefore,ifafailureoccurs,theuseristotallyunaware.Thisprogramisdenitelynotrobust.Asyoucansee,eveninasimpleprogramtherearealotofthingsthatcangowrongthataprogrammermustcontendwith.Inalargeprogram,itgetsmuchmoreproblematic.Thereareusuallymanymorepossibleerrorconditionsthanpossiblesuccessfulconditions.Therefore,youshouldalwaysexpecttospendthemajorityofyourtimecheckingstatuscodes,writingerrorhandlers,andperformingsimilartaskstomakeyourprogramrobust.Ifittakestwoweekstodevelopaprogram,itwilllikelytakeatleasttwomoretomakeitrobust.Rememberthateveryerrormessagethatpopsuponyourscreenhadtobeprogrammedinbysomeone.SomeTipsforDevelopingRobustProgramsUserTestingTestingisoneofthemostessentialthingsaprogrammerdoes.Ifyouhaven'ttestedsomething,youshouldassumeitdoesn'twork.However,testingisn'tjustaboutmakingsureyourprogramworks,it'saboutmakingsureyourprogramdoesn'tbreak.Forexample,ifIhaveaprogramthatisonlysupposedtodealwithpositivenumbers,youneedtotestwhathappensiftheuserentersanegativenumber.Oraletter.Orthenumberzero.Youmusttestwhathappensiftheyputspacesbeforetheirnumbers,spacesaftertheirnumbers,andotherlittle 118 PAGE 125 Chapter7.DevelopingRobustPrograms possibilities.Youneedtomakesurethatyouhandletheuser'sdatainawaythatmakessensetotheuser,andthatyoupassonthatdatainawaythatmakessensetotherestofyourprogram.Whenyourprogramndsinputthatdoesn'tmakesense,itneedstoperformappropriateactions.Dependingonyourprogram,thismayincludeendingtheprogram,promptingtheusertore-entervalues,notifyingacentralerrorlog,rollingbackanoperation,orignoringitandcontinuing.Notonlyshouldyoutestyourprograms,youneedtohaveotherstestitaswell.Youshouldenlistotherprogrammersandusersofyourprogramtohelpyoutestyourprogram.Ifsomethingisaproblemforyourusers,evenifitseemsokaytoyou,itneedstobexed.Iftheuserdoesn'tknowhowtouseyourprogramcorrectly,thatshouldbetreatedasabugthatneedstobexed.Youwillndthatusersndalotmorebugsinyourprogramthanyouevercould.Thereasonisthatusersdon'tknowwhatthecomputerexpects.Youknowwhatkindsofdatathecomputerexpects,andthereforearemuchmorelikelytoenterdatathatmakessensetothecomputer.Usersenterdatathatmakessensetothem.Allowingnon-programmerstouseyourprogramfortestingpurposesusuallygivesyoumuchmoreaccurateresultsastohowrobustyourprogramtrulyis.DataTestingWhendesigningprograms,eachofyourfunctionsneedstobeveryspecicaboutthetypeandrangeofdatathatitwillorwon'taccept.Youthenneedtotestthesefunctionstomakesurethattheyperformtospecicationwhenhandedtheappropriatedata.Mostimportantistestingcornercasesoredgecases.Cornercasesaretheinputsthataremostlikelytocauseproblemsorbehaveunexpectedly.Whentestingnumericdata,thereareseveralcornercasesyoualwaysneedtotest: Thenumber0 Thenumber1 119 PAGE 126 Chapter7.DevelopingRobustPrograms Anumberwithintheexpectedrange Anumberoutsidetheexpectedrange Therstnumberintheexpectedrange Thelastnumberintheexpectedrange Therstnumberbelowtheexpectedrange TherstnumberabovetheexpectedrangeForexample,ifIhaveaprogramthatissupposedtoacceptvaluesbetween5and200,Ishouldtest0,1,4,5,153,200,201,and255ataminimumand255wererandomlychoseninsideandoutsidetherange,respectively.Thesamegoesforanylistsofdatayouhave.Youneedtotestthatyourprogrambehavesasexpectedforlistsof0items,1item,massivenumbersofitems,andsoon.Inaddition,youshouldalsotestanyturningpointsyouhave.Forexample,ifyouhavedifferentcodetohandlepeopleunderandoverage30,forexample,youwouldneedtotestitonpeopleofages29,30,and31atleast.Therewillbesomeinternalfunctionsthatyouassumegetgooddatabecauseyouhavecheckedforerrorsbeforethispoint.However,whileindevelopmentyouoftenneedtocheckforerrorsanyway,asyourothercodemayhaveerrorsinit.Toverifytheconsistencyandvalidityofdataduringdevelopment,mostlanguageshaveafacilitytoeasilycheckassumptionsaboutdatacorrectness.IntheClanguagethereistheassertmacro.Youcansimplyputinyourcodeasserta>b;,anditwillgiveanerrorifitreachesthatcodewhentheconditionisnottrue.Inaddition,sincesuchacheckisawasteoftimeafteryourcodeisstable,theassertmacroallowsyoutoturnoffassertsatcompile-time.Thismakessurethatyourfunctionsarereceivinggooddatawithoutcausingunnecessaryslowdownsforcodereleasedtothepublic.ModuleTestingNotonlyshouldyoutestyourprogramasawhole,youneedtotesttheindividual 120 PAGE 127 Chapter7.DevelopingRobustPrograms piecesofyourprogram.Asyoudevelopyourprogram,youshouldtestindividualfunctionsbyprovidingitwithdatayoucreatetomakesureitrespondsappropriately.Inordertodothiseffectively,youhavetodevelopfunctionswhosesolepurposeistocallfunctionsfortesting.Thesearecalleddriversnottobeconfusedwithhardwaredrivers.Theysimplyloadsyourfunction,supplyitwithdata,andchecktheresults.Thisisespeciallyusefulifyouareworkingonpiecesofanunnishedprogram.Sinceyoucan'ttestallofthepiecestogether,youcancreateadriverprogramthatwilltesteachfunctionindividually.Also,thecodeyouaretestingmaymakecallstofunctionsnotdevelopedyet.Inordertoovercomethisproblem,youcanwriteasmallfunctioncalledastubwhichsimplyreturnsthevaluesthatfunctionneedstoproceed.Forexample,inane-commerceapplication,Ihadafunctioncalledis_ready_to_checkout.BeforeIhadtimetoactuallywritethefunctionIjustsetittoreturntrueoneverycallsothatthefunctionswhichreliedonitwouldhaveananswer.Thisallowedmetotestfunctionswhichreliedonis_ready_to_checkoutwithoutthefunctionbeingfullyimplemented.HandlingErrorsEffectivelyNotonlyisitimportanttoknowhowtotest,butitisalsoimportanttoknowwhattodowhenanerrorisdetected.HaveanErrorCodeforEverythingTrulyrobustsoftwarehasauniqueerrorcodeforeverypossiblecontingency.Bysimplyknowingtheerrorcode,youshouldbeabletondthelocationinyourcodewherethaterrorwassignalled. 121 PAGE 128 Chapter7.DevelopingRobustPrograms Thisisimportantbecausetheerrorcodeisusuallyalltheuserhastogoonwhenreportingerrors.Therefore,itneedstobeasusefulaspossible.Errorcodesshouldalsobeaccompaniedbydescriptiveerrormessages.However,onlyinrarecircumstancesshouldtheerrormessagetrytopredictwhytheerroroccurred.Itshouldsimplyrelatewhathappened.Backin1995IworkedforanInternetServiceProvider.Oneofthewebbrowserswesupportedtriedtoguessthecauseforeverynetworkerror,ratherthanjustreportingtheerror.Ifthecomputerwasn'tconnectedtotheInternetandtheusertriedtoconnecttoawebsite,itwouldsaythattherewasaproblemwiththeInternetServiceProvider,thattheserverwasdown,andthattheusershouldcontacttheirInternetServiceProvidertocorrecttheproblem.Nearlyaquarterofourcallswerefrompeoplewhohadreceivedthismessage,butmerelyneededtoconnecttotheInternetbeforetryingtousetheirbrowser.Asyoucansee,tryingtodiagnosewhattheproblemiscanleadtoalotmoreproblemsthanitxes.Itisbettertojustreporterrorcodesandmessages,andhaveseparateresourcesfortheusertotroubleshootingtheapplication.Atroubleshootingguide,nottheprogramitself,isanappropriateplacetolistpossiblereasonsandcoursesforactionforeacherrormessage.RecoveryPointsInordertosimplifyerrorhandling,itisoftenusefultobreakyourprogramapartintodistinctunits,whereeachunitfailsandisrecoveredasawhole.Forexample,youcouldbreakyourprogramupsothatreadingthecongurationlewasaunit.Ifreadingthecongurationlefailedatanypointopeningthele,readingthele,tryingtodecodethele,etc.thentheprogramwouldsimplytreatitasacongurationleproblemandskiptotherecoverypointforthatproblem.Thiswayyougreatlyreducethenumberoferror-handlingmechanismyouneedforyourprogram,becauseerrorrecoveryisdoneonamuchmoregenerallevel.Notethatevenwithrecoverypoints,yourerrormessagesneedtobespecicastowhattheproblemwas.Recoverypointsarebasicunitsforerrorrecovery,notforerrordetection.Errordetectionstillneedstobeextremelyexact,andtheerror 122 PAGE 129 Chapter7.DevelopingRobustPrograms reportsneedexacterrorcodesandmessages.Whenusingrecoverypoints,youoftenneedtoincludecleanupcodetohandledifferentcontingencies.Forexample,inourcongurationleexample,therecoveryfunctionwouldneedtoincludecodetocheckandseeifthecongurationlewasstillopen.Dependingonwheretheerroroccurred,thelemayhavebeenleftopen.Therecoveryfunctionneedstocheckforthiscondition,andanyotherconditionthatmightleadtosysteminstability,andreturntheprogramtoaconsistentstate.Thesimplestwaytohandlerecoverypointsistowrapthewholeprogramintoasinglerecoverypoint.Youwouldjusthaveasimpleerror-reportingfunctionthatyoucancallwithanerrorcodeandamessage.Thefunctionwouldprintthemandandsimplyexittheprogram.Thisisnotusuallythebestsolutionforreal-worldsituations,butitisagoodfall-back,lastresortmechanism.MakingOurProgramMoreRobustThissectionwillgothroughmakingtheadd-year.sprogramfrom Chapter6 alittlemorerobust.Sincethisisaprettysimpleprogram,wewilllimitourselvestoasinglerecoverypointthatcoversthewholeprogram.Theonlythingwewilldotorecoveristoprinttheerrorandexit.Thecodetodothatisprettysimple:.include"linux.s".equST_ERROR_CODE,8.equST_ERROR_MSG,12.globlerror_exit.typeerror_exit,@functionerror_exit:pushl%ebpmovl%esp,%ebp 123 PAGE 130 Chapter7.DevelopingRobustPrograms #WriteouterrorcodemovlST_ERROR_CODE%ebp,%ecxpushl%ecxcallcount_charspopl%ecxmovl%eax,%edxmovl$STDERR,%ebxmovl$SYS_WRITE,%eaxint$LINUX_SYSCALL#WriteouterrormessagemovlST_ERROR_MSG%ebp,%ecxpushl%ecxcallcount_charspopl%ecxmovl%eax,%edxmovl$STDERR,%ebxmovl$SYS_WRITE,%eaxint$LINUX_SYSCALLpushl$STDERRcallwrite_newline#Exitwithstatus1movl$SYS_EXIT,%eaxmovl$1,%ebxint$LINUX_SYSCALLEnteritinalecallederror-exit.s.Tocallit,youjustneedtopushtheaddressofanerrormessage,andthenanerrorcodeontothestack,andcallthefunction.Nowlet'slookforpotentialerrorspotsinouradd-yearprogram.Firstofall,wedon'tchecktoseeifeitherofouropensystemcallsactuallycompleteproperly. 124 PAGE 131 Chapter7.DevelopingRobustPrograms Linuxreturnsitsstatuscodein%eax,soweneedtocheckandseeifthereisanerror.#Openfileforreadingmovl$SYS_OPEN,%eaxmovl$input_file_name,%ebxmovl$0,%ecxmovl$0666,%edxint$LINUX_SYSCALLmovl%eax,INPUT_DESCRIPTOR%ebp#Thiswilltestandseeif%eaxis#negative.Ifitisnotnegative,it#willjumptocontinue_processing.#Otherwiseitwillhandletheerror#conditionthatthenegativenumber#represents.cmpl$0,%eaxjlcontinue_processing#Sendtheerror.section.datano_open_file_code:.ascii"0001:"no_open_file_msg:.ascii"Can'tOpenInputFile".section.textpushl$no_open_file_msgpushl$no_open_file_codecallerror_exitcontinue_processing: 125 PAGE 132 Chapter7.DevelopingRobustPrograms #RestofprogramSo,aftercallingthesystemcall,wecheckandseeifwehaveanerrorbycheckingtoseeiftheresultofthesystemcallislessthanzero.Ifso,wecallourerrorreportingandexitroutine.Aftereverysystemcall,functioncall,orinstructionwhichcanhaveerroneousresultsyoushouldadderrorcheckingandhandlingcode.Toassembleandlinktheles,do:asadd-year.s-oadd-year.oaserror-exit.s-oerror-exit.oldadd-year.owrite-newline.oerror-exit.oread-record.owrite-record.ocount-chars.o-oadd-yearNowtrytorunitwithoutthenecessaryles.Itnowexitscleanlyandgracefully!ReviewKnowtheConcepts Whatarethereasonsprogrammer'shavetroublewithscheduling? Findyourfavoriteprogram,andtrytouseitinacompletelywrongmanner.Openuplesofthewrongtype,chooseinvalidoptions,closewindowsthataresupposedtobeopen,etc.Counthowmanydifferenterrorscenariostheyhadtoaccountfor. Whatarecornercases?Canyoulistexamplesofnumericcornercases? Whyisusertestingsoimportant? Whatarestubsanddriversusedfor?What'sthedifferencebetweenthetwo? 126 PAGE 133 Chapter7.DevelopingRobustPrograms Whatarerecoverypointsusedfor? Howmanydifferenterrorcodesshouldaprogramhave?UsetheConcepts Gothroughtheadd-year.sprogramandadderror-checkingcodeaftereverysystemcall. Findoneotherprogramwehavedonesofar,andadderror-checkingtothatprogram. Addarecoverymechanismforadd-year.sthatallowsittoreadfromSTDINifitcannotopenthestandardle.GoingFurther What,ifanything,shouldyoudoifyourerror-reportingfunctionfails?Why? Trytondbugsinatleastoneopen-sourceprogram.Fileabugreportforit. Trytoxthebugyoufoundinthepreviousexercise. 127 PAGE 134 Chapter7.DevelopingRobustPrograms 128 PAGE 135 Chapter8.SharingFunctionswithCodeLibrariesBynowyoushouldrealizethatthecomputerhastodoalotofworkevenforsimpletasks.Becauseofthat,youhavetodoalotofworktowritethecodeforacomputertoevendosimpletasks.Inaddition,programmingtasksareusuallynotverysimple.Therefore,weneeedawaytomakethisprocesseasieronourselves.Thereareseveralwaystodothis,including: Writecodeinahigh-levellanguageinsteadofassemblylanguage Havelotsofpre-writtencodethatyoucancutandpasteintoyourownprograms HaveasetoffunctionsonthesystemthataresharedamonganyprogramthatwishestouseitAllthreeoftheseareusuallyusedtosomedegreeinanygivenproject.Therstoptionwillbeexploredfurtherin Chapter11 .Thesecondoptionisusefulbutitsuffersfromsomedrawbacks,including: Codethatiscopiedoftenhastobemajorlymodiedtotthesurroundingcode. Everyprogramcontainingthecopiedcodehasthesamecodeinit,thuswastingalotofspace. Ifabugisfoundinanyofthecopiedcodeithastobexedineveryapplicationprogram.Therefore,thesecondoptionisusuallyusedsparingly.Itisusuallyonlyusedincaseswhereyoucopyandpasteskeletoncodeforaspecictypeoftask,andaddinyourprogram-specicdetails.Thethirdoptionistheonethatisusedthemostoften.Thethirdoptionincludeshavingacentralrepositoryofsharedcode.Then,insteadofeachprogramwastingspacestoringthesamecopiesoffunctions,theycansimplypointtothesharedlibrarieswhichcontainthefunctionstheyneed.Ifa 129 PAGE 136 Chapter8.SharingFunctionswithCodeLibraries bugisfoundinoneofthesefunctions,itonlyhastobexedwithinthesinglefunctionlibraryle,andallapplicationswhichuseitareautomaticallyupdated.Themaindrawbackwiththisapproachisthatitcreatessomedependencyproblems,including: Ifmultipleapplicationsareallusingthesharedle,howdoweknowwhenitissafetodeletethele?Forexample,ifthreeapplicationsaresharingaleoffunctionsand2oftheprogramsaredeleted,howdoesthesystemknowthattherestillexistsanapplicationthatusesthatcode,andthereforeitshouldn'tbedeleted? Someprogramsinadvertantlyrelyonbugswithinsharedfunctions.Therefore,ifupgradingthesharedprogramxesabugthataprogramdependedon,itcouldcausethatapplicationtoceasefunctioning.Theseproblemsarewhatleadtowhatisknownas"DLLhell".However,itisgenerallyassumedthattheadvantagesoutweighthedisadvantages.Inprogramming,thesesharedcodelesarereferredtoassharedlibraries,sharedobjects,dynamic-linklibraries,DLLs,or.soles.Wewillrefertothemassharedlibraries.UsingaSharedLibraryTheprogramwewillexaminehereissimple-itwritesthecharactershelloworldtothescreenandexits.Theregularprogram,helloworld-nolib.s,lookslikethis:#PURPOSE:Thisprogramwritesthemessage"helloworld"and#exits#.include"linux.s" 130 PAGE 137 Chapter8.SharingFunctionswithCodeLibraries .section.datahelloworld:.ascii"helloworldn"helloworld_end:.equhelloworld_len,helloworld_end-helloworld.section.text.globl_start_start:movl$STDOUT,%ebxmovl$helloworld,%ecxmovl$helloworld_len,%edxmovl$SYS_WRITE,%eaxint$LINUX_SYSCALLmovl$0,%ebxmovl$SYS_EXIT,%eaxint$LINUX_SYSCALLThat'snottoolong.However,takealookathowshorthelloworld-libiswhichusesalibrary:#PURPOSE:Thisprogramwritesthemessage"helloworld"and#exits#.section.datahelloworld: 131 PAGE 138 Chapter8.SharingFunctionswithCodeLibraries .ascii"helloworldn".section.text.globl_start_start:pushl$helloworldcallprintfpushl$0callexitIt'sevenshorter!Now,buildingprogramswhichusesharedlibrariesisalittledifferentthannormal.Youcanbuildtherstprogramnormallybydoingthis:ashelloworld-nolib.s-ohelloworld-nolib.oldhelloworld-nolib.o-ohelloworld-nolibHowever,inordertobuildthesecondprogram,youhavetodothis:ashelloworld-lib.s-ohelloworld-lib.old-dynamic-linker/lib/ld-linux.so.2-ohelloworld-libhelloworld-lib.o-lcRemember,thebackslashintherstlinesimplymeansthatthecommandcontinuesonthenextline.Theoption-dynamic-linker/lib/ld-linux.so.2allowsourprogramtobelinkedtolibraries.Thisbuildstheexecutablesothatbeforeexecuting,theoperatingsystemwillloadtheprogram/lib/ld-linux.so.2toloadinexternallibrariesandlinkthemwiththeprogram.Thisprogramisknownasadynamiclinker.The-lcoptionsaystolinktotheclibrary,namedlibc.soonGNU/Linuxsystems.Givenalibraryname,cinthiscaseusuallylibrarynamesarelongerthanasingleletter,theGNU/Linuxlinkerprependsthestringlibtothebeginningof 132 PAGE 139 Chapter8.SharingFunctionswithCodeLibraries thelibrarynameandappends.sototheendofittoformthelibrary'slename.Thislibrarycontainsmanyfunctionstoautomatealltypesoftasks.Thetwoweareusingareprintf,whichprintsstrings,andexit,whichexitstheprogram.Noticethatthesymbolsprintfandexitaresimplyreferredtobynamewithintheprogram.Inpreviouschapters,thelinkerwouldresolveallofthenamestophysicalmemoryaddresses,andthenameswouldbethrownaway.Whenusingdynamiclinking,thenameitselfresideswithintheexecutable,andisresolvedbythedynamiclinkerwhenitisrun.Whentheprogramisrunbytheuser,thedynamiclinkerloadsthesharedlibrarieslistedinourlinkstatement,andthenndsallofthefunctionandvariablenamesthatwerenamedbyourprogrambutnotfoundatlinktime,andmatchesthemupwithcorrespondingentriesinthesharedlibrariesitloads.Itthenreplacesallofthenameswiththeaddresseswhichtheyareloadedat.Thissoundstime-consuming.Itistoasmalldegree,butitonlyhappensonce-atprogramstartuptime.HowSharedLibrariesWorkInourrstprograms,allofthecodewascontainedwithinthesourcele.Suchprogramsarecalledstatically-linkedexecutables,becausetheycontainedallofthenecessaryfunctionalityfortheprogramthatwasn'thandledbythekernel.Intheprogramswewrotein Chapter6 ,weusedbothourmainprogramleandlescontainingroutinesusedbymultipleprograms.Inthesecases,wecombinedallofthecodetogetherusingthelinkeratlink-time,soitwasstillstatically-linked.However,inthehelloworld-libprogram,westartedusingsharedlibraries.Whenyouusesharedlibraries,yourprogramisthendynamically-linked,whichmeansthatnotallofthecodeneededtoruntheprogramisactuallycontainedwithintheprogramleitself,butinexternallibraries.Whenweputthe-lconthecommandtolinkthehelloworldprogram,ittoldthelinkertousetheclibrarylibc.sotolookupanysymbolsthatweren'talreadydenedinhelloworld.o.However,itdoesn'tactuallyaddanycodeto 133 PAGE 140 Chapter8.SharingFunctionswithCodeLibraries ourprogram,itjustnotesintheprogramwheretolook.Whenthehelloworldprogrambegins,thele/lib/ld-linux.so.2isloadedrst.Thisisthedynamiclinker.Thislooksatourhelloworldprogramandseesthatitneedstheclibrarytorun.So,itsearchesforalecalledlibc.sointhestandardplaceslistedin/etc/ld.so.confandinthecontentsoftheLD_LIBRARY_PATHenvironmentvariable,thenlooksinitforalltheneededsymbolsprintfandexitinthiscase,andthenloadsthelibraryintotheprogram'svirtualmemory.Finally,itreplacesallinstancesofprintfintheprogramwiththeactuallocationofprintfinthelibrary.Runthefollowingcommand:ldd./helloworld-nolibItshouldreportbacknotadynamicexecutable.Thisisjustlikewesaid-helloworld-nolibisastatically-linkedexecutable.However,trythis:ldd./helloworld-libItwillreportbacksomethinglikelibc.so.6=>/lib/libc.so.6x4001d000/lib/ld-linux.so.2=>/lib/ld-linux.so.2x400000000Thenumbersinparenthesismaybedifferentonyoursystem.Thismeansthattheprogramhelloworldislinkedtolibc.so.6the.6istheversionnumber,whichisfoundat/lib/libc.so.6,and/lib/ld-linux.so.2isfoundat/lib/ld-linux.so.2.Theselibrarieshavetobeloadedbeforetheprogramcanberun.Ifyouareinterested,runthelddprogramonvariousprogramsthatareonyourLinuxdistribution,andseewhatlibrariestheyrelyon.FindingInformationaboutLibrariesOkay,sonowthatyouknowaboutlibraries,thequestionis,howdoyoundout 134 PAGE 141 Chapter8.SharingFunctionswithCodeLibraries whatlibrariesyouhaveonyoursystemandwhattheydo?Well,let'sskipthatquestionforaminuteandaskanotherquestion:Howdoprogrammersdescribefunctionstoeachotherintheirdocumentation?Let'stakealookatthefunctionprintf.It'scallinginterfaceusuallyreferredtoasaprototypelookslikethis:intprintfchar*string,...;InLinux,functionsaredescribedintheCprogramminglanguage.Infact,mostLinuxprogramsarewritteninC.ThatiswhymostdocumentationandbinarycompatibilityisdenedusingtheClanguage.TheinterfacetotheprintffunctionaboveisdescribedusingtheCprogramminglanguage.Thisdenitionmeansthatthereisafunctionprintf.Thethingsinsidetheparenthesisarethefunction'sparametersorarguments.Therstparameterhereischar*string.Thismeansthereisaparameternamedstringthenameisn'timportant,excepttousefortalkingaboutit,whichhasatypechar*.charmeansthatitwantsasingle-bytecharacter.The*afteritmeansthatitdoesn'tactuallywantacharacterasanargument,butinsteaditwantstheaddressofacharacterorsequenceofcharacters.Ifyoulookbackatourhelloworldprogram,youwillnoticethatthefunctioncalllookedlikethis:pushl$hellocallprintfSo,wepushedtheaddressofthehellostring,ratherthantheactualcharacters.Youmightnoticethatwedidn'tpushthelengthofthestring.Thewaythatprintffoundtheendofthestringwasbecauseweendeditwithanullcharacter.Manyfunctionsworkthatway,especiallyClanguagefunctions.Theintbeforethefunctiondenitiontellwhattypeofvaluethefunctionwillreturnin%eaxwhenitreturns.printfwillreturnanintwhenit'sthrough.Now,afterthechar*string,wehaveaseriesofperiods,....Thismeansthatitcantakeanindenitenumberofadditionalargumentsafterthestring.Mostfunctionscanonlytakeaspeciednumberofarguments.printf,however,cantakemany.Itwilllookintothestringparameter,andeverywhereitseesthecharacters%s,itwill 135 PAGE 142 Chapter8.SharingFunctionswithCodeLibraries lookforanotherstringfromthestacktoinsert,andeverywhereitsees%ditwilllookforanumberfromthestacktoinsert.Thisisbestdescribedusinganexample:#PURPOSE:Thisprogramistodemonstratehowtocallprintf#.section.data#Thisstringiscalledtheformatstring.It'sthefirst#parameter,andprintfusesittofindouthowmanyparameters#itwasgiven,andwhatkindtheyare.firststring:.ascii"Hello!%sisa%swholovesthenumber%dn"name:.ascii"Jonathan"personstring:.ascii"person"#Thiscouldalsohavebeenan.equ,butwedecidedtogiveit#arealmemorylocationjustforkicksnumberloved:.long3.section.text.globl_start_start:#notethattheparametersarepassedinthe#reverseorderthattheyarelistedinthe#function'sprototype.pushlnumberloved#Thisisthe%dpushl$personstring#Thisisthesecond%spushl$name#Thisisthefirst%spushl$firststring#Thisistheformatstring#intheprototypecallprintf 136 PAGE 143 Chapter8.SharingFunctionswithCodeLibraries pushl$0callexitTypeitinwiththelenameprintf-example.s,andthendothefollowingcommands:asprintf-example.s-oprintf-example.oldprintf-example.o-oprintf-example-lc-dynamic-linker/lib/ld-linux.so.2Thenruntheprogramwith./printf-example,anditshouldsaythis:Hello!Jonathanisapersonwholovesthenumber3Now,ifyoulookatthecode,you'llseethatweactuallypushtheformatstringlast,eventhoughit'stherstparameterlisted.Youalwayspushafunctionsparametersinreverseorder.1Youmaybewonderinghowtheprintffunctionknowshowmanyparametersthereare.Well,itsearchesthroughyourstring,andcountshowmany%dsand%ssitnds,andthengrabsthatnumberofparametersfromthestack.Iftheparametermatchesa%d,ittreatsitasanumber,andifitmatchesa%s,ittreatsitasapointertoanull-terminatedstring.printfhasmanymorefeaturesthanthis,butthesearethemost-usedones.So,asyoucansee,printfcanmakeoutputaloteasier,butitalsohasalotofoverhead,becauseithastocountthenumberofcharactersinthestring,lookthroughitforallofthecontrolcharactersitneedstoreplace,pullthemoffthestack,convertthemtoa 1. Thereasonthatparametersarepushedinthereverseorderisbecauseoffunctionswhichtakeavariablenumberofparameterslikeprintf.Theparameterspushedinlastwillbeinaknownpositionrelativetothetopofthestack.Theprogramcanthenusetheseparameterstodeterminewhereonthestacktheadditionalargumentsare,andwhattypetheyare.Forexample,printfusestheformatstringtodeterminehowmanyotherparametersarebeingsent.Ifwepushedtheknownargumentsrst,youwouldn'tbeabletotellwheretheywereonthestack. 137 PAGE 144 Chapter8.SharingFunctionswithCodeLibraries suitablerepresentationnumbershavetobeconvertedtostrings,etc,andstickthemalltogetherappropriately.We'veseenhowtousetheCprogramminglanguageprototypestocalllibraryfunctions.Tousethemeffectively,however,youneedtoknowseveralmoreofthepossibledatatypesforreadingfunctions.Herearethemainones:int Anintisanintegernumberbytesonx86processor.long Alongisalsoanintegernumberbytesonanx86processor.longlong Alonglongisanintegernumberthat'slargerthanalongbytesonanx86processor.short Ashortisanintegernumberthat'sshorterthananintbytesonanx86processor.char Acharisasingle-byteintegernumber.Thisismostlyusedforstoringcharacterdata,sinceASCIIstringsusuallyarerepresentedwithonebytepercharacter.float Afloatisaoating-pointnumberbytesonanx86processor.Floating-pointnumberswillbeexplainedinmoredepthin theSectioncalledFloating-pointNumbersinChapter10 138 PAGE 145 Chapter8.SharingFunctionswithCodeLibraries double Adoubleisaoating-pointnumberthatislargerthanaoatbytesonanx86processor.unsigned unsignedisamodierusedforanyoftheabovetypeswhichkeepsthemfrombeingusedassignedquantities.Thedifferencebetweensignedandunsignednumberswillbediscussedin Chapter10 .* Anasterisk*isusedtodenotethatthedataisn'tanactualvalue,butinsteadisapointertoalocationholdingthegivenvaluebytesonanx86processor.So,let'ssayinmemorylocationmy_locationyouhavethenumber20stored.Iftheprototypesaidtopassanint,youwouldusedirectaddressingmodeanddopushlmy_location.However,iftheprototypesaidtopassanint*,youwoulddopushl$my_location-animmediatemodepushoftheaddressthatthevalueresidesin.Inadditiontoindicatingtheaddressofasinglevalue,pointerscanalsobeusedtopassasequenceofconsecutivelocations,startingwiththeonepointedtobythegivenvalue.Thisiscalledanarray.struct Astructisasetofdataitemsthathavebeenputtogetherunderaname.Forexampleyoucoulddeclare:structteststruct{inta;char*b;};andanytimeyouranintostructteststructyouwouldknowthatitisactuallytwowordsrightnexttoeachother,therstbeinganinteger,andthesecondapointertoacharacterorgroupofcharacters.Youneverseestructs 139 PAGE 146 Chapter8.SharingFunctionswithCodeLibraries passedasargumentstofunctions.Instead,youusuallyseepointerstostructspassedasarguments.Thisisbecausepassingstructstofunctionsisfairlycomplicated,sincetheycantakeupsomanystoragelocations.typedef Atypedefbasicallyallowsyoutorenameatype.Forexample,Icandotypedefintmyowntype;inaCprogram,andanytimeItypedmyowntype,itwouldbejustasifItypedint.Thiscangetkindofannoying,becauseyouhavetolookupwhatallofthetypedefsandstructsinafunctionprototypereallymean.However,typedefsareusefulforgivingtypesmoremeaningfulanddescriptivenames.CompatibilityNote:Thelistedsizesareforintel-compatiblex86machines.Othermachineswillhavedifferentsizes.Also,evenwhenparametersshorterthanawordarepassedtofunctions,theyarepassedaslongsonthestack.That'showtoreadfunctiondocumentation.Now,let'sgetbacktothequestionofhowtondoutaboutlibraries.Mostofyoursystemlibrariesarein/usr/libor/lib.Ifyouwanttojustseewhatsymbolstheydene,justrunobjdump-RFILENAMEwhereFILENAMEisthefullpathtothelibrary.Theoutputofthatisn'ttoohelpful,though,forndinganinterfacethatyoumightneed.Usually,youhavetoknowwhatlibraryyouwantatthebeginning,andthenjustreadthedocumentation.Mostlibrarieshavemanualsormanpagesfortheirfunctions.Thewebisthebestsourceofdocumentationforlibraries.MostlibrariesfromtheGNUprojectalsohaveinfopagesonthem,whicharealittlemorethoroughthanmanpages. 140 PAGE 147 Chapter8.SharingFunctionswithCodeLibraries UsefulFunctionsSeveralusefulfunctionsyouwillwanttobeawareoffromtheclibraryinclude: size_tstrlenconstchar*scalculatesthesizeofnull-terminatedstrings. intstrcmpconstchar*s1,constchar*s2comparestwostringsalphabetically. char*strdupconstchar*stakesthepointertoastring,andcreatesanewcopyinanewlocation,andreturnsthenewlocation. FILE*fopenconstchar*filename,constchar*opentypeopensamanaged,bufferedleallowseasierreadingandwritingthanusingledescriptorsdirectly.23 intfcloseFILE*streamclosesaleopenedwithfopen. char*fgetschar*s,intcount,FILE*streamfetchesalineofcharactersintostrings. intfputsconstchar*s,FILE*streamwritesastringtothegivenopenle. intfprintfFILE*stream,constchar*template,...isjustlikeprintf,butitusesanopenleratherthandefaultingtousingstandardoutput.Youcanndthecompletemanualonthislibrarybygoingtohttp://www.gnu.org/software/libc/manual/ 2. stdin,stdout,andstderralllowercasecanbeusedintheseprogramstorefertothelesoftheircorrespondingledescriptors. 3. FILEisastruct.Youdon'tneedtoknowit'scontentstouseit.Youonlyhavetostorethepointerandpassittotherelevantotherfunctions. 141 PAGE 148 Chapter8.SharingFunctionswithCodeLibraries BuildingaSharedLibraryLet'ssaythatwewantedtotakeallofoursharedcodefrom Chapter6 andbuilditintoasharedlibrarytouseinourprograms.Therstthingwewoulddoisassemblethemlikenormal:aswrite-record.s-owrite-record.oasread-record.s-oread-record.oNow,insteadoflinkingthemintoaprogram,wewanttolinkthemintoasharedlibrary.Thischangesourlinkercommandtothis:ld-sharedwrite-record.oread-record.o-olibrecord.soThislinksbothoftheselestogetherintoasharedlibrarycalledlibrecord.so.Thislecannowbeusedformultipleprograms.Ifweneedtoupdatethefunctionscontainedwithinit,wecanjustupdatethisoneleandnothavetoworryaboutwhichprogramsuseit.Let'slookathowwewouldlinkagainstthislibrary.Tolinkthewrite-recordsprogram,wewoulddothefollowing:aswrite-records.s-owrite-recordsld-L.-dynamic-linker/lib/ld-linux.so.2-owrite-records-lrecordwrite-records.oInthiscommand,-L.toldthelinkertolookforlibrariesinthecurrentdirectoryitusuallyonlysearches/libdirectory,/usr/libdirectory,andafewothers.Aswe'veseen,theoption-dynamic-linker/lib/ld-linux.so.2speciedthedynamiclinker.Theoption-lrecordtellsthelinkertosearchforfunctionsinthelenamedlibrecord.so.Nowthewrite-recordsprogramisbuilt,butitwillnotrun.Ifwetryit,wewillgetanerrorlikethefollowing:./write-records:errorwhileloadingsharedlibraries: 142 PAGE 149 Chapter8.SharingFunctionswithCodeLibraries librecord.so:cannotopensharedobjectfile:NosuchfileordirectoryThisisbecause,bydefault,thedynamiclinkeronlysearches/lib,/usr/lib,andwhateverdirectoriesarelistedin/etc/ld.so.confforlibraries.Inordertoruntheprogram,youeitherneedtomovethelibrarytooneofthesedirectories,orexecutethefollowingcommand:LD_LIBRARY_PATH=.exportLD_LIBRARY_PATHAlternatively,ifthatgivesyouanerror,dothisinstead:setenvLD_LIBRARY_PATH.Now,youcanrunwrite-recordsnormallybytyping./write-records.SettingLD_LIBRARY_PATHtellsthelinkertoaddwhateverpathsyougiveittothelibrarysearchpathfordynamiclibrariesForfurtherinformationaboutdynamiclinking,seethefollowingsourcesontheInternet: Themanpageforld.socontainsalotofinformationabouthowtheLinuxdynamiclinkerworks. http://www.benyossef.com/presentations/dlink/isagreatpresentationondynamiclinkinginLinux. http://www.linuxjournal.com/article.php?sid=1059andhttp://www.linuxjournal.com/article.php?sid=1060provideagoodintroductiontotheELFleformat,withmoredetailavailableathttp://www.cs.ucdavis.edu/~haungs/paper/node10.html http://www.iecc.com/linker/linker10.htmlcontainsagreatdescriptionofhowdynamiclinkingworkswithELFles. 143 PAGE 150 Chapter8.SharingFunctionswithCodeLibraries ReviewKnowtheConcepts Whataretheadvantagesanddisadvantagesofsharedlibraries? Givenalibrarynamed'foo',whatwouldthelibrary'slenamebe? Whatdoesthelddcommanddo? Let'ssaywehadthelesfoo.oandbar.o,andyouwantedtolinkthemtogether,anddynamicallylinkthemtothelibrary'kramer'.Whatwouldthelinkingcommandbetogeneratethenalexecutable? Whatistypedeffor? Whatarestructsfor? Whatisthedifferencebetweenadataelementoftypeintandint*?Howwouldyouaccessthemdifferentlyinyourprogram? Ifyouhadaobjectlecalledfoo.o,whatwouldbethecommandtocreateasharedlibrarycalled'bar'? WhatisthepurposeofLD_LIBRARY_PATH?UsetheConcepts Rewriteoneormoreoftheprogramsfromthepreviouschapterstoprinttheirresultstothescreenusingprintfratherthanreturningtheresultastheexitstatuscode.Also,maketheexitstatuscodebe0. Usethefactorialfunctionyoudevelopedin theSectioncalledRecursiveFunctionsinChapter4 tomakeasharedlibrary.Thenre-writethemainprogramsothatitlinkswiththelibrarydynamically. 144 PAGE 151 Chapter8.SharingFunctionswithCodeLibraries Rewritetheprogramabovesothatitalsolinkswiththe'c'library.Usethe'c'library'sprintffunctiontodisplaytheresultofthefactorialcall. Rewritethetoupperprogramsothatitusestheclibraryfunctionsforlesratherthansystemcalls.GoingFurther MakealistofalltheenvironmentvariablesusedbytheGNU/Linuxdynamiclinker. Researchthedifferenttypesofexecutableleformatsinusetodayandinthehistoryofcomputing.Tellthestrengthsandweaknessesofeach. Whatkindsofprogrammingareyouinterestedingraphics,databbases,science,etc.?Findalibraryforworkinginthatarea,andwriteaprogramthatmakessomebasicuseofthatlibrary. ResearchtheuseofLD_PRELOAD.Whatisitusedfor?Trybuildingasharedlibrarythatcontainedtheexitfunction,andhaveitwriteamessagetoSTDERRbeforeexitting.UseLD_PRELOADandrunvariousprogramswithit.Whataretheresults? 145 PAGE 152 Chapter8.SharingFunctionswithCodeLibraries 146 PAGE 153 Chapter9.IntermediateMemoryTopicsHowaComputerViewsMemoryLet'sreviewhowmemorywithinacomputerworks.Youmayalsowanttore-read Chapter2 .Acomputerlooksatmemoryasalongsequenceofnumberedstoragelocations.Asequenceofmillionsofnumberedstoragelocations.Everythingisstoredintheselocations.Yourprogramsarestoredthere,yourdataisstoredthere,everything.Eachstoragelocationlookslikeeveryotherone.Thelocationsholdingyourprogramarejustliketheonesholdingyourdata.Infact,thecomputerhasnoideawhicharewhich,exceptthattheexecutableletellsitwheretostartexecuting.Thesestoragelocationsarecalledbytes.Thecomputercancombineuptofourofthemtogetherintoasingleword.Normallynumericdataisoperatedonawordatatime.Aswementioned,instructionsarealsostoredinthissamememory.Eachinstructionisadifferentlength.Mostinstructionstakeuponeortwostoragelocationsfortheinstructionitself,andthenstoragelocationsfortheinstruction'sarguments.Forexample,theinstructionmovldata_items,%edi,4,%ebxtakesup7storagelocations.Thersttwoholdtheinstruction,thethirdonetellswhichregisterstouse,andthenextfourholdthestoragelocationofdata_items.Inmemory,instructionslookjustlikealltheothernumbers,andtheinstructionsthemselvescanbemovedintoandoutofregistersjustlikenumbers,becausethat'swhattheyare.Thischapterisfocusedonthedetailsofcomputermemory.Togetstartedlet'sreviewsomebasictermsthatwewillbeusinginthischapter: 147 PAGE 154 Chapter9.IntermediateMemoryTopics Byte Thisisthesizeofastoragelocation.Onx86processors,abytecanholdnumbersbetween0and255.Word Thisisthesizeofanormalregister.Onx86processors,awordisfourbyteslong.Mostcomputeroperationshandleawordatatime.Address Anaddressisanumberthatreferstoabyteinmemory.Forexample,therstbyteonacomputerhasanaddressof0,thesecondhasanaddressof1,andsoon.1Everypieceofdataonthecomputernotinaregisterhasanaddress.Theaddressofdatawhichspansseveralbytesisthesameastheaddressofitsrstbyte.Normally,wedon'tevertypethenumericaddressofanything,butwelettheassemblerdoitforus.Whenweuselabelsincode,thesymbolusedinthelabelwillbeequivalenttotheaddressitislabelling.Theassemblerwillthenreplacethatsymbolwithitsaddresswhereveryouuseitinyourprogram.Forexample,sayyouhavethefollowingcode:.section.datamy_data:.long2,3,4Now,anytimeintheprogramthatmy_dataisused,itwillbereplacedbytheaddressoftherstvalueofthe.longdirective.Pointer Apointerisaregisterormemorywordwhosevalueisanaddress.Inourprogramsweuse%ebpasapointertothecurrentstackframe.Allbase 1. Youactuallyneveruseaddressesthislow,butitworksfordiscussion. 148 PAGE 155 Chapter9.IntermediateMemoryTopics pointeraddressinginvolvespointers.Programmingusesalotofpointers,soit'sanimportantconcepttograsp.TheMemoryLayoutofaLinuxProgramWhenyouprogramisloadedintomemory,each.sectionisloadedintoitsownregionofmemory.Allofthecodeanddatadeclaredineachsectionisbroughttogether,eveniftheywereseparatedinyoursourcecode.Theactualinstructionsthe.textsectionareloadedattheaddress0x08048000numbersstartingwith0xareinhexadecimal,whichwillbediscussedin Chapter10 .The.datasectionisloadedimmediatelyafterthat,followedbythe.bsssection.ThelastbytethatcanbeaddressedonLinuxislocation0xbfffffff.Linuxstartsthestackhereandgrowsitdownwardtowardtheothersections.Betweenthemisahugegap.Theinitiallayoutofthestackisasfollows:Atthebottomofthestackthebottomofthestackisthetopaddressofmemory-see Chapter4 ,thereisawordofmemorythatiszero.Afterthatcomesthenull-terminatednameoftheprogramusingASCIIcharacters.Aftertheprogramnamecomestheprogram'senvironmentvariablesthesearenotimportanttousinthisbook.Thencometheprogram'scommand-linearguments.Thesearethevaluesthattheusertypedinonthecommandlinetorunthisprogram.Whenwerunas,forexample,wegiveitseveralarguments-as,sourcefile.s,-o,andobjectfile.o.Afterthese,wehavethenumberofargumentsthatwereused.Whentheprogrambegins,thisiswherethestackpointer,%esp,ispointing.Furtherpushesonthestackmove%espdowninmemory.Forexample,theinstructionpushl%eaxisequivalenttomovl%eax,%esp 149 PAGE 156 Chapter9.IntermediateMemoryTopics subl$4,%espLikewise,theinstructionpopl%eaxisthesameasmovl%esp,%eaxaddl$4,%espYourprogram'sdataregionstartsatthebottomofmemoryandgoesup.Thestackstartsatthetopofmemory,andmovesdownwardwitheachpush.Thismiddlepartbetweenthestackandyourprogram'sdatasectionsisinaccessiblememory-youarenotallowedtoaccessituntilyoutellthekernelthatyouneedit.2Ifyoutry,youwillgetanerrortheerrormessageisusually"segmentationfault".Thesamewillhappenifyoutrytoaccessdatabeforethebeginningofyourprogram,0x08048000.Thelastaccessiblememoryaddresstoyourprogramiscalledthesystembreakalsocalledthecurrentbreakorjustthebreak. 2. Thestackcanaccessitasitgrowsdownward,andyoucanaccessthestackregionsthrough%esp.However,yourprogram'sdatasectiondoesn'tgrowthatway.Thewaytogrowthatwillbeexplainedshortly. 150 PAGE 157 Chapter9.IntermediateMemoryTopics MemoryLayoutofaLinuxProgramatStartupEveryMemoryAddressisaLieSo,whydoesthecomputernotallowyoutoaccessmemoryinthebreakarea?Toanswerthisquestion,wewillhavetodelveintothedepthsofhowyourcomputerreallyhandlesmemory.Youmayhavewondered,sinceeveryprogramgetsloadedintothesameplacein 151 PAGE 158 Chapter9.IntermediateMemoryTopics memory,don'ttheysteponeachother,oroverwriteeachother?Itwouldseemso.However,asaprogramwriter,youonlyaccessvirtualmemory.PhysicalmemoryreferstotheactualRAMchipsinsideyourcomputerandwhattheycontain.It'susuallybetween16and512Megabytesonmoderncomputers.Ifwetalkaboutaphysicalmemoryaddress,wearetalkingaboutwhereexactlyonthesechipsapieceofmemoryislocated.Virtualmemoryisthewayyourprogramthinksaboutmemory.Beforeloadingyourprogram,Linuxndsanemptyphysicalmemoryspacelargeenoughtotyourprogram,andthentellstheprocessortopretendthatthismemoryisactuallyattheaddress0x0804800toloadyourprograminto.Confusedyet?Letmeexplainfurther.Eachprogramgetsitsownsandboxtoplayin.Everyprogramrunningonyourcomputerthinksthatitwasloadedatmemoryaddress0x0804800,andthatit'sstackstartsat0xbffffff.WhenLinuxloadsaprogram,itndsasectionofunusedmemory,andthentellstheprocessortousethatsectionofmemoryastheaddress0x0804800forthisprogram.Theaddressthataprogrambelievesitusesiscalledthevirtualaddress,whiletheactualaddressonthechipsthatitreferstoiscalledthephysicaladdress.Theprocessofassigningvirtualaddressestophysicaladdressesiscalledmapping.Earlierwetalkedabouttheinaccessiblememorybetweenthe.bssandthestack,butwedidn'ttalkaboutwhyitwasthere.Thereasonisthatthisregionofvirtualmemoryaddresseshasn'tbeenmappedontophysicalmemoryaddresses.Themappingprocesstakesupconsiderabletimeandspace,soifeverypossiblevirtualaddressofeverypossibleprogramweremapped,youwouldnothaveenoughphysicalmemorytoevenrunoneprogram.So,thebreakisthebeginningoftheareathatcontainsunmappedmemory.Withthestack,however,Linuxwillautomaticallymapinmemorythatisaccessedfromstackpushes.Ofcourse,thisisaverysimpliedviewofvirtualmemory.Thefullconceptismuchmoreadvanced.Forexample,Virtualmemorycanbemappedtomorethanjustphysicalmemory;itcanbemappedtodiskaswell.SwappartitionsonLinuxallowLinux'svirtualmemorysystemtomapmemorynotonlytophysicalRAM, 152 PAGE 159 Chapter9.IntermediateMemoryTopics butalsotodiskblocksaswell.Forexample,let'ssayyouonlyhave16Megabytesofphysicalmemory.Let'salsosaythat8MegabytesarebeingusedbyLinuxandsomebasicapplications,andyouwanttorunaprogramthatrequires20Megabytesofmemory.Canyou?Theanswerisyes,butonlyifyouhavesetupaswappartition.Whathappensisthatafterallofyourremaining8Megabytesofphysicalmemoryhavebeenmappedintovirtualmemory,Linuxstartsmappingpartsofyourapplication'svirtualmemorytodiskblocks.So,ifyouaccessa"memory"locationinyourprogram,thatlocationmaynotactuallybeinmemoryatall,butondisk.Astheprogrammeryouwon'tknowthedifference,though,becauseitisallhandledbehindthescenesbyLinux.Now,x86processorscannotruninstructionsdirectlyfromdisk,norcantheyaccessdatadirectlyfromdisk.Thisrequiresthehelpoftheoperatingsystem.Whenyoutrytoaccessmemorythatismappedtodisk,theprocessornoticesthatitcan'tserviceyourmemoryrequestdirectly.ItthenasksLinuxtostepin.Linuxnoticesthatthememoryisactuallyondisk.Therefore,itmovessomedatathatiscurrentlyinmemoryontodisktomakeroom,andthenmovesthememorybeingaccessedfromthediskbackintophysicalmemory.Itthenadjuststheprocessor'svirtual-to-physicalmemorylookuptablessothatitcanndthememoryinthenewlocation.Finally,Linuxreturnscontroltotheprogramandrestartsitattheinstructionwhichwastryingtoaccessthedataintherstplace.Thisinstructioncannowbecompletedsuccessfully,becausethememoryisnowinphysicalRAM.3HereisanoverviewofthewaymemoryaccessesarehandledunderLinux: Theprogramtriestoloadmemoryfromavirtualaddress. Theprocessor,usingtablessuppliedbyLinux,transformsthevirtualmemoryaddressintoaphysicalmemoryaddressonthey. 3. NotethatnotonlycanLinuxhaveavirtualaddressmaptoadifferentphysicaladdress,itcanalsomovethosemappingsaroundasneeded. 153 PAGE 160 Chapter9.IntermediateMemoryTopics Iftheprocessordoesnothaveaphysicaladdresslistedforthememoryaddress,itsendsarequesttoLinuxtoloadit. Linuxlooksattheaddress.Ifitismappedtoadisklocation,itcontinuesontothenextstep.Otherwise,itterminatestheprogramwithasegmentationfaulterror. Ifthereisnotenoughroomtoloadthememoryfromdisk,Linuxwillmoveanotherpartoftheprogramoranotherprogramontodisktomakeroom. Linuxthenmovesthedataintoafreephysicalmemoryaddress. Linuxupdatestheprocessor'svirtual-to-physicalmemorymappingtablestoreectthechanges. Linuxrestorescontroltotheprogram,causingittore-issuetheinstructionwhichcausedthisprocesstohappen. Theprocessorcannowhandletheinstructionusingthenewly-loadedmemoryandtranslationtables.It'salotofworkfortheoperatingsystem,butitgivestheuserandtheprogrammergreatexibilitywhenitcomestomemorymanagement.Now,inordertomaketheprocessmoreefcient,memoryisseparatedoutintogroupscalledpages.WhenrunningLinuxonx86processors,apageis4096bytesofmemory.Allofthememorymappingsaredoneapageatatime.Physicalmemoryassignment,swapping,mapping,etc.arealldonetomemorypagesinsteadofindividualmemoryaddresses.Whatthismeanstoyouasaprogrammeristhatwheneveryouareprogramming,youshouldtrytokeepmostmemoryaccesseswithinthesamebasicrangeofmemory,soyouwillonlyneedapageortwoofmemoryatatime.Otherwise,Linuxmayhavetokeepmovingpagesonandoffofdisktosatisfyyourmemoryneeds.Diskaccessisslow,sothiscanreallyslowdownyourprogram.Sometimessomanyprogramscanbeloadedthatthereishardlyenoughphysicalmemoryforthem.Theywindupspendingmoretimejustswappingmemoryon 154 PAGE 161 Chapter9.IntermediateMemoryTopics andoffofdiskthantheydoactuallyprocessingit.Thisleadstoaconditioncalledswapdeathwhichleadstoyoursystembeingunresponsiveandunproductive.It'susuallyusuallyrecoverableifyoustartterminatingyourmemory-hungryprograms,butit'sapain.ResidentSetSize:Theamountofmemorythatyourprogramcurrentlyhasinphysicalmemoryiscalledit'sresidentsetsize,andcanbeviewedbyusingtheprogramtop.Theresidentsetsizeislistedunderthecolumnlabelled"RSS".GettingMoreMemoryWenowknowthatLinuxmapsallofourvirtualmemoryintophysicalmemoryorswap.Ifyoutrytoaccessapieceofvirtualmemorythathasn'tbeenmappedyet,ittriggersanerrorknownasasegmentationfault,whichwillterminateyourprogram.Theprogrambreakpoint,ifyouremember,isthelastvalidaddressyoucanuse.Now,thisisallgreatifyouknowbeforehandhowmuchstorageyouwillneed.Youcanjustaddallthememoryyouneedtoyour.dataor.bsssections,anditwillallbethere.However,let'ssayyoudon'tknowhowmuchmemoryyouwillneed.Forexample,withatexteditor,youdon'tknowhowlongtheperson'slewillbe.Youcouldtrytondamaximumlesize,andjusttelltheuserthattheycan'tgobeyondthat,butthat'sawasteiftheleissmall.ThereforeLinuxhasafacilitytomovethebreakpointtoaccomodateanapplication'smemoryneeds.Ifyouneedmorememory,youcanjusttellLinuxwhereyouwantthenewbreakpointtobe,andLinuxwillmapallthememoryyouneedbetweenthecurrentandnewbreakpoint,andthenmovethebreakpointtothespotyouspecify.Thatmemoryisnowavailableforyourprogramtouse.ThewaywetellLinuxtomovethebreakpointisthroughthebrksystemcall.Thebrksystemcalliscallnumber 155 PAGE 162 Chapter9.IntermediateMemoryTopics 45whichwillbein%eax.%ebxshouldbeloadedwiththerequestedbreakpoint.Thenyoucallint$0x80tosignalLinuxtodoitswork.Aftermappinginyourmemory,Linuxwillreturnthenewbreakpointin%eax.Thenewbreakpointmightactuallybelargerthanwhatyouaskedfor,becauseLinuxroundsuptothenearestpage.Ifthereisnotenoughphysicalmemoryorswaptofulllyourrequest,Linuxwillreturnazeroin%eax.Also,ifyoucallbrkwithazeroin%ebx,itwillsimplyreturnthelastusablememoryaddress.Theproblemwiththismethodiskeepingtrackofthememorywerequest.Let'ssayIneedtomovethebreaktohaveroomtoloadale,andthenneedtomoveabreakagaintoloadanotherle.Let'ssayIthengetridoftherstle.Younowhaveagiantgapinmemorythat'smapped,butthatyouaren'tusing.Ifyoucontinuetomovethebreakinthiswayforeachleyouload,youcaneasilyrunoutofmemory.So,whatisneededisamemorymanager.Amemorymanagerisasetofroutinesthattakescareofthedirtyworkofgettingyourprogrammemoryforyou.Mostmemorymanagershavetwobasicfunctions-allocateanddeallocate.4Wheneveryouneedacertainamountofmemory,youcansimplytellallocatehowmuchyouneed,anditwillgiveyoubackanaddresstothememory.Whenyou'redonewithit,youtelldeallocatethatyouarethroughwithit.allocatewillthenbeabletoreusethememory.Thispatternofmemorymanagementiscalleddynamicmemoryallocation.Thisminimizesthenumberof"holes"inyourmemory,makingsurethatyouaremakingthebestuseofityoucan.Thepoolofmemoryusedbymemorymanagersiscommonlyreferredtoastheheap.Thewaymemorymanagersworkisthattheykeeptrackofwherethesystembreakis,andwherethememorythatyouhaveallocatedis.Theymarkeachblockofmemoryintheheapasbeingusedorunused.Whenyourequestmemory,thememorymanagercheckstoseeifthereareanyunusedblocksoftheappropriatesize.Ifnot,itcallsthebrksystemcalltorequestmorememory.Whenyoufree 4. Thefunctionnamesusuallyaren'tallocateanddeallocate,butthefunctionalitywillbethesame.IntheCprogramminglanguage,forexample,theyarenamedmallocandfree. 156 PAGE 163 Chapter9.IntermediateMemoryTopics memoryitmarkstheblockasunusedsothatfuturerequestscanretrieveit.Inthenextsectionwewilllookatbuildingourownmemorymanager.ASimpleMemoryManagerHereIwillshowyouasimplememorymanager.Itisveryprimitivebutitshowstheprinciplesquitewell.Asusual,Iwillgiveyoutheprogramrstforyoutolookthrough.Afterwardswillfollowanin-depthexplanation.Itlookslong,butitismostlycomments.#PURPOSE:Programtomanagememoryusage-allocates#anddeallocatesmemoryasrequested##NOTES:Theprogramsusingtheseroutineswillask#foracertainsizeofmemory.Weactually#usemorethanthatsize,butweputit#atthebeginning,beforethepointer#wehandback.Weaddasizefieldand#anAVAILABLE/UNAVAILABLEmarker.So,the#memorylookslikethis#############################################################AvailableMarker#Sizeofmemory#Actualmemorylocations############################################################^--Returnedpointer#pointshere#Thepointerwereturnonlypointstotheactual#locationsrequestedtomakeiteasierforthe#callingprogram.Italsoallowsustochangeour#structurewithoutthecallingprogramhavingto#changeatall..section.data 157 PAGE 164 Chapter9.IntermediateMemoryTopics #######GLOBALVARIABLES#########Thispointstothebeginningofthememorywearemanagingheap_begin:.long0#Thispointstoonelocationpastthememorywearemanagingcurrent_break:.long0######STRUCTUREINFORMATION#####sizeofspaceformemoryregionheader.equHEADER_SIZE,8#Locationofthe"available"flagintheheader.equHDR_AVAIL_OFFSET,0#Locationofthesizefieldintheheader.equHDR_SIZE_OFFSET,4###########CONSTANTS###########.equUNAVAILABLE,0#Thisisthenumberwewillusetomark#spacethathasbeengivenout.equAVAILABLE,1#Thisisthenumberwewillusetomark#spacethathasbeenreturned,andis#availableforgiving.equSYS_BRK,45#systemcallnumberforthebreak#systemcall.equLINUX_SYSCALL,0x80#makesystemcallseasiertoread.section.text 158 PAGE 165 Chapter9.IntermediateMemoryTopics ##########FUNCTIONS##############allocate_init###PURPOSE:callthisfunctiontoinitializethe#functionsspecifically,thissetsheap_beginand#current_break.Thishasnoparametersandno#returnvalue..globlallocate_init.typeallocate_init,@functionallocate_init:pushl%ebp#standardfunctionstuffmovl%esp,%ebp#Ifthebrksystemcalliscalledwith0in%ebx,it#returnsthelastvalidusableaddressmovl$SYS_BRK,%eax#findoutwherethebreakismovl$0,%ebxint$LINUX_SYSCALLincl%eax#%eaxnowhasthelastvalid#address,andwewantthe#memorylocationafterthatmovl%eax,current_break#storethecurrentbreakmovl%eax,heap_begin#storethecurrentbreakasour#firstaddress.Thiswillcause#theallocatefunctiontoget#morememoryfromLinuxthe#firsttimeitisrunmovl%ebp,%esp#exitthefunctionpopl%ebp 159 PAGE 166 Chapter9.IntermediateMemoryTopics ret#####ENDOFFUNCTION#########allocate###PURPOSE:Thisfunctionisusedtograbasectionof#memory.Itcheckstoseeifthereareany#freeblocks,and,ifnot,itasksLinux#foranewone.##PARAMETERS:Thisfunctionhasoneparameter-thesize#ofthememoryblockwewanttoallocate##RETURNVALUE:#Thisfunctionreturnstheaddressofthe#allocatedmemoryin%eax.Ifthereisno#memoryavailable,itwillreturn0in%eax#######PROCESSING#########Variablesused:##%ecx-holdthesizeoftherequestedmemory#first/onlyparameter#%eax-currentmemoryregionbeingexamined#%ebx-currentbreakposition#%edx-sizeofcurrentmemoryregion##Wescanthrougheachmemoryregionstartingwith#heap_begin.Welookatthesizeofeachone,andif#ithasbeenallocated.Ifit'sbigenoughforthe#requestedsize,anditsavailable,itgrabsthatone.#Ifitdoesnotfindaregionlargeenough,itasks#Linuxformorememory.Inthatcase,itmoves#current_breakup 160 PAGE 167 Chapter9.IntermediateMemoryTopics .globlallocate.typeallocate,@function.equST_MEM_SIZE,8#stackpositionofthememorysize#toallocateallocate:pushl%ebp#standardfunctionstuffmovl%esp,%ebpmovlST_MEM_SIZE%ebp,%ecx#%ecxwillholdthesize#wearelookingforwhichisthefirst#andonlyparametermovlheap_begin,%eax#%eaxwillholdthecurrent#searchlocationmovlcurrent_break,%ebx#%ebxwillholdthecurrent#breakalloc_loop_begin:#hereweiteratethrougheach#memoryregioncmpl%ebx,%eax#needmorememoryiftheseareequaljemove_break#grabthesizeofthismemorymovlHDR_SIZE_OFFSET%eax,%edx#Ifthespaceisunavailable,gotothecmpl$UNAVAILABLE,HDR_AVAIL_OFFSET%eaxjenext_location#nextonecmpl%edx,%ecx#Ifthespaceisavailable,comparejleallocate_here#thesizetotheneededsize.Ifits#bigenough,gotoallocate_here 161 PAGE 168 Chapter9.IntermediateMemoryTopics next_location:addl$HEADER_SIZE,%eax#Thetotalsizeofthememoryaddl%edx,%eax#regionisthesumofthesize#requestedcurrentlystored#in%edx,plusanother8bytes#fortheheaderforthe#AVAILABLE/UNAVAILABLEflag,#and4forthesizeofthe#region.So,adding%edxand$8#to%eaxwillgettheaddress#ofthenextmemoryregionjmpalloc_loop_begin#golookatthenextlocationallocate_here:#ifwe'vemadeithere,#thatmeansthatthe#regionheaderoftheregion#toallocateisin%eax#markspaceasunavailablemovl$UNAVAILABLE,HDR_AVAIL_OFFSET%eaxaddl$HEADER_SIZE,%eax#move%eaxpasttheheaderto#theusablememorysince#that'swhatwereturnmovl%ebp,%esp#returnfromthefunctionpopl%ebpretmove_break:#ifwe'vemadeithere,that#meansthatwehaveexhausted#alladdressablememory,and#weneedtoaskformore.#%ebxholdsthecurrent 162 PAGE 169 Chapter9.IntermediateMemoryTopics #endpointofthedata,#and%ecxholdsitssize#weneedtoincrease%ebxto#wherewe_want_memory#toend,soweaddl$HEADER_SIZE,%ebx#addspacefortheheaders#structureaddl%ecx,%ebx#addspacetothebreakfor#thedatarequested#nowitstimetoaskLinux#formorememorypushl%eax#saveneededregisterspushl%ecxpushl%ebxmovl$SYS_BRK,%eax#resetthebreak%ebxhas#therequestedbreakpointint$LINUX_SYSCALL#undernormalconditions,thisshould#returnthenewbreakin%eax,which#willbeeither0ifitfails,or#itwillbeequaltoorlargerthan#weaskedfor.Wedon'tcare#inthisprogramwhereitactually#setsthebreak,soaslongas%eax#isn't0,wedon'tcarewhatitiscmpl$0,%eax#checkforerrorconditionsjeerrorpopl%ebx#restoresavedregisterspopl%ecx 163 PAGE 170 Chapter9.IntermediateMemoryTopics popl%eax#setthismemoryasunavailable,sincewe'reaboutto#giveitawaymovl$UNAVAILABLE,HDR_AVAIL_OFFSET%eax#setthesizeofthememorymovl%ecx,HDR_SIZE_OFFSET%eax#move%eaxtotheactualstartofusablememory.#%eaxnowholdsthereturnvalueaddl$HEADER_SIZE,%eaxmovl%ebx,current_break#savethenewbreakmovl%ebp,%esp#returnthefunctionpopl%ebpreterror:movl$0,%eax#onerror,wereturnzeromovl%ebp,%esppopl%ebpret########ENDOFFUNCTION##########deallocate###PURPOSE:#Thepurposeofthisfunctionistogiveback#aregionofmemorytothepoolafterwe'redone#usingit.##PARAMETERS:#Theonlyparameteristheaddressofthememory#wewanttoreturntothememorypool. 164 PAGE 171 Chapter9.IntermediateMemoryTopics ##RETURNVALUE:#Thereisnoreturnvalue##PROCESSING:#Ifyouremember,weactuallyhandtheprogramthe#startofthememorythattheycanuse,whichis#8storagelocationsaftertheactualstartofthe#memoryregion.Allwehavetodoisgoback#8locationsandmarkthatmemoryasavailable,#sothattheallocatefunctionknowsitcanuseit..globldeallocate.typedeallocate,@function#stackpositionofthememoryregiontofree.equST_MEMORY_SEG,4deallocate:#sincethefunctionissosimple,we#don'tneedanyofthefancyfunctionstuff#gettheaddressofthememorytofree#normallythisis8%ebp,butsince#wedidn'tpush%ebpormove%espto#%ebp,wecanjustdo4%espmovlST_MEMORY_SEG%esp,%eax#getthepointertotherealbeginningofthememorysubl$HEADER_SIZE,%eax#markitasavailablemovl$AVAILABLE,HDR_AVAIL_OFFSET%eax#returnret########ENDOFFUNCTION########## 165 PAGE 172 Chapter9.IntermediateMemoryTopics Therstthingtonoticeisthatthereisno_startsymbol.Thereasonisthatthisisjustasetoffunctions.Amemorymanagerbyitselfisnotafullprogram-itdoesn'tdoanything.Itissimplyautilitytobeusedbyotherprograms.Toassembletheprogram,dothefollowing:asalloc.s-oalloc.oOkay,nowlet'slookatthecode.VariablesandConstantsAtthebeginningoftheprogram,wehavetwolocationssetup:heap_begin:.long0current_break:.long0Remember,thesectionofmemorybeingmanagediscommonlyreferredtoastheheap.Whenweassembletheprogram,wehavenoideawherethebeginningoftheheapis,norwherethecurrentbreakis.Therefore,wereservespacefortheiraddresses,butjustllthemwitha0forthetimebeing.Nextwehaveasetofconstantstodenethestructureoftheheap.Thewaythismemorymanagerworksisthatbeforeeachregionofmemoryallocated,wewillhaveashortrecorddescribingthememory.Thisrecordhasawordreservedfortheavailableagandawordfortheregion'ssize.Theactualmemoryallocatedimmediatelyfollowsthisrecord.Theavailableagisusedtomarkwhetherthisregionisavailableforallocations,orifitiscurrentlyinuse.Thesizeeldletsusknowbothwhetherornotthisregionisbigenoughforanallocationrequest,as 166 PAGE 173 Chapter9.IntermediateMemoryTopics wellasthelocationofthenextmemoryregion.Thefollowingconstantsdescribethisrecord:.equHEADER_SIZE,8.equHDR_AVAIL_OFFSET,0.equHDR_SIZE_OFFSET,4Thissaysthattheheaderis8bytestotal,theavailableagisoffset0bytesfromthebeginning,andthesizeeldisoffset4bytesfromthebeginning.Ifwearecarefultoalwaysusetheseconstants,thenweprotectourselvesfromhavingtodotoomuchworkifwelaterdecidetoaddmoreinformationtotheheader.Thevaluesthatwewilluseforouravailableeldareeither0forunavailable,or1foravailable.Tomakethiseasiertoread,wehavethefollowingdenitions:.equUNAVAILABLE,0.equAVAILABLE,1Finally,wehaveourLinuxsystemcalldenitions:.equBRK,45.equLINUX_SYSCALL,0x80Theallocate_initfunctionOkay,thisisasimplefunction.Allitdoesissetuptheheap_beginandcurrent_breakvariableswediscussedearlier.So,ifyourememberthediscussionearlier,thecurrentbreakcanbefoundusingthebrksystemcall.So,thefunctionstartslikethis:pushl%ebpmovl%esp,%ebpmovl$SYS_BRK,%eax 167 PAGE 174 Chapter9.IntermediateMemoryTopics movl$0,%ebxint$LINUX_SYSCALLAnyway,afterint$LINUX_SYSCALL,%eaxholdsthelastvalidaddress.Weactuallywanttherstinvalidaddressinsteadofthelastvalidaddress,sowejustincrement%eax.Thenwemovethatvaluetotheheap_beginandcurrent_breaklocations.Thenweleavethefunction.Thecodelookslikethis:incl%eaxmovl%eax,current_breakmovl%eax,heap_beginmovl%ebp,%esppopl%ebpretTheheapconsistsofthememorybetweenheap_beginandcurrent_break,sothissaysthatwestartoffwithaheapofzerobytes.Ourallocatefunctionwillthenextendtheheapasmuchasitneedstowhenitiscalled.TheallocatefunctionThisisthedoozyfunction.Let'sstartbylookingatanoutlineofthefunction: 1. Startatthebeginningoftheheap. 2. Checktoseeifwe'reattheendoftheheap. 3. Ifweareattheendoftheheap,grabthememoryweneedfromLinux,markitas"unavailable"andreturnit.IfLinuxwon'tgiveusanymore,returna0. 4. Ifthecurrentmemoryregionismarked"unavailable",gotothenextone,andgobacktostep2. 5. Ifthecurrentmemoryregionistoosmalltoholdtherequestedamountofspace,gobacktostep2. 168 PAGE 175 Chapter9.IntermediateMemoryTopics 6. Ifthememoryregionisavailableandlargeenough,markitas"unavailable"andreturnit.Now,lookbackthroughthecodewiththisinmind.Besuretoreadthecommentssoyou'llknowwhichregisterholdswhichvalue.Nowthatyou'velookedbackthroughthecode,let'sexamineitonelineatatime.Westartofflikethis:pushl%ebpmovl%esp,%ebpmovlST_MEM_SIZE%ebp,%ecxmovlheap_begin,%eaxmovlcurrent_break,%ebxThispartinitializesallofourregisters.Thersttwolinesarestandardfunctionstuff.Thenextmovepullsthesizeofthememorytoallocateoffofthestack.Thisisouronlyfunctionparameter.Afterthat,itmovesthebeginningheapaddressandtheendoftheheapintoregisters.Iamnowreadytodoprocessing.Thenextsectionismarkedalloc_loop_begin.Inthisloopwearegoingtoexaminememoryregionsuntilweeitherndanopenmemoryregionordeterminethatweneedmorememory.Ourrstinstructionschecktoseeifweneedmorememory:cmpl%ebx,%eaxjemove_break%eaxholdsthecurrentmemoryregionbeingexaminedand%ebxholdsthelocationpasttheendoftheheap.Thereforeifthenextregiontobeexaminedispasttheendoftheheap,itmeansweneedmorememorytoallocatearegionofthissize.Let'sskipdowntomove_breakandseewhathappensthere:move_break:addl$HEADER_SIZE,%ebxaddl%ecx,%ebx 169 PAGE 176 Chapter9.IntermediateMemoryTopics pushl%eaxpushl%ecxpushl%ebxmovl$SYS_BRK,%eaxint$LINUX_SYSCALLWhenwereachthispointinthecode,%ebxholdswherewewantthenextregionofmemorytobe.So,weaddourheadersizeandregionsizeto%ebx,andthat'swherewewantthesystembreaktobe.Wethenpushalltheregisterswewanttosaveonthestack,andcallthebrksystemcall.Afterthatwecheckforerrors:cmpl$0,%eaxjeerrorIftherewerenoerrorswepoptheregistersbackoffthestack,markthememoryasunavailable,recordthesizeofthememory,andmakesure%eaxpointstothestartofusablememorywhichisaftertheheader.popl%ebxpopl%ecxpopl%eaxmovl$UNAVAILABLE,HDR_AVAIL_OFFSET%eaxmovl%ecx,HDR_SIZE_OFFSET%eaxaddl$HEADER_SIZE,%eaxThenwestorethenewprogrambreakandreturnthepointertotheallocatedmemory.movl%ebx,current_breakmovl%ebp,%esppopl%ebpretTheerrorcodejustreturns0in%eax,sowewon'tdiscussit. 170 PAGE 177 Chapter9.IntermediateMemoryTopics Let'sgobacklookattherestoftheloop.Whathappensifthecurrentmemorybeinglookedatisn'tpasttheendoftheheap?Well,let'slook.movlHDR_SIZE_OFFSET%eax,%edxcmpl$UNAVAILABLE,HDR_AVAIL_OFFSET%eaxjenext_locationThisrstgrabsthesizeofthememoryregionandputsitin%edx.ThenitlooksattheavailableagtoseeifitissettoUNAVAILABLE.Ifso,thatmeansthatmemoryregionisinuse,sowe'llhavetoskipoverit.So,iftheavailableagissettoUNAVAILABLE,yougotothecodelabelednext_location.IftheavailableagissettoAVAILABLE,thenwekeepongoing.Let'ssaythatthespacewasavailable,andsowekeepgoing.Thenwechecktoseeifthisspaceisbigenoughtoholdtherequestedamountofmemory.Thesizeofthisregionisbeingheldin%edx,sowedothis:cmpl%edx,%ecxjleallocate_hereIftherequestedsizeislessthanorequaltothecurrentregion'ssize,wecanusethisblock.Itdoesn'tmatterifthecurrentregionislargerthanrequested,becausetheextraspacewilljustbeunused.So,let'sjumpdowntoallocate_hereandseewhathappens:movl$UNAVAILABLE,HDR_AVAIL_OFFSET%eaxaddl$HEADER_SIZE,%eaxmovl%ebp,%esppopl%ebpretItmarksthememoryasbeingunavailable.Thenitmovesthepointer%eaxpasttheheader,andusesitasthereturnvalueforthefunction.Remember,thepersonusingthisfunctiondoesn'tneedtoevenknowaboutourmemoryheaderrecord.Theyjustneedapointertousablememory. 171 PAGE 178 Chapter9.IntermediateMemoryTopics Okay,solet'ssaytheregionwasn'tbigenough.Whatthen?Well,wewouldthenbeatthecodelabelednext_location.Thissectionofcodeisusedanytimethatwegureoutthatthecurrentmemoryregionwon'tworkforallocatingmemory.Allitdoesisadvance%eaxtothenextpossiblememoryregion,andgoesbacktothebeginningoftheloop.Rememberthat%edxisholdingthesizeofthecurrentmemoryregion,andHEADER_SIZEisthesymbolforthesizeofthememoryregion'sheader.Sothiscodewillmoveustothenextmemoryregion:addl$HEADER_SIZE,%eaxaddl%edx,%eaxjmpalloc_loop_beginAndnowthefunctionrunsanotherloop.Wheneveryouhavealoop,youmustmakesurethatitwillalwaysend.Thebestwaytodothatistoexamineallofthepossibilities,andmakesurethatallofthemeventuallyleadtotheloopending.Inourcase,wehavethefollowingpossibilities: Wewillreachtheendoftheheap Wewillndamemoryregionthat'savailableandlargeenough WewillgotothenextlocationThersttwoitemsareconditionsthatwillcausethelooptoend.Thethirdonewillkeepitgoing.However,evenifweneverndanopenregion,wewilleventuallyreachtheendoftheheap,becauseitisanitesize.Therefore,weknowthatnomatterwhichconditionistrue,theloophastoeventuallyhitaterminatingcondition.ThedeallocatefunctionThedeallocatefunctionismucheasierthantheallocateone.That'sbecauseitdoesn'thavetodoanysearchingatall.Itcanjustmarkthecurrentmemory 172 PAGE 179 Chapter9.IntermediateMemoryTopics regionasAVAILABLE,andallocatewillnditnexttimeitiscalled.Sowehave:movlST_MEMORY_SEG%esp,%eaxsubl$HEADER_SIZE,%eaxmovl$AVAILABLE,HDR_AVAIL_OFFSET%eaxretInthisfunction,wedon'thavetosave%ebpor%espsincewe'renotchangingthem,nordowehavetorestorethemattheend.Allwe'redoingisreadingtheaddressofthememoryregionfromthestack,backinguptothebeginningoftheheader,andmarkingtheregionasavailable.Thisfunctionhasnoreturnvalue,sowedon'tcarewhatweleavein%eax.PerformanceIssuesandOtherProblemsOursimplisticmemorymanagerisnotreallyusefulforanythingmorethananacademicexercise.Thissectionlooksattheproblemswithsuchasimplisticallocator.Thebiggestproblemhereisspeed.Now,ifthereareonlyafewallocationsmade,thenspeedwon'tbeabigissue.Butthinkaboutwhathappensifyoumakeathousandallocations.Onallocationnumber1000,youhavetosearchthrough999memoryregionstondthatyouhavetorequestmorememory.Asyoucansee,that'sgettingprettyslow.Inaddition,rememberthatLinuxcankeeppagesofmemoryondiskinsteadofinmemory.So,sinceyouhavetogothrougheverypieceofmemoryyourprogram'smemory,thatmeansthatLinuxhastoloadeverypartofmemorythat'scurrentlyondisktochecktoseeifitsavailable.Youcanseehowthiscouldgetreally,reallyslow.5Thismethodissaidtoruninlineartime,whichmeansthateveryelementyouhavetomanagemakesyourprogramtake 5. Thisiswhyaddingmorememorytoyourcomputermakesitrunfaster.Themoremem-oryyourcomputerhas,thelessitputsondisk,soitdoesn'thavetoalwaysbeinterruptingyourprogramstoretreivepagesoffthedisk. 173 PAGE 180 Chapter9.IntermediateMemoryTopics longer.Aprogramthatrunsinconstanttimetakesthesameamountoftimenomatterhowmanyelementsyouaremanaging.Takethedeallocatefunction,forinstance.Itonlyruns4instructions,nomatterhowmanyelementswearemanaging,orwheretheyareinmemory.Infact,althoughourallocatefunctionisoneoftheslowestofallmemorymanagers,thedeallocatefunctionisoneofthefastest.Anotherperformanceproblemisthenumberoftimeswe'recallingthebrksystemcall.Systemcallstakealongtime.Theyaren'tlikefunctions,becausetheprocessorhastoswitchmodes.Yourprogramisn'tallowedtomapitselfmemory,buttheLinuxkernelis.So,theprocessorhastoswitchintokernelmode,thenLinuxmapsthememory,andthenswitchesbacktousermodeforyourapplicationtocontinuerunning.Thisisalsocalledacontextswitch.Contextswitchesarerelativelyslowonx86processors.Generally,youshouldavoidcallingthekernelunlessyoureallyneedto.Anotherproblemthatwehaveisthatwearen'trecordingwhereLinuxactuallysetsthebreak.PreviouslywementionedthatLinuxmightactuallysetthebreakpastwherewerequestedit.Inthisprogram,wedon'tevenlookatwhereLinuxactuallysetsthebreak-wejustassumeitsetsitwherewerequested.That'snotreallyabug,butitwillleadtounnecessarybrksystemcallswhenwealreadyhavethememorymappedin.Anotherproblemwehaveisthatifwearelookingfora5-byteregionofmemory,andtherstopenonewecometois1000bytes,wewillsimplymarkthewholethingasallocatedandreturnit.Thisleaves995bytesofunused,butallocated,memory.Itwouldbeniceinsuchsituationstobreakitapartsotheother995bytescanbeusedlater.Itwouldalsobenicetocombineconsecutivefreespaceswhenlookingforlargeallocations. 174 PAGE 181 Chapter9.IntermediateMemoryTopics UsingourAllocatorTheprogramswedointhisbookaren'tcomplicatedenoughtonecessitateamemorymanager.Therefore,wewilljustuseourmemorymanagertoallocateabufferforoneofourlereading/writingprogramsinsteadofassigningitinthe.bss.Theprogramwewilldemonstratethisonisread-records.sfrom Chapter6 .Thisprogramusesabuffernamedrecord_buffertohandleitsinput/outputneeds.Wewillsimplychangethisfrombeingabufferdenedin.bsstobeingapointertoadynamically-allocatedbufferusingourmemorymanager.Youwillneedtohavethecodefromthatprogramhandyaswewillonlybediscussingthechangesinthissection.Therstchangeweneedtomakeisinthedeclaration.Currentlyitlookslikethis:.section.bss.lcomm,record_buffer,RECORD_SIZEItwouldbeamisnomertokeepthesamename,sinceweareswitchingitfrombeinganactualbuffertobeingapointertoabuffer.Inaddition,itnowonlyneedstobeonewordbigenoughtoholdapointer.Thenewdeclarationwillstayinthe.datasectionandlooklikethis:record_buffer_ptr:.long0Ournextchangeisweneedtoinitializeourmemorymanagerimmediatelyafterwestartourprogram.Therefore,rightafterthestackissetup,thefollowingcallneedstobeadded:callallocate_initAfterthat,thememorymanagerisreadytostartservicingmemoryallocationrequests.Weneedtoallocateenoughmemorytoholdtheserecordsthatweare 175 PAGE 182 Chapter9.IntermediateMemoryTopics reading.Therefore,wewillcallallocatetoallocatethismemory,andthensavethepointeritreturnsintorecord_buffer_ptr.Likethis:pushl$RECORD_SIZEcallallocatemovl%eax,record_buffer_ptrNow,whenwemakethecalltoread_record,itisexpectingapointer.Intheoldcode,thepointerwastheimmediate-modereferencetorecord_buffer.Now,record_buffer_ptrjustholdsthepointerratherthanthebufferitself.Therefore,wemustdoadirectmodeloadtogetthevalueinrecord_buffer_ptr.Weneedtoremovethisline:pushl$record_bufferAndputthislineinitsplace:pushlrecord_buffer_ptrThenextchangecomeswhenwearetryingtondtheaddressoftherstnameeldofourrecord.Intheoldcode,itwas$RECORD_FIRSTNAME+record_buffer.However,thatonlyworksbecauseitisaconstantoffsetfromaconstantaddress.Inthenewcode,itistheoffsetofanaddressstoredinrecord_buffer_ptr.Togetthatvalue,wewillneedtomovethepointerintoaregister,andthenadd$RECORD_FIRSTNAMEtoittogetthepointer.Sowherewehavethefollowingcode:pushl$RECORD_FIRSTNAME+record_bufferWeneedtoreplaceitwiththis:movlrecord_buffer_ptr,%eaxaddl$RECORD_FIRSTNAME,%eaxpushl%eax 176 PAGE 183 Chapter9.IntermediateMemoryTopics Similarly,weneedtochangethelinethatsaysmovl$RECORD_FIRSTNAME+record_buffer,%ecxsothatitreadslikethis:movlrecord_buffer_ptr,%ecxaddl$RECORD_FIRSTNAME,%ecxFinally,onechangethatweneedtomakeistodeallocatethememoryoncewearedonewithitinthisprogramit'snotnecessary,butit'sagoodpracticeanyway.Todothat,wejustsendrecord_buffer_ptrtothedeallocatefunctionrightbeforeexitting:pushlrecord_buffer_ptrcalldeallocateNowyoucanbuildyourprogramwiththefollowingcommands:asread-records.s-oread-records.oldalloc.oread-record.oread-records.owrite-newline.ocount-chars.o-oread-recordsYoucanthenrunyourprogrambydoing./read-records.Theusesofdynamicmemoryallocationmaynotbeapparenttoyouatthispoint,butasyougofromacademicexercisestoreal-lifeprogramsyouwilluseitcontinually.MoreInformationMoreinformationonmemoryhandlinginLinuxandotheroperatingsystemscanbefoundatthefollowinglocations: 177 PAGE 184 Chapter9.IntermediateMemoryTopics MoreinformationaboutthememorylayoutofLinuxprogramscanbefoundinKonstantinBoldyshev'sdocument,"StartupstateofaLinux/i386ELFbinary",availableathttp://linuxassembly.org/startup.html Agoodoverviewofvirtualmemoryinmanydifferentsystemsisavailableathttp://cne.gmu.edu/modules/vm/ Severalin-deptharticlesonLinux'svirtualmemorysubsystemisavailableathttp://www.nongnu.org/lkdp/les.html DougLeahaswrittenupadescriptionofhispopularmemoryallocatorathttp://gee.cs.oswego.edu/dl/html/malloc.html Apaperonthe4.4BSDmemoryallocatorisavailableathttp://docs.freebsd.org/44doc/papers/malloc.htmlReviewKnowtheConcepts DescribethelayoutofmemorywhenaLinuxprogramstarts. Whatistheheap? Whatisthecurrentbreak? Whichdirectiondoesthestackgrowin? Whichdirectiondoestheheapgrowin? Whathappenswhenyouaccessunmappedmemory? Howdoestheoperatingsystempreventprocessesfromwritingovereachother'smemory? Describetheprocessthatoccursifapieceofmemoryyouareusingiscurrentlyresidingondisk? 178 PAGE 185 Chapter9.IntermediateMemoryTopics Whydoyouneedanallocator?UsetheConcepts Modifythememorymanagersothatitcallsallocate_initautomaticallyifithasn'tbeeninitialized. Modifythememorymanagersothatiftherequestedsizeofmemoryissmallerthantheregionchosen,itwillbreakuptheregionintomultipleparts.Besuretotakeintoaccountthesizeofthenewheaderrecordwhenyoudothis. Modifyoneofyourprogramsthatusesbufferstousethememorymanagertogetbuffermemoryratherthanusingthe.bss.GoingFurther Researchgarbagecollection.Whatadvantagesanddisadvantagesdoesthishaveoverthestyleofmemorymanagementusedhere? Researchreferencecounting.Whatadvantagesanddisadvantagesdoesthishaveoverthestyleofmemorymanagementusedhere? Changethenameofthefunctionstomallocandfree,andbuildthemintoasharedlibrary.UseLD_PRELOADtoforcethemtobeusedasyourmemorymanagerinsteadofthedefaultone.AddsomewritesystemcallstoSTDOUTtoverifythatyourmemorymanagerisbeingusedinsteadofthedefaultone. 179 PAGE 186 Chapter9.IntermediateMemoryTopics 180 PAGE 187 Chapter10.CountingLikeaComputerCountingCountingLikeaHumanInmanyways,computerscountjustlikehumans.So,beforewestartlearninghowcomputerscount,let'stakeadeeperlookathowwecount.Howmanyngersdoyouhave?No,it'snotatrickquestion.Humansnormallyhavetenngers.Whyisthatsignicant?Lookatournumberingsystem.Atwhatpointdoesaone-digitnumberbecomeatwo-digitnumber?That'sright,atten.Humanscountanddomathusingabasetennumberingsystem.Basetenmeansthatwegroupeverythingintens.Let'ssaywe'recountingsheep.1,2,3,4,5,6,7,8,9,10.Whydidweallofasuddennowhavetwodigits,andre-usethe1?That'sbecausewe'regroupingournumbersbyten,andwehave1groupoftensheep.Okay,let'sgotothenextnumber11.Thatmeanswehave1groupoftensheep,and1sheepleftungrouped.Sowecontinue-12,13,14,15,16,17,18,19,20.Nowwehave2groupsoften.21-2groupsoften,and1sheepungrouped.22-2groupsoften,and2sheepungrouped.So,let'ssaywekeepcounting,andgetto97,98,99,and100.Look,ithappenedagain!Whathappensat100?Wenowhavetengroupsoften.At101wehavetengroupsoften,and1ungroupedsheep.Sowecanlookatanynumberlikethis.Ifwecounted60879sheep,thatwouldmeanthatwehad6groupsoftengroupsoftengroupsoftengroupsoften,0groupsoftengroupsoftengroupsoften,8groupsoftengroupsoften,7groupsoften,and9sheepleftungrouped.So,isthereanythingsignicantaboutgroupingthingsbyten?No!It'sjustthatgroupingbytenishowwe'vealwaysdoneit,becausewehavetenngers.Wecouldhavegroupedatnineorateleveninwhichcasewewouldhavehadtomakeupanewsymbol.Theonlydifferencebetweenthedifferentgroupingsofnumbersisthatwehavetore-learnourmultiplication,addition,subtraction,and 181 PAGE 188 Chapter10.CountingLikeaComputer divisiontablesforeachgrouping.Theruleshaven'tchanged,justthewaywerepresentthem.Also,someofourtricksthatwelearneddon'talwaysapply,either.Forexample,let'ssaywegroupedbynineinsteadoften.Movingthedecimalpointonedigittotherightnolongermultipliesbyten,itnowmultipliesbynine.Inbasenine,500isonlyninetimesaslargeas50.CountingLikeaComputerThequestionis,howmanyngersdoesthecomputerhavetocountwith?Thecomputeronlyhastwongers.Sothatmeansallofthegroupsaregroupsoftwo.So,let'scountinbinary-0zero,1one,10two-onegroupoftwo,11three-onegroupoftwoandoneleftover,100four-twogroupsoftwo,101ve-twogroupsoftwoandoneleftover,110six-twogroupsoftwoandonegroupoftwo,andsoon.Inbasetwo,movingthedecimalonedigittotherightmultipliesbytwo,andmovingittotheleftdividesbytwo.Basetwoisalsoreferredtoasbinary.Thenicethingaboutbasetwoisthatthebasicmathtablesareveryshort.Inbaseten,themultiplicationtablesaretencolumnswide,andtencolumnstall.Inbasetwo,itisverysimple:Tableofbinaryaddition+|0|1--+-----+-----0|0|0--+-----+-----1|1|10Tableofbinarymultiplication 182 PAGE 189 Chapter10.CountingLikeaComputer *|0|1--+-----+-----0|0|0--+-----+-----1|0|1So,let'saddthenumbers10010101with1100101:10010101+1100101-----------11111010Now,let'smultiplythem:10010101*1100101-----------10010101000000001001010100000000000000001001010110010101---------------11101011001001 183 PAGE 190 Chapter10.CountingLikeaComputer ConversionsBetweenBinaryandDecimalLet'slearnhowtoconvertnumbersfrombinarybasetwotodecimalbaseten.Thisisactuallyarathersimpleprocess.Ifyouremember,eachdigitstandsforsomegroupingoftwo.So,wejustneedtoaddupwhateachdigitrepresents,andwewillhaveadecimalnumber.Takethebinarynumber10010101.Tondoutwhatitisindecimal,wetakeitapartlikethis:10010101|||||||||||||||Individualunits^0||||||0groupsof2^1|||||1groupof4^2||||0groupsof8^3|||1groupof16^4||0groupsof32^5|0groupsof64^61groupof128^7andthenweaddallofthepiecestogether,likethis:1*128+0*64+0*32+1*16+0*8+1*4+0*2+1*1=128+16+4+1=149So10010101inbinaryis149indecimal.Let'slookat1100101.Itcanbewrittenas1*64+1*32+0*16+0*8+1*4+0*2+1*1=64+32+4+1=101Soweseethat1100101inbinaryis101indecimal.Let'slookatonemorenumber,11101011001001.Youcanconvertittodecimalbydoing1*8192+1*4096+1*2048+0*1024+1*512+0*256 184 PAGE 191 Chapter10.CountingLikeaComputer +1*128+1*64+0*32+0*16+1*8+0*4+0*2+1*1=8192+4096+2048+512+128+64+8+1=15049Now,ifyou'vebeenpayingattention,youhavenoticedthatthenumberswejustconvertedarethesameonesweusedtomultiplywithearlier.So,let'scheckourresults:101*149=15049.Itworked!Nowlet'slookatgoingfromdecimalbacktobinary.Inordertodotheconversion,youhavetodividethenumberintogroupsoftwo.So,let'ssayyouhadthenumber17.Ifyoudivideitbytwo,youget8with1leftover.Sothatmeansthereare8groupsoftwo,and1ungrouped.Thatmeansthattherightmostdigitwillbe1.Now,wehavetherigtmostdigitguredout,and8groupsof2leftover.Now,let'sseehowmanygroupsoftwogroupsoftwowehave,bydividing8by2.Weget4,withnothingleftover.Thatmeansthatallgroupstwocanbefurtherdividedintomoregroupsoftwo.So,wehave0groupsofonlytwo.Sothenextdigittotheleftis0.So,wedivide4by2andgettwo,with0leftover,sothenextdigitis0.Then,wedivide2by2andget1,with0leftover.Sothenextdigitis0.Finally,wedivide1by2andget0with1leftover,sothenextdigittotheleftis1.Now,there'snothingleft,sowe'redone.So,thenumberwewoundupwithis10001.Previously,weconvertedtobinary11101011001001todecimal15049.Let'sdothereversetomakesurethatwediditright:15049/2=7524Remaining17524/2=3762Remaining03762/2=1881Remaining01881/2=940Remaining1940/2=470Remaining0470/2=235Remaining0235/2=117Remaining1117/2=58Remaining1 185 PAGE 192 Chapter10.CountingLikeaComputer 58/2=29Remaining029/2=14Remaining114/2=7Remaining07/2=3Remaining13/2=1Remaining11/2=0Remaining1Then,weputtheremainingnumbersbacktogether,andwehavetheoriginalnumber!Remembertherstdivisionremaindergoestothefarright,sofromthebottomupyouhave11101011001001.Eachdigitinabinarynumberiscalledabit,whichstandsforbinarydigit.Remember,computersdivideuptheirmemoryintostoragelocationscalledbytes.Eachstoragelocationonanx86processorandmostothersis8bitslong.Earlierwesaidthatabytecanholdanynumberbetween0and255.Thereasonforthisisthatthelargestnumberyoucantinto8bitsis255.Youcanseethisforyourselfifyouconvertbinary11111111intodecimal:11111111=*2^7+*2^6+*2^5+*2^4+*2^3+*2^2+*2^1+1*2^0=128+64+32+16+8+4+2+1=255Thelargestnumberthatyoucanholdin16bitsis65535.Thelargestnumberyoucanholdin32bitsis4294967295billion.Thelargestnumberyoucanholdin64bitsis18,446,744,073,709,551,615.Thelargestnumberyoucanholdin128bitsis340,282,366,920,938,463,463,374,607,431,768,211,456.Anyway,youseethepicture.Forx86processors,mostofthetimeyouwilldealwith4-bytenumbersbits,becausethat'sthesizeoftheregisters. 186 PAGE 193 Chapter10.CountingLikeaComputer Truth,Falsehood,andBinaryNumbersNowwe'veseenthatthecomputerstoreseverythingassequencesof1'sand0's.Let'slookatsomeotherusesofthis.Whatif,insteadoflookingatasequenceofbitsasanumber,weinsteadlookedatitasasetofswitches.Forexample,let'ssaytherearefourswitchesthatcontrollightinginthehouse.Wehaveaswitchforoutsidelights,aswitchforthehallwaylights,aswitchforthelivingroomlights,andaswitchforthebedroomlights.Wecouldmakealittletableshowingwhichofthesewereonandoff,likeso:OutsideHallwayLivingRoomBedroomOnOffOnOnIt'sobviousfromlookingatthisthatallofthelightsareonexceptthehallwayones.Now,insteadofusingthewords"On"and"Off",let'susethenumbers1and0.1willrepresenton,and0willrepresentoff.So,wecouldrepresentthesameinformationasOutsideHallwayLivingRoomBedroom1011Now,insteadofhavinglabelsonthelightswitches,let'ssaywejustmemorizedwhichpositionwentwithwhichswitch.Then,thesameinformationcouldberepresentedas1011oras1011Thisisjustoneofmanywaysyoucanusethecomputersstoragelocationstorepresentmorethanjustnumbers.Thecomputersmemoryjustseesnumbers,butprogrammerscanusethesenumberstorepresentanythingtheirimaginationscan 187 PAGE 194 Chapter10.CountingLikeaComputer comeupwith.Theyjustsometimeshavetobecreativewhenguringoutthebestrepresentation.Notonlycanyoudoregulararithmeticwithbinarynumbers,theyalsohaveafewoperationsoftheirown,calledbinaryorlogicaloperations.Thestandardbinaryoperationsare AND OR NOT XORBeforewelookatexamples,I'lldescribethemforyou.ANDtakestwobitsandreturnsonebit.ANDwillreturna1onlyifbothbitsare1,anda0otherwise.Forexample,1AND1is1,but1AND0is0,0AND1is0,and0AND0is0.ORtakestwobitsandreturnsonebit.Itwillreturn1ifeitheroftheoriginalbitsis1.Forexample,1OR1is1,1OR0isone,0OR1is1,but0OR0is0.NOTonlytakesonebit,andreturnsit'soppositeNOT1is0andNOT0is1.Finally,XORislikeOR,exceptitreturns0ifbothbitsare1.Computerscandotheseoperationsonwholeregistersatatime.Forexample,ifaregisterhas10100010101010010101101100101010andanotheronehas10001000010101010101010101111010,youcanrunanyoftheseoperationsonthewholeregisters.Forexample,ifweweretoANDthem,thecomputerwillrunfromtherstbittothe32ndandruntheANDoperationonthatbitinbothregisters.Inthiscase:10100010101010010101101100101010AND10001000010101010101010101111010--------------------------------10000000000000010101000100101010 188 PAGE 195 Chapter10.CountingLikeaComputer You'llseethattheresultingsetofbitsonlyhasaonewherebothnumbershadaone,andineveryotherpositionithasazero.Let'slookatwhatanORlookslike:10100010101010010101101100101010OR10001000010101010101010101111010--------------------------------10101010111111010101111101111010Inthiscase,theresultingnumberhasa1whereeithernumberhasa1inthegivenposition.Let'slookattheNOToperation:NOT10100010101010010101101100101010------------------------------------01011101010101101010010011010101Thisjustreverseseachdigit.Finally,wehaveXOR,whichislikeanOR,exceptifbothdigitsare1,itreturns0.10100010101010010101101100101010XOR10001000010101010101010101111010--------------------------------00101010111111000000111001010000ThisisthesametwonumbersusedintheORoperation,soyoucancomparehowtheywork.Also,ifyouXORanumberwithitself,youwillalwaysget0,likethis:10100010101010010101101100101010XOR10100010101010010101101100101010--------------------------------00000000000000000000000000000000Theseoperationsareusefulfortworeasons: Thecomputercandothemextremelyfast 189 PAGE 196 Chapter10.CountingLikeaComputer YoucanusethemtocomparemanytruthvaluesatthesametimeYoumaynothaveknownthatdifferentinstructionsexecuteatdifferentspeeds.It'strue,theydo.Andtheseoperationsarethefastestonmostprocessors.Forexample,yousawthatXORinganumberwithitselfproduces0.Well,theXORoperationisfasterthantheloadingoperation,somanyprogrammersuseittoloadaregisterwithzero.Forexample,thecodemovl$0,%eaxisoftenreplacedbyxorl%eax,%eaxWe'lldiscussspeedmorein Chapter12 ,butIwantyoutoseehowprogrammersoftendotrickythings,especiallywiththesebinaryoperators,tomakethingsfast.Nowlet'slookathowwecanusetheseoperatorstomanipulatetrue/falsevalues.Earlierwediscussedhowbinarynumberscanbeusedtorepresentanynumberofthings.Let'susebinarynumberstorepresentwhatthingsmyDadandIlike.First,let'slookatthethingsIlike:Food:yesHeavyMetalMusic:yesWearingDressyClothes:noFootball:yesNow,let'slookatwhatmyDadlikes:Food:yesHeavyMetalMusic:noWearingDressyClothes:yesFootball:yesNow,let'susea1tosayyeswelikesomething,anda0tosaynowedon't.Nowwehave: 190 PAGE 197 Chapter10.CountingLikeaComputer MeFood:1HeavyMetalMusic:1WearingDressyClothes:0Football:1DadFood:1HeavyMetalMusic:0WearingDressyClothes:1Football:1Now,ifwejustmemorizewhichpositioneachofthesearein,wehaveMe1101Dad1011Now,let'sseewewanttogetalistofthingsbothmyDadandIlike.YouwouldusetheANDoperation.So1101AND1011--------1001WhichtranslatestoThingswebothlikeFood:yesHeavyMetalMusic:noWearingDressyClothes:noFootball:yes 191 PAGE 198 Chapter10.CountingLikeaComputer Remember,thecomputerhasnoideawhattheonesandzeroesrepresent.That'syourjobandyourprogram'sjob.Ifyouwroteaprogramaroundthisrepresentationyourprogramwouldatsomepointexamineeachbitandhavecodetotelltheuserwhatit'sforifyouaskedacomputerwhattwopeopleagreedonanditanswered1001,itwouldn'tbeveryuseful.Anyway,let'ssaywewanttoknowthethingsthatwedisagreeon.ForthatwewoulduseXOR,becauseitwillreturn1onlyifoneortheotheris1,butnotboth.So1101XOR1011--------0110AndI'llletyoutranslatethatbackout.Thepreviousoperations:AND,OR,NOT,andXORarecalledbooleanoperatorsbecausetheywererststudiedbyGeorgeBoole.So,ifsomeonementionesbooleanoperatorsorbooleanalgebra,younowknowwhattheyaretalkingabout.Inadditiontothebooleanoperations,therearealsotwobinaryoperatorsthataren'tboolean,shiftandrotate.Shiftsandrotateseachdowhattheirnameimplies,andcandosototherightortheleft.Aleftshiftmoveseachdigitofabinarynumberonespacetotheleft,putsazerointheonesspot,andchopsoffthefurthestdigittotheleft.Aleftrotatedoesthesamething,buttakesthefurthestdigittotheleftandputsitintheonesspot.Forexample,Shiftleft10010111=00101110Rotateleft10010111=00101111Noticethatifyourotateanumberforeverydigitithasi.e.-rotatinga32-bitnumber32times,youwindupwiththesamenumberyoustartedwith.However,ifyoushiftanumberforeverydigityouhave,youwindupwith0.So,whataretheseshiftsusefulfor?Well,ifyouhavebinarynumbersrepresentingthings,youuseshiftstopeekateachindividualvalue.Let'ssay,forinstance,thatwehadmyDad'slikesstoredinaregisterbits.Itwouldlooklikethis: 192 PAGE 199 Chapter10.CountingLikeaComputer 00000000000000000000000000001011Now,aswesaidpreviously,thisdoesn'tworkasprogramoutput.So,inordertodooutput,wewouldneedtodoshiftingandmasking.Maskingistheprocessofeliminatingeverythingyoudon'twant.Inthiscase,foreveryvaluewearelookingfor,wewillshiftthenumbersothatvalueisintheonesplace,andthenmaskthatdigitsothatitisallwesee.MaskingisaccomplishedbydoinganANDwithanumberthathasthebitsweareinterestedinsetto1.Forexample,let'ssaywewantedtoprintoutwhethermyDadlikesdressyclothesornot.Thatdataisthesecondvaluefromtheright.So,wehavetoshiftthenumberright1digitsoitlookslikethis:00000000000000000000000000000101andthen,wejustwanttolookatthatdigit,sowemaskitbyANDingitwith00000000000000000000000000000001.00000000000000000000000000000101AND00000000000000000000000000000001-----------------------------------00000000000000000000000000000001Thiswillmakethevalueoftheregister1ifmyDadlikesdressyclothes,and0ifhedoesn't.Thenwecandoacomparisonto1andprinttheresults.Thecodewouldlooklikethis:#NOTE-assumethattheregister%ebxholds#myDad'spreferencesmovl%ebx,%eax#Thiscopiestheinformationinto%eaxso#wedon'tlosetheoriginaldatashrl$1,%eax#Thisistheshiftoperator.Itstands#forShiftRightLong.Thisfirstnumber#isthenumberofpositionstoshift, 193 PAGE 200 Chapter10.CountingLikeaComputer #andthesecondistheregistertoshift#Thisdoesthemaskingandl$0b00000000000000000000000000000001,%eax#Checktoseeiftheresultis1or0cmpl$0b00000000000000000000000000000001,%eaxjeyes_he_likes_dressy_clothesjmpno_he_doesnt_like_dressy_clothesAndthenwewouldhavetwolabelswhichprintedsomethingaboutwhetherornothelikesdressyclothesandthenexits.The0bnotationmeansthatwhatfollowsisabinarynumber.Inthiscaseitwasn'tneeded,because1isthesameinanynumberingsystem,butIputitthereforclarity.Wealsodidn'tneedthe31zeroes,butIputthemintomakeapointthatthenumberyouareusingis32bits.Whenanumberrepresentsasetofoptionsforafunctionorsystemcall,theindividualtrue/falseelementsarecalledags.Manysystemcallshavenumerousoptionsthatareallsetinthesameregisterusingamechanismlikewe'vedescribed.Theopensystemcall,forexample,hasasitssecondparameteralistofagstotelltheoperatingsystemhowtoopenthele.Someoftheagsinclude:O_WRONLY Thisagis0b00000000000000000000000000000001inbinary,or01inoctaloranynumbersystemforthatmatter.Thissaystoopentheleinwrite-onlymode.O_RDWR Thisagis0b00000000000000000000000000000010inbinary,or02inoctal.Thissaystoopentheleforbothreadingandwriting. 194 PAGE 201 Chapter10.CountingLikeaComputer O_CREAT Thisagis0b00000000000000000000000001000000inbinary,or0100inoctal.Itmeanstocreatetheleifitdoesn'talreadyexist.O_TRUNC Thisagis0b00000000000000000000001000000000inbinary,or01000inoctal.Itmeanstoerasethecontentsoftheleifthelealreadyexists.O_APPEND Thisagis0b00000000000000000000010000000000inbinary,or02000inoctal.Itmeanstostartwritingattheendoftheleratherthanatthebeginning.Tousetheseags,yousimplyORthemtogetherinthecombinationthatyouwant.Forexample,toopenaleinwrite-onlymode,andhaveitcreatetheleifitdoesn'texist,IwoulduseO_WRONLYandO_CREAT.OR'dtogether,Iwouldhave0101.Notethatifyoudon'tseteitherO_WRONLYorO_RDWR,thentheleisautomaticallyopenedinread-onlymodeO_RDONLY,exceptthatitisn'treallyaagsinceit'szero.Manyfunctionsandsystemcallsuseagsforoptions,asitallowsasinglewordtoholdupto32possibleoptionsifeachoptionisrepresentedbyasinglebit.TheProgramStatusRegisterWe'veseenhowbitsonaregistercanbeusedtogivetheanswersofyes/noandtrue/falsestatements.Onyourcomputer,thereisaregistercalledtheprogramstatusregister.Thisregisterholdsalotofinformationaboutwhathappensinacomputation.Forexample,haveyoueverwonderedwhatwouldhappenifyou 195 PAGE 202 Chapter10.CountingLikeaComputer addedtwonumbersandtheresultwaslargerthanwouldtinaregister?Theprogramstatusregisterhasaagcalledthecarryag.Youcantestittoseeifthelastcomputationoverowedtheregister.Thereareagsforanumberofdifferentstatuses.Infact,whenyoudoacomparecmplinstruction,theresultisstoredinthisregister.Theconditionaljumpinstructionsjge,jne,etcusetheseresultstotellwhetherornottheyshouldjump.jmp,theunconditionaljump,doesn'tcarewhatisinthestatusregister,sinceitisunconditional.Let'ssayyouneededtostoreanumberlargerthan32bits.So,let'ssaythenumberis2registerswide,or64bits.Howcouldyouhandlethis?Ifyouwantedtoaddtwo64bitnumbers,youwouldaddtheleastsignicantregistersrst.Then,ifyoudetectedancarry,youcouldadd1tothemostsignicantregister.Infact,thisisprobablythewayyoulearnedtododecimaladdition.Iftheresultinonecolumnismorethan9,yousimplycarriedthenumbertothenextmostsignicantcolumn.Ifyouadded65and37,rstyouadd7and4toget12.Youkeepthe2intherightcolumn,andcarrytheonetothenextcolumn.Thereyouadd6,3,andthe1youcarried.Thisresultsin10.So,youkeepthezerointhatcolumnandcarrytheonetothenextmostsignicantcolumn,whichisempty,soyoujustputtheonethere.Luckily,32bitsisusuallybigenoughtoholdthenumbersweuseregularly.Additionalprogramstatusregisteragsareexaminedin AppendixB .OtherNumberingSystemsWhatwehavestudiedsofaronlyappliestopositiveintegers.However,real-worldnumbersarenotalwayspositiveintegers.Negativenumbersandnumberswithdecimalsarealsoused.Floating-pointNumbersSofar,theonlynumberswe'vedealtwithareintegers-numberswithnodecimalpoint.Computershaveageneralproblemwithnumberswithdecimalpoints, 196 PAGE 203 Chapter10.CountingLikeaComputer becausecomputerscanonlystorexed-size,nitevalues.Decimalnumberscanbeanylength,includinginnitelengththinkofarepeatingdecimal,liketheresultof1/3.Thewayacomputerhandlesdecimalsisbystoringthemataxedprecisionnumberofsignicantbits.Acomputerstoresdecimalnumbersintwoparts-theexponentandthemantissa.Themantissacontainstheactualdigitsthatwillbeused,andtheexponentiswhatmagnitudethenumberis.Forexample,12345.2isstoredas1.23452*10^4.Themantissais1.23452andtheexponentis4.AllnumbersarestoredasX.XXXXX*10^XXXX.Thenumber1isstoredas1.00000*10^0.Now,themantissaandtheexponentareonlysolong,whichleadstosomeinterestingproblems.Forexample,whenacomputerstoresaninteger,ifyouadd1toit,theresultingnumberisonelarger.Thisdoesnotnecessarilyhappenwithoatingpointnumbers.Ifthenumberissufcientlybig,like5.234*10^5000,adding1toitmightnotevenregisterinthemantissaremember,bothpartsareonlysolong.Thisaffectsseveralthings,especiallyorderofoperations.Let'ssaythatIadd1to5.234*10^5000afewbillionortrilliontimes.Guesswhat-thenumberwon'tchangeatall.However,ifIaddonetoitselfenoughtimes,andthenaddittotheoriginalnumber,itmightmakeadent.Youshouldnotethatittakesmostcomputersalotlongertodooating-pointarithmeticthanitdoesintegerarithmetic.So,forprogramsthatreallyneedspeed,integersaremostlyused.NegativeNumbersHowwouldyouthinkthatnegativenumbersonacomputermightberepresented?Onethoughtmightbetousetherstdigitofanumberasthesign,so00000000000000000000000000000001wouldrepresentthenumber1,and10000000000000000000000000000001wouldrepresent-1.Thismakesalotofsense,andinfactsomeoldprocessorsworkthisway.However,ithassome 197 PAGE 204 Chapter10.CountingLikeaComputer problems.Firstofall,ittakesalotmorecircuitrytoaddandsubtractsignednumbersrepresentedthisway.Evenmoreproblematic,thisrepresentationhasaproblemwiththenumber0.Inthissystem,youcouldhavebothanegativeandapositive0.Thisleadstoalotofquestions,like"shouldnegativezerobeequaltopositivezero?",and"Whatshouldthesignofzerobeinvariouscircumstances?".Theseproblemswereovercomebyusingarepresentationofnegativenumberscalledtwo'scomplementrepresentation.Togetthenegativerepresentationofanumberintwo'scomplementform,youmustperformthefollowingsteps: 1. PerformaNOToperationonthenumber 2. AddonetotheresultingnumberSo,togetthenegativeof00000000000000000000000000000001,youwouldrstdoaNOToperation,whichgives11111111111111111111111111111110,andthenaddone,giving11111111111111111111111111111111.Togetnegativetwo,rsttake00000000000000000000000000000010.TheNOTofthatnumberis11111111111111111111111111111101.Addingonegives11111111111111111111111111111110.Withthisrepresentation,youcanaddnumbersjustasiftheywerepositive,andcomeoutwiththerightanswers.Forexample,ifyouaddoneplusnegativeoneinbinary,youwillnoticethatallofthenumbersiptozero.Also,therstdigitstillcarriesthesignbit,makingitsimpletodeterminewhetherornotthenumberispositiveornegative.Negativenumberswillalwayshavea1intheleftmostbit.Thisalsochangeswhichnumbersarevalidforagivennumberofbits.Withsignednumbers,thepossiblemagnitudeofthevaluesissplittoallowforbothpositiveandnegativenumbers.Forexample,abytecannormallyhavevaluesupto255.Asignedbyte,however,canstorevaluesfrom-128to127.Onethingtonoteaboutthetwo'scomplementrepresentationofsignednumbersisthat,unlikeunsignedquantities,ifyouincreasethenumberofbits,youcan'tjustaddzeroestotheleftofthenumber.Forexample,let'ssaywearedealingwithfour-bitquantitiesandwehadthenumber-3,1101.Ifweweretoextendthisinto 198 PAGE 205 Chapter10.CountingLikeaComputer aneight-bitregister,wecouldnotrepresentitas00001101asthiswouldrepresent13,not-3.Whenyouincreasethesizeofasignedquantityintwo'scomplementrepresentation,youhavetoperformsignextension.Signextensionmeansthatyouhavetopadtheleft-handsideofthequantitywithwhateverdigitisinthesigndigitwhenyouaddbits.So,ifweextendanegativenumberby4digits,weshouldllthenewdigitswitha1.Ifweextendapositivenumberby4digits,weshouldllthenewdigitswitha0.So,theextensionof-3fromfourtoeightbitswillyield11111101.Thex86processorhasdifferentformsofseveralinstructionsdependingonwhethertheyexpectthequantitiestheyoperateontobesignedorunsigned.Thesearelistedin AppendixB .Forexample,thex86processorhasbothasign-preservingshift-right,sarl,andashift-rightwhichdoesnotpreservethesignbit,shrl.OctalandHexadecimalNumbersThenumberingsystemsdiscussedsofarhavebeendecimalandbinary.However,twoothersareusedcommonincomputing-octalandhexadecimal.Infact,theyareprobablywrittenmoreoftenthanbinary.Octalisarepresentationthatonlyusesthenumbers0through7.Sotheoctalnumber10isactually8indecimalbecauseitisonegroupofeight.Octal121isdecimal81onegroupof64^2,twogroupsof8,andoneleftover.Whatmakesoctalniceisthatevery3binarydigitsmakeoneoctaldigitthereisnosuchgroupingofbinarydigitsintodecimal.So0is000,1is001,2is010,3is011,4is100,5is101,6is110,and7is111.PermissionsinLinuxaredoneusingoctal.ThisisbecauseLinuxpermissionsarebasedontheabilitytoread,writeandexecute.Therstbitisthereadpermission,thesecondbitisthewritepermission,andthethirdbitistheexecutepermission.So,0givesnopermissions,6givesreadandwritepermission,and5givesreadandexecutepermissions.Thesenumbersarethenusedforthe 199 PAGE 206 Chapter10.CountingLikeaComputer threedifferentsetsofpermissions-theowner,thegroup,andeveryoneelse.Thenumber0644meansreadandwritefortherstpermissionset,andread-onlyforthesecondandthirdset.Therstpermissionsetisfortheownerofthele.Thethirdpermissionsetisforthegroupownerofthele.Thelastpermissionsetisforeveryoneelse.So,0751meansthattheownerofthelecanread,write,andexecutethele,thegroupmemberscanreadandexecutethele,andeveryoneelsecanonlyexecutethele.Anyway,asyoucansee,octalisusedtogroupbitsbinarydigitsintothrees.Thewaytheassemblerknowsthatanumberisoctalisbecauseoctalnumbersareprexedwithazero.Forexample010means10inoctal,whichis8indecimal.Ifyoujustwrite10thatmeans10indecimal.Thebeginningzeroiswhatdifferentiatesthetwo.So,becarefulnottoputanyleadingzeroesinfrontofdecimalnumbers,ortheywillbeinterepretedasoctalnumbers!Hexadecimalnumbersalsocalledjust"hex"usethenumbers1-15foreachdigit.however,since10-15don'thavetheirownnumbers,hexadecimalusesthelettersathroughftorepresentthem.Forexample,theletterarepresents10,theletterbrepresents11,andsoon.10inhexadecimalis16indecimal.Inoctal,eachdigitrepresentedthreebits.Inhexadecimal,eachdigitrepresentsfourbits.Everytwodigitsisafullbyte,andeightdigitsisa32-bitword.Soyousee,itisconsiderablyeasiertowriteahexadecimalnumberthanitistowriteabinarynumber,becauseit'sonlyaquarterasmanydigits.Themostimportantnumbertorememberinhexadecimalisf,whichmeansthatallbitsareset.So,ifIwanttosetallofthebitsofaregisterto1,Icanjustdomovl$0xFFFFFFFF,%eaxWhichisconsiderablyeasierandlesserror-pronethanwritingmovl$0b11111111111111111111111111111111,%eaxNotealsothathexadecimalnumbersareprexedwith0x.So,whenwedo 200 PAGE 207 Chapter10.CountingLikeaComputer int$0x80Wearecallinginterruptnumber128groupsof16,orinterruptnumber0b00000000000000000000000010000000.Hexadecimalandoctalnumberstakesomegettingusedto,buttheyareheavilyusedincomputerprogramming.Itmightbeworthwhiletomakeupsomenumbersinhexandtrytoconvertthembackandforthtobinary,decimal,andoctal.OrderofBytesinaWordOnethingthatconfusesmanypeoplewhendealingwithbitsandbytesonalowlevelisthat,whenbytesarewrittenfromregisterstomemory,theirbytesarewrittenoutleast-signicant-portion-rst.1Whatmostpeopleexpectisthatiftheyhaveawordinaregister,say0x5d23efeethespacingissoyoucanseewherethebytesare,thebyteswillbewrittentomemoryinthatorder.However,onx86processors,thebytesareactuallywritteninreverseorder.Inmemorythebyteswouldbe0xeeef235donx86processors.Thebytesarewritteninreverseorderfromwhattheywouldappearconceptually,butthebitswithinthebytesareorderednormally.Notallprocessorsbehavethisway.Thex86processorisalittle-endianprocessor,whichmeansthatitstoresthe"littleend",orleast-signicantbyteofitswordsrst. 1. Signicanceinthiscontextisreferringtowhichdigittheyrepresent.Forexample,inthenumber294,thedigit2isthemostsignicantbecauseitrepresentsthehundredsplace,9isthenextmostsignicant,and4istheleastsignicant. 201 PAGE 208 Chapter10.CountingLikeaComputer Register-to-memorytransfersonlittle-endiansystemsOtherprocessorsarebig-endianprocessors,whichmeansthattheystorethe"bigend",ormostsignicantbyte,oftheirwordsrst,thewaywewouldnaturallyreadanumber. 202 PAGE 209 Chapter10.CountingLikeaComputer Register-to-memorytransfersonbig-endiansystemsThisdifferenceisnotnormallyaproblemalthoughithassparkedmanytechnicalcontroversiesthroughouttheyears.Becausethebytesarereversedagainornot,ifitisabig-endianprocessorwhenbeingreadbackintoaregister,theprogrammerusuallynevernoticeswhatorderthebytesarein.Thebyte-switchingmagichappensautomaticallybehindthescenesduringregister-to-memorytransfers.However,thebyteordercancauseproblemsinseveralinstances: Ifyoutrytoreadinseveralbytesatatimeusingmovlbutdealwiththemonabyte-by-bytebasisusingtheleastsignicantbytei.e.-byusing%aland/orshiftingoftheregister,thiswillbeinadifferentorderthantheyappearinmemory. 203 PAGE 210 Chapter10.CountingLikeaComputer Ifyoureadorwriteleswrittenfordifferentarchitectures,youmayhavetoaccountforwhateverordertheywritetheirbytesin. Ifyoureadorwritetonetworksockets,youmayhavetoaccountforadifferentbyteorderintheprotocol.Aslongasyouareawareoftheissue,itusuallyisn'tabigdeal.Formorein-depthlookatbyteorderissues,youshouldreadDAV'sEndianFAQathttp://www.rdrop.com/~cary/html/endian_faq.html,especiallythearticle"OnHolyWarsandaPleaforPeace"byDanielCohen.ConvertingNumbersforDisplaySofar,wehavebeenunabletodisplayanynumberstoredtotheuser,exceptbytheextremelylimittedmeansofpassingitthroughexitcodes.Inthissection,wewilldiscussconvertingpositivenumbersintostringsfordisplay.Thefunctionwillbecalledinteger2string,anditwilltaketwoparameters-anintegertoconvertandastringbufferlledwithnullcharacterszeroes.Thebufferwillbeassumedtobebigenoughtostoretheentirenumberasastring.atleast11characterslong,toincludeatrailingnullcharacter.Rememberthatthewaythatweseenumbersisinbase10.Therefore,toaccesstheindividualdecimaldigitsofanumber,weneedtobedividingby10anddisplayingtheremainderforeachdigit.Therefore,theprocesswilllooklikethis: Dividethenumberbyten Theremainderisthecurrentdigit.Convertittoacharacterandstoreit. Wearenishedifthequotientiszero. Otherwise,takethequotientandthenextlocationinthebufferandrepeattheprocess. 204 PAGE 211 Chapter10.CountingLikeaComputer Theonlyproblemisthatsincethisprocessdealswiththeone'splacerst,itwillleavethenumberbackwards.Therefore,wewillhavetonishbyreversingthecharacters.Wewilldothisbystoringthecharactersonthestackaswecomputethem.Thisway,aswepopthembackofftollinthebuffer,itwillbeinthereverseorderthatwepushedthemon.Thecodeforthefunctionshouldbeputinalecalledinteger-to-string.sandshouldbeenteredasfollows:#PURPOSE:Convertanintegernumbertoadecimalstring#fordisplay##INPUT:Abufferlargeenoughtoholdthelargest#possiblenumber#Anintegertoconvert##OUTPUT:Thebufferwillbeoverwrittenwiththe#decimalstring##Variables:##%ecxwillholdthecountofcharactersprocessed#%eaxwillholdthecurrentvalue#%ediwillholdthebase0#.equST_VALUE,8.equST_BUFFER,12.globlinteger2string.typeinteger2string,@functioninteger2string:#Normalfunctionbeginningpushl%ebpmovl%esp,%ebp 205 PAGE 212 Chapter10.CountingLikeaComputer #Currentcharactercountmovl$0,%ecx#MovethevalueintopositionmovlST_VALUE%ebp,%eax#Whenwedivideby10,the10#mustbeinaregisterormemorylocationmovl$10,%ediconversion_loop:#Divisionisactuallyperformedonthe#combined%edx:%eaxregister,sofirst#clearout%edxmovl$0,%edx#Divide%edx:%eaxwhichareimpliedby10.#Storethequotientin%eaxandtheremainder#in%edxbothofwhichareimplied.divl%edi#Quotientisintherightplace.%edxhas#theremainder,whichnowneedstobeconverted#intoanumber.So,%edxhasanumberthatis#0through9.Youcouldalsointerpretthisas#anindexontheASCIItablestartingfromthe#character'0'.Theasciicodefor'0'pluszero#isstilltheasciicodefor'0'.Theasciicode#for'0'plus1istheasciicodeforthe#character'1'.Therefore,thefollowing#instructionwillgiveusthecharacterforthe#numberstoredin%edxaddl$'0',%edx#Nowwewilltakethisvalueandpushitonthe 206 PAGE 213 Chapter10.CountingLikeaComputer #stack.Thisway,whenwearedone,wecanjust#popoffthecharactersone-by-oneandtheywill#beintherightorder.Notethatwearepushing#thewholeregister,butweonlyneedthebyte#in%dlthelastbyteofthe%edxregisterfor#thecharacter.pushl%edx#Incrementthedigitcountincl%ecx#Checktoseeif%eaxiszeroyet,gotonext#stepifso.cmpl$0,%eaxjeend_conversion_loop#%eaxalreadyhasitsnewvalue.jmpconversion_loopend_conversion_loop:#Thestringisnowonthestack,ifwepopit#offacharacteratatimewecancopyitinto#thebufferandbedone.#Getthepointertothebufferin%edxmovlST_BUFFER%ebp,%edxcopy_reversing_loop:#Wepushedawholeregister,butweonlyneed#thelastbyte.Sowearegoingtopopoffto#theentire%eaxregister,butthenonlymovethe#smallpart%alintothecharacterstring.popl%eaxmovb%al,%edx 207 PAGE 214 Chapter10.CountingLikeaComputer #Decreasing%ecxsoweknowwhenwearefinisheddecl%ecx#Increasing%edxsothatitwillbepointingto#thenextbyteincl%edx#Checktoseeifwearefinishedcmpl$0,%ecx#Ifso,jumptotheendofthefunctionjeend_copy_reversing_loop#Otherwise,repeattheloopjmpcopy_reversing_loopend_copy_reversing_loop:#Donecopying.Nowwriteanullbyteandreturnmovb$0,%edxmovl%ebp,%esppopl%ebpretToshowthisusedinafullprogram,usethefollowingcode,alongwiththecount_charsandwrite_newlinefunctionswrittenaboutinpreviouschapters.Thecodeshouldbeinalecalledconversion-program.s..include"linux.s".section.data#Thisiswhereitwillbestoredtmp_buffer:.ascii"0" 208 PAGE 215 Chapter10.CountingLikeaComputer .section.text.globl_start_start:movl%esp,%ebp#Storagefortheresultpushl$tmp_buffer#Numbertoconvertpushl$824callinteger2stringaddl$8,%esp#Getthecharactercountforoursystemcallpushl$tmp_buffercallcount_charsaddl$4,%esp#Thecountgoesin%edxforSYS_WRITEmovl%eax,%edx#Makethesystemcallmovl$SYS_WRITE,%eaxmovl$STDOUT,%ebxmovl$tmp_buffer,%ecxint$LINUX_SYSCALL#Writeacarriagereturnpushl$STDOUTcallwrite_newline#Exitmovl$SYS_EXIT,%eax 209 PAGE 216 Chapter10.CountingLikeaComputer movl$0,%ebxint$LINUX_SYSCALLTobuildtheprogram,issuethefollowingcommands:asinteger-to-string.s-ointeger-to-number.oascount-chars.s-ocount-chars.oaswrite-newline.s-owrite-newline.oasconversion-program.s-oconversion-program.oldinteger-to-number.ocount-chars.owrite-newline.oconversion-program.o-oconversion-programTorunjusttype./conversion-programandtheoutputshouldsay824.ReviewKnowtheConcepts Convertthedecimalnumber5,294tobinary. Whatnumberdoes0x0234aeffrepresent?Specifyinbinary,octal,anddecimal. Addthebinarynumbers10111001and101011. Multiplythebinarynumbers11001010110. Converttheresultsoftheprevioustwoproblemsintodecimal. DescribehowAND,OR,NOT,andXORwork. Whatismaskingfor? Whatnumberwouldyouusefortheagsoftheopensystemcallifyouwantedtoopentheleforwriting,andcreatetheleifitdoesn'texist? 210 PAGE 217 Chapter10.CountingLikeaComputer Howwouldyourepresent-55inathirty-twobitregister? Sign-extendthepreviousquantityintoa64-bitregister. Describethedifferencebetweenlittle-endianandbig-endianstorageofwordsinmemory.UsetheConcepts Gobacktopreviousprogramsthatreturnednumericresultsthroughtheexitstatuscode,andrewritethemtoprintouttheresultsinsteadusingourintegertostringconversionfunction. Modifytheinteger2stringcodetoreturnresultsinoctalratherthandecimal. Modifytheinteger2stringcodesothattheconversionbaseisaparameterratherthanhardcoded. Writeafunctioncalledis_negativethattakesasingleintegerasaparameterandreturns1iftheparameterisnegative,and0iftheparameterispositive.GoingFurther Modifytheinteger2stringcodesothattheconversionbasecanbegreaterthan10thisrequiresyoutouselettersfornumberspast9. Createafunctionthatdoesthereverseofinteger2stringcallednumber2integerwhichtakesacharacterstringandconvertsittoaregister-sizedinteger.Testitbyrunningthatintegerbackthroughtheinteger2stringfunctionanddisplayingtheresults. Writeaprogramthatstoreslikesanddislikesintoasinglemachineword,andthencomparestwosetsoflikesanddislikesforcommonalities. 211 PAGE 218 Chapter10.CountingLikeaComputer WriteaprogramthatreadsastringofcharactersfromSTDINandconvertsthemtoanumber. 212 PAGE 219 Chapter11.High-LevelLanguagesInthischapterwewillbegintolookatourrst"real-world"programminglanguage.Assemblylanguageisthelanguageusedatthemachine'slevel,butmostpeoplendcodinginassemblylanguagetoocumbersomeforeverydayuse.Manycomputerlanguageshavebeeninventedtomaketheprogrammingtaskeasier.Knowingawidevarietyoflanguagesisusefulformanyreasons,including Differentlanguagesarebasedondifferentconcepts,whichwillhelpyoutolearndifferentandbetterprogrammingmethodsandideas. Differentlanguagesaregoodfordifferenttypesofprojects. Differentcompanieshavedifferentstandardlanguages,soknowingmorelanguagesmakesyourskillsmoremarketable. Themorelanguagesyouknow,theeasieritistopickupnewones.Asaprogrammer,youwilloftenhavetopickupnewlanguages.Professionalprogrammerscanusuallypickupanewlanguagewithaboutaweeksworthofstudyandpractice.Languagesaresimplytools,andlearningtouseanewtoolshouldnotbesomethingaprogrammerinchesat.Infact,ifyoudocomputerconsultingyouwilloftenhavetolearnnewlanguagesonthespotinordertokeepyourselfemployed.Itwilloftenbeyourcustomer,notyou,whodecideswhatlanguageisused.Thischapterwillintroduceyoutoafewofthelanguagesavailabletoyou.Iencourageyoutoexploreasmanylanguagesasyouareinterestedin.Ipersonallytrytolearnanewlanguageeveryfewmonths.CompiledandInterpretedLanguagesManylanguagesarecompiledlanguages.Whenyouwriteassemblylanguage,eachinstructionyouwriteistranslatedintoexactlyonemachineinstructionforprocessing.Withcompilers,astatementcantranslateintooneorhundredsof 213 PAGE 220 Chapter11.High-LevelLanguages machineinstructions.Infact,dependingonhowadvancedyourcompileris,itmightevenrestructurepartsofyourcodetomakeitfaster.Inassemblylanguagewhatyouwriteiswhatyouget.Therearealsolanguagesthatareinterpretedlanguages.Theselanguagesrequirethattheuserrunaprogramcalledaninterpreterthatinturnrunsthegivenprogram.Theseareusuallyslowerthancompiledprograms,sincetheinterpreterhastoreadandinterpretthecodeasitgoesalong.However,inwell-madeinterpreters,thistimecanbefairlynegligible.Thereisalsoaclassofhybridlanguageswhichpartiallycompileaprogrambeforeexecutionintobyte-codes.Thisisdonebecausetheinterpretercanreadthebyte-codesmuchfasterthanitcanreadtheregularlanguage.Therearemanyreasonstochooseoneortheother.Compiledprogramsarenice,becauseyoudon'thavetoalreadyhaveaninterpreterinstalledintheuser'smachine.Youhavetohaveacompilerforthelanguage,buttheusersofyourprogramdon't.Inaninterpretedlanguage,youhavetobesurethattheuserhasaninterpreterinstalledforyourprogram,andthatthecomputerknowswhichinterpretertorunyourprogramwith.However,interpetedlanguagestendtobemoreexible,whilecompiledlanguagesaremorerigid.Languagechoiceisusuallydrivenbyavailabletoolsandsupportforprogrammingmethodsratherthanbywhetheralanguageiscompiledorinterpretted.Infactmanylanguageshaveoptionsforeitherone.High-levellanguages,whethercompiledorinterpreted,areorientedaroundyou,theprogrammer,insteadofaroundthemachine.Thisopensthemuptoawidevarietyoffeatures,whichcanincludethefollowing: Beingabletogroupmultipleoperationsintoasingleexpression Beingabletouse"bigvalues"-valuesthataremuchmoreconceptualthanthe4-bytewordsthatcomputersnormallydealwithforexample,beingabletoviewtextstringsasasinglevalueratherthanasastringofbytes. 214 PAGE 221 Chapter11.High-LevelLanguages Havingaccesstobetterowcontrolconstructsthanjustjumps. Havingacompilertochecktypesofvalueassignmentsandotherassertions. Havingmemoryhandledautomatically. Beingabletoworkinalanguagethatresemblestheproblemdomainratherthanthecomputerhardware.Sowhydoesonechooseonelanguageoveranother?Forexample,manychoosePerlbecauseithasavastlibraryoffunctionsforhandlingjustabouteveryprotocolortypeofdataontheplanet.Python,however,hasacleanersyntaxandoftenlendsitselftomorestraightforwardsolutions.It'scross-platformGUItoolsarealsoexcellent.PHPmakeswritingwebapplicationssimple.CommonLISPhasmorepowerandfeaturesthananyotherenvironmentforthosewillingtolearnit.Schemeisthemodelofsimplicityandpowercombinedtogether.Ciseasytointerfacewithotherlanguages.Eachlanguageisdifferent,andthemorelanguagesyouknowthebetterprogrammeryouwillbe.Knowingtheconceptsofdifferentlanguageswillhelpyouinallprogramming,becauseyoucanmatchtheprogramminglanguagetotheproblembetter,andyouhavealargersetoftoolstoworkwith.Evenifcertainfeaturesaren'tdirectlysupportedinthelanguageyouareusing,oftentheycanbesimulated.However,ifyoudon'thaveabroadexperiencewithlanguages,youwon'tknowofallthepossibilitiesyouhavetochoosefrom.YourFirstCProgramHereisyourrstCprogram,whichprints"Helloworld"tothescreenandexits.Typeitin,andgiveitthenameHello-World.c#include PAGE 222 Chapter11.High-LevelLanguages /*"HelloWorld!"tothescreenand*//*exit.*//*MainProgram*/intmainintargc,char**argv{/*Printourstringtostandardoutput*/puts"HelloWorld!n";/*Exitwithstatus0*/return0;}Asyoucansee,it'saprettysimpleprogram.Tocompileit,runthecommandgcc-oHelloWorldHello-World.cToruntheprogram,do./HelloWorldLet'slookathowthisprogramwasputtogether.CommentsinCarestartedwith/*andendedwith*/.Commentscanspanmultiplelines,butmanypeopleprefertostartandendcommentsonthesamelinesotheydon'tgetconfused.#include PAGE 223 Chapter11.High-LevelLanguages Now,everythinginstdio.hisnowinyourprogramjustasifyoutypeditthereyourself.Theanglebracketsaroundthelenametellthecompilertolookinit'sstandardpathsforthele/usr/includeand/usr/local/include,usually.Ifitwasinquotes,like#include"stdio.h"itwouldlookinthecurrentdirectoryforthele.Anyway,stdio.hcontainsthedeclarationsforthestandardinputandoutputfunctionsandvariables.Thesedeclarationstellthecompilerwhatfunctionsareavailableforinputandoutput.Thenextfewlinesaresimplycommentsabouttheprogram.Thenthereisthelineintmainintargc,char**argv.Thisisthestartofafunction.CFunctionsaredeclaredwiththeirname,argumentsandreturntype.Thisdeclarationsaysthatthefunction'snameismain,itreturnsanintinteger-4byteslongonthex86platform,andhastwoarguments-anintcalledargcandachar**calledargv.Youdon'thavetoworryaboutwheretheargumentsarepositionedonthestack-theCcompilertakescareofthatforyou.Youalsodon'thavetoworryaboutloadingvaluesintoandoutofregistersbecausethecompilertakescareofthat,too.ThemainfunctionisaspecialfunctionintheClanguage-itisthestartofallCprogramsmuchlike_startinourassembly-languageprograms.Italwaystakestwoparameters.Therstparameteristhenumberofargumentsgiventothiscommand,andthesecondparameterisalistoftheargumentsthatweregiven.Thenextlineisafunctioncall.Inassemblylanguage,youhadtopushtheargumentsofafunctionontothestack,andthencallthefunction.Ctakescareofthiscomplexityforyou.Yousimplyhavetocallthefunctionwiththeparametersinparenthesis.Inthiscase,wecallthefunctionputs,withasingleparameter.Thisparameteristhecharacterstringwewanttoprint.Wejusthavetotypeinthestringinquotations,andthecompilertakescareofdeningstorageandmovingthepointerstothatstorageontothestackbeforecallingthefunction.Asyoucansee,it'salotlesswork.Finallyourfunctionreturnsthenumber0.Inassemblylanguage,westoredourreturnvaluein%eax,butinCwejustusethereturncommandandittakescare 217 PAGE 224 Chapter11.High-LevelLanguages ofthatforus.Thereturnvalueofthemainfunctioniswhatisusedastheexitcodefortheprogram.Asyoucansee,usinghigh-levellanguagesmakeslifemucheasier.Italsoallowsourprogramstorunonmultipleplatformsmoreeasily.Inassemblylanguage,yourprogramistiedtoboththeoperatingsystemandthehardwareplatform,whileincompiledandinterpretedlanguagesthesamecodecanusuallyrunonmultipleoperatingsystemsandhardwareplatforms.Forexample,thisprogramcanbebuiltandexecutedonx86hardwarerunningLinux,Windows,UNIX,ormostotheroperatingsystems.Inaddition,itcanalsorunonMacintoshhardwarerunninganumberofoperatingsystems.AdditionalinformationontheCprogramminglanguagecanbefoundin AppendixE .PerlPerlisaninterpretedlanguage,existingmostlyonLinuxandUNIX-basedplatforms.Itactuallyrunsonalmostallplatforms,butyounditmostoftenonLinuxandUNIX-basedones.Anyway,hereisthePerlversionoftheprogram,whichshouldbetypedintoalenamedHello-World.pl:#!/usr/bin/perlprint"Helloworld!n";SincePerlisinterpreted,youdon'tneedtocompileorlinkit.Justruninwiththefollowingcommand:perlHello-World.plAsyoucansee,thePerlversionisevenshorterthantheCversion.WithPerlyoudon'thavetodeclareanyfunctionsorprogramentrypoints.Youcanjuststart 218 PAGE 225 Chapter11.High-LevelLanguages typingcommandsandtheinterpreterwillrunthemasitcomestothem.Infactthisprogramonlyhastwolinesofcode,oneofwhichisoptional.Therst,optionallineisusedforUNIXmachinestotellwhichinterpretertousetoruntheprogram.The#!tellsthecomputerthatthisisaninterpretedprogram,andthe/usr/bin/perltellsthecomputertousetheprogram/usr/bin/perltointerprettheprogram.However,sincewerantheprogrambytypinginperlHello-World.pl,wehadalreadyspeciedthatwewereusingtheperlinterpreter.ThenextlinecallsaPerlbuiltinfunction,print.Thishasoneparameter,thestringtoprint.Theprogramdoesn'thaveanexplicitreturnstatement-itknowstoreturnsimplybecauseitrunsofftheendofthele.Italsoknowstoreturn0becausetherewerenoerrorswhileitran.Youcanseethatinterpretedlanguagesareoftenfocusedonlettingyougetworkingcodeasquicklyaspossible,withouthavingtodoalotofextralegwork.OnethingaboutPerlthatisn'tsoevidentfromthisexampleisthatPerltreatsstringsasasinglevalue.Inassemblylanguage,wehadtoprogramaccordingtothecomputer'smemoryarchitecture,whichmeantthatstringshadtobetreatedasasequenceofmultiplevalues,withapointertotherstletter.Perlpretendsthatstringscanbestoreddirectlyasvalues,andthushidesthecomplicationofmanipulatingthemforyou.Infact,oneofPerl'smainstrengthsisit'sabilityandspeedatmanipulatingtext.PythonThePythonversionoftheprogramlooksalmostexactlylikethePerlone.However,PythonisreallyaverydifferentlanguagethanPerl,evenifitdoesn'tseemsofromthistrivialexample.TypetheprogramintoalenamedHello-World.py.Theprogramfollows:#!/usr/bin/python 219 PAGE 226 Chapter11.High-LevelLanguages print"HelloWorld"Youshouldbeabletotellwhatthedifferentlinesoftheprogramdo.ReviewKnowtheConcepts Whatisthedifferencebetweenaninteprettedlanguageandacompiledlanguage? Whatreasonsmightcauseyoutoneedtolearnanewprogramminglanguage?UsetheConcepts Learnthebasicsyntaxofanewprogramminglanguage.Re-codeoneoftheprogramsinthisbookinthatlanguage. Intheprogramyouwroteinthequestionabove,whatspecicthingswereautomatedintheprogramminglanguageyouchose? Modifyyourprogramsothatitruns10,000timesinarow,bothinassemblylanguageandinyournewlanguage.Thenrunthetimecommandtoseewhichisfaster.Whichdoescomeoutahead?Whydoyouthinkthatis? Howdoestheprogramminglanguage'sinput/outputmethodsdifferfromthatoftheLinuxsystemcalls? 220 PAGE 227 Chapter11.High-LevelLanguages GoingFurther HavingseenlanguageswhichhavesuchbrevityasPerl,whydoyouthinkthisbookstartedyouwithalanguageasverboseasassemblylanguage? Howdoyouthinkhighlevellanguageshaveaffectedtheprocessofprogramming? Whydoyouthinksomanylanguagesexist? Learntwonewhighlevellanguages.Howdotheydifferfromeachother?Howaretheysimilar?Whatapproachtoproblem-solvingdoeseachtake? 221 PAGE 228 Chapter11.High-LevelLanguages 222 PAGE 229 Chapter12.OptimizationOptimizationistheprocessofmakingyourapplicationrunmoreeffectively.Youcanoptimizeformanythings-speed,memoryspaceusage,diskspaceusage,etc.Thischapter,however,focusesonspeedoptimization.WhentoOptimizeItisbettertonotoptimizeatallthantooptimizetoosoon.Whenyouoptimize,yourcodegenerallybecomeslessclear,becauseitbecomesmorecomplex.Readersofyourcodewillhavemoretroublediscoveringwhyyoudidwhatyoudidwhichwillincreasethecostofmaintenanceofyourproject.Evenwhenyouknowhowandwhyyourprogramrunsthewayitdoes,optimizedcodeishardertodebugandextend.Itslowsthedevelopmentprocessdownconsiderably,bothbecauseofthetimeittakestooptimizethecode,andthetimeittakestomodifyyouroptimizedcode.Compoundingthisproblemisthatyoudon'tevenknowbeforehandwherethespeedissuesinyourprogramwillbe.Evenexperiencedprogrammershavetroublepredictingwhichpartsoftheprogramwillbethebottleneckswhichneedoptimization,soyouwillprobablyendupwastingyourtimeoptimizingthewrongparts. theSectioncalledWheretoOptimize willdiscusshowtondthepartsofyourprogramthatneedoptimization.Whileyoudevelopyourprogram,youneedtohavethefollowingpriorities: Everythingisdocumented Everythingworksasdocumented Thecodeiswritteninanmodular,easilymodiableformDocumentationisessential,especiallywhenworkingingroups.Theproperfunctioningoftheprogramisessential.You'llnoticeapplicationspeedwasnot 223 PAGE 230 Chapter12.Optimization anywhereonthatlist.Optimizationisnotnecessaryduringearlydevelopmentforthefollowingreasons: Minorspeedproblemscanbeusuallysolvedthroughhardware,whichisoftenmuchcheaperthanaprogrammer'stime. Yourapplicationwillchangedramaticallyasyoureviseit,thereforewastingmostofyoureffortstooptimizeit.1 Speedproblemsareusuallylocalizedinafewplacesinyourcode-ndingtheseisdifcultbeforeyouhavemostoftheprogramnished.Therefore,thetimetooptimizeistowardtheendofdevelopment,whenyouhavedeterminedthatyourcorrectcodeactuallyhasperformanceproblems.Inaweb-basede-commerceprojectIwasinvolvedin,Ifocusedentirelyoncorrectness.Thiswasmuchtothedismayofmycolleagues,whowereworriedaboutthefactthateachpagetooktwelvesecondstoprocessbeforeiteverstartedloadingmostwebpagesprocessinunderasecond.However,Iwasdeterminedtomakeittherightwayrst,andputoptimizationasalastpriority.Whenthecodewasnallycorrectafter3monthsofwork,ittookonlythreedaystondandeliminatethebottlenecks,bringingtheaverageprocessingtimeunderaquarterofasecond.Byfocusingonthecorrectorder,Iwasabletonishaprojectthatwasbothcorrectandefcient.WheretoOptimizeOnceyouhavedeterminedthatyouhaveaperformanceissueyouneedtodeterminewhereinthecodetheproblemsoccur.Youcandothisbyrunningaproler.Aprolerisaprogramthatwillletyourunyourprogram,anditwilltellyouhowmuchtimeisspentineachfunction,andhowmanytimestheyarerun. 1. Manynewprojectsoftenhavearstcodebasewhichiscompletelyrewrittenasdevel-operslearnmoreabouttheproblemtheyaretryingtosolve.Anyoptimizationdoneontherstcodebaseiscompletelywasted. 224 PAGE 231 Chapter12.Optimization gprofisthestandardGNU/Linuxprolingtool,butadiscussionofusingprolersisoutsidethescopeofthistext.Afterrunningaproler,youcandeterminewhichfunctionsarecalledthemostorhavethemosttimespentinthem.Thesearetheonesyoushouldfocusyouroptimizationeffortson.Ifaprogramonlyspends1%ofitstimeinagivenfunction,thennomatterhowmuchyouspeeditupyouwillonlyachieveamaximumofa1%overallspeedimprovement.However,ifaprogramspends20%ofitstimeinagivenfunction,thenevenminorimprovementstothatfunctionsspeedwillbenoticeable.Therefore,prolinggivesyoutheinformationyouneedtomakegoodchoicesaboutwheretospendyourprogrammingtime.Inordertooptimizefunctions,youneedtounderstandinwhatwaystheyarebeingcalledandused.Themoreyouknowabouthowandwhenafunctioniscalled,thebetterpositionyouwillbeintooptimizeitappropriately.Therearetwomaincategoriesofoptimization-localoptimizationsandglobaloptimizations.Localoptimizationsconsistofoptimizationsthatareeitherhardwarespecic-suchasthefastestwaytoperformagivencomputation-orprogram-specic-suchasmakingaspecicpieceofcodeperformthebestforthemostoften-occuringcase.Globaloptimizationconsistofoptimizationswhicharestructural.Forexample,ifyouweretryingtondthebestwayforthreepeopleindifferentcitiestomeetinSt.Louis,alocaloptimizationwouldbendingabetterroadtogetthere,whileaglobaloptimizationwouldbetodecidetoteleconferenceinsteadofmeetinginperson.Globaloptimizationofteninvolvesrestructuringcodetoavoidperformanceproblems,ratherthantryingtondthebestwaythroughthem.LocalOptimizationsThefollowingaresomewell-knownmethodsofoptimizingpiecesofcode.Whenusinghighlevellanguages,someofthesemaybedoneautomaticallybyyourcompiler'soptimizer. 225 PAGE 232 Chapter12.Optimization PrecomputingCalculations Sometimesafunctionhasalimittednumberofpossibleinputsandoutputs.Infact,itmaybesofewthatyoucanactuallyprecomputeallofthepossibleanswersbeforehand,andsimplylookuptheanswerwhenthefunctioniscalled.Thistakesupsomespacesinceyouhavetostorealloftheanswers,butforsmallsetsofdatathisworksoutreallywell,especiallyifthecomputationnormallytakesalongtime.RememberingCalculationResults Thisissimilartothepreviousmethod,butinsteadofcomputingresultsbeforehand,theresultofeachcalculationrequestedisstored.Thiswaywhenthefunctionstarts,iftheresulthasbeencomputedbeforeitwillsimplyreturnthepreviousanswer,otherwiseitwilldothefullcomputationandstoretheresultforlaterlookup.Thishastheadvantageofrequiringlessstoragespacebecauseyouaren'tprecomputingallresults.Thisissometimestermedcachingormemoizing.LocalityofReference Localityofreferenceisatermforwhereinmemorythedataitemsyouareaccessingare.Withvirtualmemory,youmayaccesspagesofmemorywhicharestoredondisk.Insuchacase,theoperatingsystemhastoloadthatmemorypagefromdisk,andunloadotherstodisk.Let'ssay,forinstance,thattheoperatingsystemwillallowyoutohave20kofmemoryinphysicalmemoryandforcestherestofittobeondisk,andyourapplicationuses60kofmemory.Let'ssayyourprogramhastodo5operationsoneachpieceofdata.Ifitdoesoneoperationoneverypieceofdata,andthengoesthroughanddoesthenextoperationoneachpieceofdata,eventuallyeverypageofdatawillbeloadedandunloadedfromthedisk5times.Instead,ifyoudidall5operationsonagivendataitem,youonlyhavetoloadeachpagefromdiskonce.Whenyoubundleasmanyoperationsondatathatisphysicallyclosetoeachotherinmemory,thenyouaretakingadvantageoflocalityofreference. 226 PAGE 233 Chapter12.Optimization Inaddition,processorsusuallystoresomedataon-chipinacache.Ifyoukeepallofyouroperationswithinasmallareaofphysicalmemory,yourprogrammaybypassevenmainmemoryandonlyusethechip'sultra-fastcachememory.Thisisalldoneforyou-allyouhavetodoistotrytooperateonsmallsectionsofmemoryatatime,ratherthanbouncingallovertheplace.RegisterUsage Registersarethefastestmemorylocationsonthecomputer.Whenyouaccessmemory,theprocessorhastowaitwhileitisloadedfromthememorybus.However,registersarelocatedontheprocessoritself,soaccessisextremelyfast.Thereforemakingwiseusageofregistersisextremelyimportant.Ifyouhavefewenoughdataitemsyouareworkingwith,trytostorethemallinregisters.Inhighlevellanguages,youdonotalwayshavethisoption-thecompilerdecideswhatgoesinregistersandwhatdoesn't.InlineFunctions Functionsaregreatfromthepointofviewofprogrammanagement-theymakeiteasytobreakupyourprogramintoindependent,understandable,andreuseableparts.However,functioncallsdoinvolvetheoverheadofpushingargumentsontothestackanddoingthejumpsrememberlocalityofreference-yourcodemaybeswappedoutondiskinsteadofinmemory.Forhighlevellanguages,it'softenimpossibleforcompilerstodooptimizationsacrossfunction-callboundaries.However,somelanguagessupportinlinefunctionsorfunctionmacros.Thesefunctionslook,smell,taste,andactlikerealfunctions,exceptthecompilerhastheoptiontosimplyplugthecodeinexactlywhereitwascalled.Thismakestheprogramfaster,butitalsoincreasesthesizeofthecode.Therearealsomanyfunctions,likerecursivefunctions,whichcannotbeinlinedbecausetheycallthemselveseitherdirectlyorindirectly. 227 PAGE 234 Chapter12.Optimization OptimizedInstructions Oftentimestherearemultipleassemblylanguageinstructionswhichaccomplishthesamepurpose.Askilledassemblylanguageprogrammerknowswhichinstructionsarethefastest.However,thiscanchangefromprocessortoprocessor.Formoreinformationonthistopic,youneedtoseetheuser'smanualthatisprovidedforthespecicchipyouareusing.Asanexample,let'slookattheprocessofloadingthenumber0intoaregister.Onmostprocessors,doingamovl$0,%eaxisnotthequickestway.Thequickestwayistoexclusive-ortheregisterwithitself,xorl%eax,%eax.Thisisbecauseitonlyhastoaccesstheregister,anddoesn'thavetotransferanydata.Forusersofhigh-levellanguages,thecompilerhandlesthiskindofoptimizationsforyou.Forassembly-languageprogrammers,youneedtoknowyourprocessorwell.AddressingModes Differentaddressingmodesworkatdifferentspeeds.Thefastestaretheimmediateandregisteraddressingmodes.Directisthenextfastest,indirectisnext,andbasepointerandindexedindirectaretheslowest.Trytousethefasteraddressingmodes,whenpossible.Oneinterestingconsequenceofthisisthatwhenyouhaveastructuredpieceofmemorythatyouareaccessingusingbasepointeraddressing,therstelementcanbeaccessedthequickest.Sinceit'soffsetis0,youcanaccessitusingindirectaddressinginsteadofbasepointeraddressing,whichmakesitfaster.DataAlignment Someprocessorscanaccessdataonword-alignedmemoryboundariesi.e.-addressesdivisiblebythewordsizefasterthannon-aligneddata.So,whensettingupstructuresinmemory,itisbesttokeepitword-aligned.Somenon-x86processors,infact,cannotaccessnon-aligneddatainsomemodes.Thesearejustasmatteringofexamplesofthekindsoflocaloptimizationspossible.However,rememberthatthemaintainabilityandreadabilityofcodeis 228 PAGE 235 Chapter12.Optimization muchmoreimportantexceptunderextremecircumstances.GlobalOptimizationGlobaloptimizationhastwogoals.Therstoneistoputyourcodeinaformwhereitiseasytodolocaloptimiztions.Forexample,ifyouhavealargeprocedurethatperformsseveralslow,complexcalculations,youmightseeifyoucanbreakpartsofthatprocedureintotheirownfunctionswherethevaluescanbeprecomputedormemoized.Statelessfunctionsfunctionsthatonlyoperateontheparametersthatwerepassedtothem-i.e.noglobalsorsystemcallsaretheeasiesttypeoffunctionstooptimizeinacomputer.Themorestatelesspartsofyourprogramyouhave,themoreopportunitiesyouhavetooptimize.Inthee-commercesituationIwroteaboutabove,thecomputerhadtondalloftheassociatedpartsforspecicinventoryitems.Thisrequiredabout12databasecalls,andintheworstcasetookabout20seconds.However,thegoalofthisprogramwastobeinteractive,andalongwaitwoulddestroythatgoal.However,Iknewthattheseinventorycongurationsdonotchange.Therefore,Iconvertedthedatabasecallsintotheirownfunctions,whichwerestateless.Iwasthenabletomemoizethefunctions.Atthebeginningofeachday,thefunctionresultswereclearedincaseanyonehadchangedthem,andseveralinventoryitemswereautomaticallypreloaded.Fromthenonduringtheday,thersttimesomeoneaccessedaninventoryitem,itwouldtakethe20secondsitdidbeforehand,butafterwardsitwouldtakelessthanasecond,becausethedatabaseresultshadbeenmemoized.Globaloptimizationusuallyofteninvolvesachievingthefollowingpropertiesinyourfunctions:Parallelization Parallelizationmeansthatyouralgorithmcaneffectivelybesplitamongmultipleprocesses.Forexample,pregnancyisnotveryparallelizablebecause 229 PAGE 236 Chapter12.Optimization nomatterhowmanywomenyouhave,itstilltakesninemonths.However,buildingacarisparallelizablebecauseyoucanhaveoneworkerworkingontheenginewhileanotheroneisworkingontheinterior.Usually,applicationshavealimittohowparallelizabletheyare.Themoreparallelizableyourapplicationis,thebetteritcantakeadvantageofmultiprocessorandclusteredcomputercongurations.Statelessness Aswe'vediscussed,statelessfunctionsandprogramsarethosethatrelyentirelyonthedataexplicitlypassedtothemforfunctioning.Mostprocessesarenotentirelystateless,buttheycanbewithinlimits.Inmye-commerceexample,thefunctionwasn'tentirelystateless,butitwaswithintheconnesofasingleday.Therefore,Ioptimizeditasifitwereastatelessfunction,butmadeallowancesforchangesatnight.Twogreatbenetsresultingfromstatelessnessisthatmoststatelessfunctionsareparallelizableandoftenbenetfrommemoization.Globaloptimizationtakesquiteabitofpracticetoknowwhatworksandwhatdoesn't.Decidinghowtotackleoptimizationproblemsincodeinvolveslookingatalltheissues,andknowingthatxingsomeissuesmaycauseothers.ReviewKnowtheConcepts Atwhatlevelofimportanceisoptimizationcomparedtotheotherprioritiesinprogramming? Whatisthedifferencebetweenlocalandglobaloptimizations? Namesometypesoflocaloptimizations. 230 PAGE 237 Chapter12.Optimization Howdoyoudeterminewhatpartsofyourprogramneedoptimization? Atwhatlevelofimportanceisoptimizationcomparedtotheotherprioritiesinprogramming?WhydoyouthinkIrepeatedthatquestion?UsetheConcepts Gobackthrougheachprograminthisbookandtrytomakeoptimizationsaccordingtotheproceduresoutlinedinthischapter Pickaprogramfromthepreviousexerciseandtrytocalculatetheperformanceimpactonyourcodeunderspecicinputs.2GoingFurther Findanopen-sourceprogramthatyoundparticularlyfast.Contactoneofthedevelopersandaskaboutwhatkindsofoptimizationstheyperformedtoimprovethespeed. Findanopen-sourceprogramthatyoundparticularlyslow,andtrytoimaginethereasonsfortheslowness.Then,downloadthecodeandtrytoproleitusinggproforsimilartool.Findwherethecodeisspendingthemajorityofthetimeandtrytooptimizeit.Wasthereasonfortheslownessdifferentthanyouimagined? Hasthecompilereliminatedtheneedforlocaloptimizations?Whyorwhynot? Whatkindofproblemsmightacompilerrunintoifittriedtooptimizecodeacrossfunctioncallboundaries? 2. Sincetheseprogramsareusuallyshortenoughnottohavenoticeableperformanceprob-lems,loopingthroughtheprogramthousandsoftimeswillexaggeratethetimeittakestorunenoughtomakecalculations. 231 PAGE 238 Chapter12.Optimization 232 PAGE 239 Chapter13.MovingOnfromHereCongratulationsongettingthisfar.Youshouldnowhaveabasisforunderstandingtheissuesinvolvedinmanyareasofprogramming.Evenifyouneveruseassemblylanguageagain,youhavegainedavaluableperspectiveandmentalframeworkforunderstandingtherestofcomputerscience.Thereareessentiallythreemethodstolearntoprogram: FromtheBottomUp-Thisishowthisbookteaches.Itstartswithlow-levelprogramming,andworkstowardmoregeneralizedteaching. FromtheTopDown-Thisistheoppositedirection.Thisfocusesonwhatyouwanttodowiththecomputer,andteachesyouhowtobreakitdownmoreandmoreuntilyougettothelowlevels. FromtheMiddle-ThisischaracterizedbybookswhichteachaspecicprogramminglanguageorAPI.Thesearenotasconcernedwithconceptsastheyarewithspecics.Differentpeoplelikedifferentapproaches,butagoodprogrammertakesallofthemintoaccount.Thebottom-upapproacheshelpyouunderstandthemachineaspects,thetop-downapproacheshelpyouunderstandtheproblem-areaaspects,andthemiddleapproacheshelpyouwithpracticalquestionsandanswers.Toleaveanyoftheseaspectsoutwouldbeamistake.ComputerProgrammingisavastsubject.Asaprogrammer,youwillneedtobepreparedtobeconstantlylearningandpushingyourlimits.Thesebookswillhelpyoudothat.Theynotonlyteachtheirsubjects,butalsoteachvariouswaysandmethodsofthinking.AsAlanPerlissaid,"Alanguagethatdoesn'taffectthewayyouthinkaboutprogrammingisnotworthknowing"http://www.cs.yale.edu/homes/perlis-alan/quotes.html.Ifyouareconstantlylookingfornewandbetterwaysofdoingandthinking,youwillmakeasuccessfulprogrammer.Ifyoudonotseektoenhanceyourself,"Alittlesleep,alittleslumber,alittlefoldingofthehandstorest-andpovertywillcomeonyoulikea 233 PAGE 240 Chapter13.MovingOnfromHere banditandscarcitylikeanarmedman."Proverbs24:33-34NIV.Perhapsnotquitethatsevere,butstill,it'sbesttoalwaysbelearning.Thesebookswereselectedbecauseoftheircontentandtheamountofrespecttheyhaveinthecomputerscienceworld.Eachofthembringssomethingunique.Therearemanybookshere.Thebestwaytostartwouldbetolookthroughonlinereviewsofseveralofthebooks,andndastartingpointthatinterestsyou.FromtheBottomUpThislistisinthebestreadingorderIcouldnd.It'snotnecessarilyeasiesttohardest,butbasedonsubjectmatter. ProgrammingfromtheGroundUpbyJonathanBartlett IntroductiontoAlgorithmsbyThomasH.Cormen,CharlesE.Leiserson,andRonaldL.Rivest TheArtofComputerProgrammingbyDonaldKnuthvolumeset-volume1isthemostimportant ProgrammingLanguagesbySamuelN.Kamin ModernOperatingSystemsbyAndrewTanenbaum LinkersandLoadersbyJohnLevine ComputerOrganizationandDesign:TheHardware/SoftwareInterfacebyDavidPattersonandJohnHennessyFromtheTopDownThesebooksarearrangedfromthesimplesttothehardest.However,theycanbereadinanyorderyoufeelcomfortablewith. 234 PAGE 241 Chapter13.MovingOnfromHere HowtoDesignProgramsbyMatthiasFelleisen,RobertBruceFindler,MatthewFlatt,andShiramKrishnamurthi,availableonlineathttp://www.htdp.org/ SimplyScheme:AnIntroductiontoComputerSciencebyBrianHarveyandMatthewWright HowtoThinkLikeaComputerScientist:LearningwithPythonbyAllenDowney,JeffElkner,andChrisMeyers,availableonlineathttp://www.greenteapress.com/thinkpython/ StructureandInterpretationofComputerProgramsbyHaroldAbelsonandGeraldJaySussmanwithJulieSussman,availableonlineathttp://mitpress.mit.edu/sicp/ DesignPatternsbyErichGamma,RichardHelm,RalphJohnson,andJohnVlissides WhatnotHow:TheRulesApproachtoApplicationDevelopmentbyChrisDate TheAlgorithmDesignManualbySteveSkiena ProgrammingLanguagePragmaticsbyMichaelScott EssentialsofProgrammingLanguagesbyDanielP.Friedman,MitchellWand,andChristopherT.HaynesFromtheMiddleOutEachoftheseisthebestbookonitssubject.Ifyouneedtoknowtheselanguages,thesewilltellyouallyouneedtoknow. ProgrammingPerlbyLarryWall,TomChristiansen,andJonOrwant CommonLISP:TheLanguagebyGuyR.Steele ANSICommonLISPbyPaulGraham TheCProgrammingLanguagebyBrianW.KernighanandDennisM.Ritchie 235 PAGE 242 Chapter13.MovingOnfromHere TheWaiteGroup'sCPrimerPlusbyStephenPrata TheC++ProgrammingLanguagebyBjarneStroustrup ThinkinginJavabyBruceEckel,availableonlineathttp://www.mindview.net/Books/TIJ/ TheSchemeProgrammingLanguagebyKentDybvig LinuxAssemblyLanguageProgrammingbyBobNevelnSpecializedTopicsThesebooksarethebestbooksthatcovertheirtopic.Theyarethoroughandauthoritative.Togetabroadbaseofknowledge,youshouldreadseveraloutsideoftheareasyounormallyprogramin. PracticalProgramming-ProgrammingPearlsandMoreProgrammingPearlsbyJonLouisBentley Databases-UnderstandingRelationalDatabasesbyFabianPascal ProjectManagement-TheMythicalMan-MonthbyFredP.Brooks UNIXProgramming-TheArtofUNIXProgrammingbyEricS.Raymond,availableonlineathttp://www.catb.org/~esr/writings/taoup/ UNIXProgramming-AdvancedProgrammingintheUNIXEnvironmentbyW.RichardStevens NetworkProgramming-UNIXNetworkProgrammingvolumesbyW.RichardStevens GenericProgramming-ModernC++DesignbyAndreiAlexandrescu Compilers-TheArtofCompilerDesign:TheoryandPracticebyThomasPittmanandJamesPeters 236 PAGE 243 Chapter13.MovingOnfromHere Compilers-AdvancedCompilerDesignandImplementationbyStevenMuchnick DevelopmentProcess-Refactoring:ImprovingtheDesignofExistingCodebyMartinFowler,KentBeck,JohnBrant,WilliamOpdyke,andDonRoberts Typesetting-ComputersandTypesettingvolumesbyDonaldKnuth Cryptography-AppliedCryptographybyBruceSchneier Linux-ProfessionalLinuxProgrammingbyNeilMatthew,RichardStones,and14otherpeople LinuxKernel-LinuxDeviceDriversbyAlessandroRubiniandJonathanCorbet OpenSourceProgramming-TheCathedralandtheBazaar:MusingsonLinuxandOpenSourcebyanAccidentalRevolutionarybyEricS.Raymond ComputerArchitecture-ComputerArchitecture:AQuantitativeApproachbyDavidPattersonandJohnHennessyFurtherResourcesonAssemblyLanguageInassemblylanguage,yourbestresourcesareontheweb. http://www.linuxassembly.org/-agreatresourceforLinuxassemblylanguageprogrammers http://www.sandpile.org/-arepositoryofreferencematerialonx86,x86-64,andcompatibleprocessors http://www.x86.org/-Dr.Dobb'sJournalMicroprocessorResources http://www.drpaulcarter.com/pcasm/-Dr.PaulCarter'sPCAssemblyLanguagePage http://webster.cs.ucr.edu/-TheArtofAssemblyHomePage 237 PAGE 244 Chapter13.MovingOnfromHere http://www.intel.com/design/pentium/manuals/-Intel'smanualsfortheirprocessors http://www.janw.easynet.be/-JanWagemaker'sLinuxassemblylanguageexamples http://www.azillionmonkeys.com/qed/asm.html-PaulHsieh'sx86AssemblyPage 238 PAGE 245 AppendixA.GUIProgrammingIntroductiontoGUIProgrammingThepurposeofthisappendixisnottoteachyouhowtodoGraphicalUserInterfaces.Itissimplymeanttoshowhowwritinggraphicalapplicationsisthesameaswritingotherapplications,justusinganadditionallibrarytohandlethegraphicalparts.Asaprogrammeryouneedtogetusedtolearningnewlibraries.Mostofyourtimewillbespentpassingdatafromonelibrarytoanother.TheGNOMELibrariesTheGNOMEprojectsisoneofseveralprojectstoprovideacompletedesktoptoLinuxusers.TheGNOMEprojectincludesapaneltoholdapplicationlaunchersandmini-applicationscalledapplets,severalstandardapplicationstodothingssuchaslemanagement,sessionmanagement,andconguration,andanAPIforcreatingapplicationswhichtinwiththewaytherestofthesystemworks.OnethingtonoticeabouttheGNOMElibrariesisthattheyconstantlycreateandgiveyoupointerstolargedatastructures,butyouneverneedtoknowhowtheyarelaidoutinmemory.AllmanipulationoftheGUIdatastructuresaredoneentirelythroughfunctioncalls.Thisisacharacteristicofgoodlibrarydesign.Librarieschangefromversiontoversion,andsodoesthedatathateachdatastructureholds.Ifyouhadtoaccessandmanipulatethatdatayourself,thenwhenthelibraryisupdatedyouwouldhavetomodifyyourprogramstoworkwiththenewlibrary,oratleastrecompilethem.Whenyouaccessthedatathroughfunctions,thefunctionstakecareofknowingwhereinthestructureeachpieceofdatais.Thepointersyoureceivefromthelibraryareopaque-youdon'tneedtoknowspecicallywhatthestructuretheyarepointingtolookslike,youonlyneedtoknowthefunctionsthatwillproperlymanipulateit.Whendesigninglibraries,evenforusewithinonlyoneprogram,thisisagoodpracticetokeepinmind. 239 PAGE 246 AppendixA.GUIProgramming ThischapterwillnotgointodetailsabouthowGNOMEworks.Ifyouwouldliketoknowmore,visittheGNOMEdeveloperwebsiteathttp://developer.gnome.org/.Thissitecontainstutorials,mailinglists,APIdocumentation,andeverythingelseyouneedtostartprogrammingintheGNOMEenvironment.ASimpleGNOMEPrograminSeveralLanguagesThisprogramwillsimplyshowaWindowthathasabuttontoquittheapplication.Whenthatbuttonisclickeditwillaskyouifyouaresure,andifyouclickyesitwillclosetheapplication.Torunthisprogram,typeinthefollowingasgnome-example.s:#PURPOSE:Thisprogramismeanttobeanexample#ofwhatGUIprogramslooklikewritten#withtheGNOMElibraries##INPUT:Theusercanonlyclickonthe"Quit"#buttonorclosethewindow##OUTPUT:Theapplicationwillclose##PROCESS:Iftheuserclicksonthe"Quit"button,#theprogramwilldisplayadialogasking#iftheyaresure.IftheyclickYes,it#willclosetheapplication.Otherwise#itwillcontinuerunning#.section.data###GNOMEdefinitions-ThesewerefoundintheGNOME#headerfilesfortheClanguage#andconvertedintotheirassembly 240 PAGE 247 AppendixA.GUIProgramming #equivalents#GNOMEButtonNamesGNOME_STOCK_BUTTON_YES:.ascii"Button_Yes"GNOME_STOCK_BUTTON_NO:.ascii"Button_No"#GnomeMessageBoxTypesGNOME_MESSAGE_BOX_QUESTION:.ascii"question"#StandarddefinitionofNULL.equNULL,0#GNOMEsignaldefinitionssignal_destroy:.ascii"destroy"signal_delete_event:.ascii"delete_event"signal_clicked:.ascii"clicked"###Application-specificdefinitions#Applicationinformationapp_id:.ascii"gnome-example"app_version:.ascii"1.000"app_title:.ascii"GnomeExampleProgram"#TextforButtonsandwindowsbutton_quit_text: 241 PAGE 248 AppendixA.GUIProgramming .ascii"IWanttoQuittheGNOMEExampleProgram"quit_question:.ascii"Areyousureyouwanttoquit?".section.bss#Variablestosavethecreatedwidgetsin.equWORD_SIZE,4.lcommappPtr,WORD_SIZE.lcommbtnQuit,WORD_SIZE.section.text.globlmain.typemain,@functionmain:pushl%ebpmovl%esp,%ebp#InitializeGNOMElibrariespushl12%ebp#argvpushl8%ebp#argcpushl$app_versionpushl$app_idcallgnome_initaddl$16,%esp#recoverthestack#Createnewapplicationwindowpushl$app_title#Windowtitlepushl$app_id#ApplicationIDcallgnome_app_newaddl$8,%esp#recoverthestackmovl%eax,appPtr#savethewindowpointer 242 PAGE 249 AppendixA.GUIProgramming #Createnewbuttonpushl$button_quit_text#buttontextcallgtk_button_new_with_labeladdl$4,%esp#recoverthestackmovl%eax,btnQuit#savethebuttonpointer#MakethebuttonshowupinsidetheapplicationwindowpushlbtnQuitpushlappPtrcallgnome_app_set_contentsaddl$8,%esp#Makesthebuttonshowuponlyafterit'swindow#showsup,thoughpushlbtnQuitcallgtk_widget_showaddl$4,%esp#MakestheapplicationwindowshowuppushlappPtrcallgtk_widget_showaddl$4,%esp#HaveGNOMEcallourdelete_handlerfunction#whenevera"delete"eventoccurspushl$NULL#extradatatopasstoour#functionwedon'tuseanypushl$delete_handler#functionaddresstocallpushl$signal_delete_event#nameofthesignalpushlappPtr#widgettolistenforeventsoncallgtk_signal_connectaddl$16,%esp#recoverstack#HaveGNOMEcallourdestroy_handlerfunction#whenevera"destroy"eventoccurs 243 PAGE 250 AppendixA.GUIProgramming pushl$NULL#extradatatopasstoour#functionwedon'tuseanypushl$destroy_handler#functionaddresstocallpushl$signal_destroy#nameofthesignalpushlappPtr#widgettolistenforeventsoncallgtk_signal_connectaddl$16,%esp#recoverstack#HaveGNOMEcallourclick_handlerfunction#whenevera"click"eventoccurs.Notethat#theprevioussignalswerelisteningonthe#applicationwindow,whilethisoneisonly#listeningonthebuttonpushl$NULLpushl$click_handlerpushl$signal_clickedpushlbtnQuitcallgtk_signal_connectaddl$16,%esp#TransfercontroltoGNOME.Everythingthat#happensfromhereoutisinreactiontouser#events,whichcallsignalhandlers.Thismain#functionjustsetsupthemainwindowandconnects#signalhandlers,andthesignalhandlerstake#careoftherestcallgtk_main#Aftertheprogramisfinished,leavemovl$0,%eaxleaveret#A"destroy"eventhappenswhenthewidgetisbeing#removed.Inthiscase,whentheapplicationwindow 244 PAGE 251 AppendixA.GUIProgramming #isbeingremoved,wesimplywanttheeventloopto#quitdestroy_handler:pushl%ebpmovl%esp,%ebp#Thiscausesgtktoexitit'seventloop#assoonasitcan.callgtk_main_quitmovl$0,%eaxleaveret#A"delete"eventhappenswhentheapplicationwindow#getsclickedinthe"x"thatyounormallyuseto#closeawindowdelete_handler:movl$1,%eaxret#A"click"eventhappenswhenthewidgetgetsclickedclick_handler:pushl%ebpmovl%esp,%ebp#Createthe"Areyousure"dialogpushl$NULL#Endofbuttonspushl$GNOME_STOCK_BUTTON_NO#Button1pushl$GNOME_STOCK_BUTTON_YES#Button0pushl$GNOME_MESSAGE_BOX_QUESTION#Dialogtypepushl$quit_question#Dialogmesasgecallgnome_message_box_newaddl$16,%esp#recoverstack 245 PAGE 252 AppendixA.GUIProgramming #%eaxnowholdsthepointertothedialogwindow#SettingModalto1preventsanyotheruser#interactionwhilethedialogisbeingshownpushl$1pushl%eaxcallgtk_window_set_modalpopl%eaxaddl$4,%esp#Nowweshowthedialogpushl%eaxcallgtk_widget_showpopl%eax#Thissetsupallthenecessarysignalhandlers#inordertojustshowthedialog,closeitwhen#oneofthebuttonsisclicked,andreturnthe#numberofthebuttonthattheuserclickedon.#Thebuttonnumberisbasedontheorderthebuttons#werepushedoninthegnome_message_box_newfunctionpushl%eaxcallgnome_dialog_run_and_closeaddl$4,%esp#Button0istheYesbutton.Ifthisisthe#buttontheyclickedon,tellGNOMEtoquit#it'seventloop.Otherwise,donothingcmpl$0,%eaxjneclick_handler_endcallgtk_main_quitclick_handler_end:leave 246 PAGE 253 AppendixA.GUIProgramming retTobuildthisapplication,executethefollowingcommands:asgnome-example.s-ognome-example.ogccgnome-example.o`gnome-config--libsgnomeui`-ognome-exampleThentypein./gnome-exampletorunit.Thisprogram,likemostGUIprograms,makesheavyuseofpassingpointerstofunctionsasparameters.InthisprogramyoucreatewidgetswiththeGNOMEfunctionsandthenyousetupfunctionstobecalledwhencertaineventshappen.Thesefunctionsarecalledcallbackfunctions.Alloftheeventprocessingishandledbythefunctiongtk_main,soyoudon'thavetoworryabouthowtheeventsarebeingprocessed.Allyouhavetodoishavecallbackssetuptowaitforthem.HereisashortdescriptionofalloftheGNOMEfunctionsthatwereusedinthisprogram:gnome_init Takesthecommand-linearguments,argumentcount,applicationid,andapplicationversionandinitializestheGNOMElibraries.gnome_app_new Createsanewapplicationwindow,andreturnsapointertoit.Takestheapplicationidandthewindowtitleasarguments.gtk_button_new_with_label Createsanewbuttonandreturnsapointertoit.Takesoneargument-thetextthatisinthebutton. 247 PAGE 254 AppendixA.GUIProgramming gnome_app_set_contents Thistakesapointertothegnomeapplicationwindowandwhateverwidgetyouwantabuttoninthiscaseandmakesthewidgetbethecontentsoftheapplicationwindowgtk_widget_show Thismustbecalledoneverywidgetcreatedapplicationwindow,buttons,textentryboxes,etcinorderforthemtobevisible.However,inorderforagivenwidgettobevisible,allofit'sparentsmustbevisibleaswell.gtk_signal_connect Thisisthefunctionthatconnectswidgetsandtheirsignalhandlingcallbackfunctions.Thisfunctiontakesthewidgetpointer,thenameofthesignal,thecallbackfunction,andanextradatapointer.Afterthisfunctioniscalled,anytimethegiveneventistriggered,thecallbackwillbecalledwiththewidgetthatproducedthesignalandtheextradatapointer.Inthisapplication,wedon'tusetheextradatapointer,sowejustsetittoNULL,whichis0.gtk_main ThisfunctioncausesGNOMEtoenterintoit'smainloop.Tomakeapplicationprogrammingeasier,GNOMEhandlesthemainloopoftheprogramforus.GNOMEwillcheckforeventsandcalltheappropriatecallbackfunctionswhentheyoccur.Thisfunctionwillcontinuetoprocesseventsuntilgtk_main_quitiscalledbyasignalhandler.gtk_main_quit ThisfunctioncausesGNOMEtoexitit'smainloopattheearliestopportunity.gnome_message_box_new Thisfunctioncreatesadialogwindowcontainingaquestionandresponse 248 PAGE 255 AppendixA.GUIProgramming buttons.Ittakesasparametersthemessagetodisplay,thetypeofmessageitiswarning,question,etc,andalistofbuttonstodisplay.ThenalparametershouldbeNULLtoindicatethattherearenomorebuttonstodisplay.gtk_window_set_modal Thisfunctionmakesthegivenwindowamodalwindow.InGUIprogramming,amodalwindowisonethatpreventseventprocessinginotherwindowsuntilthatwindowisclosed.ThisisoftenusedwithDialogwindows.gnome_dialog_run_and_close Thisfunctiontakesadialogpointerthepointerreturnedbygnome_message_box_newcanbeusedhereandwillsetupalloftheappropriatesignalhandlerssothatitwillrununtilabuttonispressed.Atthattimeitwillclosethedialogandreturntoyouwhichbuttonwaspressed.Thebuttonnumberreferstotheorderinwhichthebuttonsweresetupingnome_message_box_new.ThefollowingisthesameprogramwrittenintheClanguage.Typeitinasgnome-example-c.c:/*PURPOSE:ThisprogramismeanttobeanexampleofwhatGUIprogramslooklikewrittenwiththeGNOMElibraries*/#include PAGE 256 AppendixA.GUIProgramming #defineMY_BUTTON_TEXT"IWanttoQuittheExampleProgram"#defineMY_QUIT_QUESTION"Areyousureyouwanttoquit?"/*Mustdeclarefunctionsbeforetheyareused*/intdestroy_handlergpointerwindow,GdkEventAny*e,gpointerdata;intdelete_handlergpointerwindow,GdkEventAny*e,gpointerdata;intclick_handlergpointerwindow,GdkEventAny*e,gpointerdata;intmainintargc,char**argv{gpointerappPtr;/*applicationwindow*/gpointerbtnQuit;/*quitbutton*//*InitializeGNOMElibraries*/gnome_initMY_APP_ID,MY_APP_VERSION,argc,argv;/*Createnewapplicationwindow*/appPtr=gnome_app_newMY_APP_ID,MY_APP_TITLE;/*Createnewbutton*/btnQuit=gtk_button_new_with_labelMY_BUTTON_TEXT;/*Makethebuttonshowupinsidetheapplicationwindow*/gnome_app_set_contentsappPtr,btnQuit;/*Makesthebuttonshowup*/gtk_widget_showbtnQuit;/*Makestheapplicationwindowshowup*/ 250 PAGE 257 AppendixA.GUIProgramming gtk_widget_showappPtr;/*Connectthesignalhandlers*/gtk_signal_connectappPtr,"delete_event",GTK_SIGNAL_FUNCdelete_handler,NULL;gtk_signal_connectappPtr,"destroy",GTK_SIGNAL_FUNCdestroy_handler,NULL;gtk_signal_connectbtnQuit,"clicked",GTK_SIGNAL_FUNCclick_handler,NULL;/*TransfercontroltoGNOME*/gtk_main;return0;}/*Functiontoreceivethe"destroy"signal*/intdestroy_handlergpointerwindow,GdkEventAny*e,gpointerdata{/*LeaveGNOMEeventloop*/gtk_main_quit;return0;}/*Functiontoreceivethe"delete_event"signal*/intdelete_handlergpointerwindow,GdkEventAny*e,gpointerdata{return0;} 251 PAGE 258 AppendixA.GUIProgramming /*Functiontoreceivethe"clicked"signal*/intclick_handlergpointerwindow,GdkEventAny*e,gpointerdata{gpointermsgbox;intbuttonClicked;/*Createthe"Areyousure"dialog*/msgbox=gnome_message_box_newMY_QUIT_QUESTION,GNOME_MESSAGE_BOX_QUESTION,GNOME_STOCK_BUTTON_YES,GNOME_STOCK_BUTTON_NO,NULL;gtk_window_set_modalmsgbox,1;gtk_widget_showmsgbox;/*Rundialogbox*/buttonClicked=gnome_dialog_run_and_closemsgbox;/*Button0istheYesbutton.Ifthisisthebuttontheyclickedon,tellGNOMEtoquitit'seventloop.Otherwise,donothing*/ifbuttonClicked==0{gtk_main_quit;}return0;}Tocompileit,type 252 PAGE 259 AppendixA.GUIProgramming gccgnome-example-c.c`gnome-config--cflags--libsgnomeui`-ognome-example-cRunitbytyping./gnome-example-c.Finally,wehaveaversioninPython.Typeitinasgnome-example.py:#PURPOSE:Thisprogramismeanttobeanexample#ofwhatGUIprogramslooklikewritten#withtheGNOMElibraries##ImportGNOMElibrariesimportgtkimportgnome.ui####DEFINECALLBACKFUNCTIONSFIRST#####InPython,functionshavetobedefinedbefore#theyareused,sowehavetodefineourcallback#functionsfirst.defdestroy_handlerevent:gtk.mainquitreturn0defdelete_handlerwindow,event:return0defclick_handlerevent:#Createthe"Areyousure"dialogmsgbox=gnome.ui.GnomeMessageBox"Areyousureyouwanttoquit?",gnome.ui.MESSAGE_BOX_QUESTION,gnome.ui.STOCK_BUTTON_YES,gnome.ui.STOCK_BUTTON_NO 253 PAGE 260 AppendixA.GUIProgramming msgbox.set_modalmsgbox.showresult=msgbox.run_and_close#Button0istheYesbutton.Ifthisisthe#buttontheyclickedon,tellGNOMEtoquit#it'seventloop.Otherwise,donothingifresult==0:gtk.mainquitreturn0####MAINPROGRAM#####Createnewapplicationwindowmyapp=gnome.ui.GnomeApp"gnome-example","GnomeExampleProgram"#Createnewbuttonmybutton=gtk.GtkButton"IWanttoQuittheGNOMEExampleprogram"myapp.set_contentsmybutton#Makesthebuttonshowupmybutton.show#Makestheapplicationwindowshowupmyapp.show#Connectsignalhandlersmyapp.connect"delete_event",delete_handlermyapp.connect"destroy",destroy_handlermybutton.connect"clicked",click_handler 254 PAGE 261 AppendixA.GUIProgramming #TransfercontroltoGNOMEgtk.mainloopTorunittypepythongnome-example.py.GUIBuildersInthepreviousexample,youhavecreatedtheuser-interfacefortheapplicationbycallingthecreatefunctionsforeachwidgetandplacingitwhereyouwantedit.However,thiscanbequiteburdensomeformorecomplexapplications.Manyprogrammingenvironments,includingGNOME,haveprogramscalledGUIbuildersthatcanbeusedtoautomaticallycreateyourGUIforyou.Youjusthavetowritethecodeforthesignalhandlersandforinitializingyourprogram.ThemainGUIbuilderforGNOMEapplicationsiscalledGLADE.GLADEshipswithmostLinuxdistributions.ThereareGUIbuildersformostprogrammingenvironments.BorlandhasarangeoftoolsthatwillbuildGUIsquicklyandeasilyonLinuxandWin32systems.TheKDEenvironmenthasatoolcalledQTDesignerwhichhelpsyouautomaticallydeveloptheGUIfortheirsystem.Thereisabroadrangeofchoicesfordevelopinggraphicalapplications,buthopefullythisappendixgaveyouatasteofwhatGUIprogrammingislike. 255 PAGE 262 AppendixA.GUIProgramming 256 PAGE 263 AppendixB.Commonx86InstructionsReadingtheTablesThetablesofinstructionspresentedinthisappendixinclude: Theinstructioncode Theoperandsused Theagsused AbriefdescriptionofwhattheinstructiondoesIntheoperandssection,itwilllistthetypeofoperandsittakes.Ifittakesmorethanoneoperand,eachoperandwillbeseparatedbyacomma.Eachoperandwillhavealistofcodeswhichtellwhethertheoperandcanbeanimmediate-modevalueI,aregisterR,oramemoryaddressM.Forexample,themovlinstructionislistedasI/R/M,R/M.Thismeansthattherstoperandcanbeanykindofvalue,whilethesecondoperandmustbearegisterormemorylocation.Note,however,thatinx86assemblylanguageyoucannothavemorethanoneoperandbeamemorylocation.Intheagssection,itliststheagsinthe%eflagsregisteraffectedbytheinstruction.Thefollowingagsarementioned:O Overowag.Thisissettotrueifthedestinationoperandwasnotlargeenoughtoholdtheresultoftheinstruction.S Signag.Thisissettothesignofthelastresult. 257 PAGE 264 AppendixB.Commonx86Instructions Z Zeroag.Thisagissettotrueiftheresultoftheinstructioniszero.A Auxiliarycarryag.Thisagissetforcarriesandborrowsbetweenthethirdandfourthbit.Itisnotoftenused.P Parityag.Thisagissettotrueifthelowbyteofthelastresulthadanevennumberof1bits.C Carryag.Usedinarithmetictosaywhetherornottheresultshouldbecarriedovertoanadditionalbyte.Ifthecarryagisset,thatusuallymeansthatthedestinationregistercouldnotholdthefullresult.Itisuptotheprogrammertodecideonwhatactiontotakei.e.-propogatetheresulttoanotherbyte,signalanerror,orignoreitentirely.Otheragsexist,buttheyaremuchlessimportant.DataTransferInstructionsTheseinstructionsperformlittle,ifanycomputation.Insteadtheyaremostlyusedformovingdatafromoneplacetoanother.TableB-1.DataTransferInstructions Instruction Operands AffectedFlags movl I/R/M,I/R/M O/S/Z/A/C Thiscopiesawordofdatafromonelocationtoanother.movl%eax,%ebxcopiesthecontentsof%eaxto%ebx 258 PAGE 265 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags movb I/R/M,I/R/M O/S/Z/A/C Sameasmovl,butoperatesonindividualbytes. leal M,I/R/M O/S/Z/A/C Thistakesamemorylocationgiveninthestandardformat,and,insteadofloadingthecontentsofthememorylocation,loadsthecomputedaddress.Forexample,leal5%ebp,%ecx,1,%eaxloadstheaddresscomputedby5+%ebp+1*%ecxandstoresthatin%eax popl R/M O/S/Z/A/C Popsthetopofthestackintothegivenlocation.Thisisequivalenttoperformingmovl%esp,R/Mfollowedbyaddl$4,%esp.popflisavariantwhichpopsthetopofthestackintothe%eflagsregister. pushl I/R/M O/S/Z/A/C Pushesthegivenvalueontothestack.Thisistheequivalenttoperformingsubl$4,%espfollowedbymovlI/R/M,%esp.pushflisavariantwhichpushesthecurrentcontentsofthe%eflagsregisterontothetopofthestack. xchgl R/M,R/M O/S/Z/A/C Exchangethevaluesofthegivenoperands. IntegerInstructionsThesearebasiccalculatinginstructionsthatoperateonsignedorunsignedintegers.TableB-2.IntegerInstructions Instruction Operands AffectedFlags adcl I/R/M,R/M O/S/Z/A/P/C 259 PAGE 266 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags Addwithcarry.Addsthecarrybitandtherstoperandtothesecond,and,ifthereisanoverow,setsoverowandcarrytotrue.Thisisusuallyusedforoperationslargerthanamachineword.Theadditionontheleast-signicantwordwouldtakeplaceusingaddl,whileadditionstotheotherwordswouldusedtheadclinstructiontotakethecarryfromthepreviousaddintoaccount.Fortheusualcase,thisisnotused,andaddlisusedinstead. addl I/R/M,R/M O/S/Z/A/P/C Addition.Addstherstoperandtothesecond,storingtheresultinthesecond.Iftheresultislargerthanthedestinationregister,theoverowandcarrybitsaresettotrue.Thisinstructionoperatesonbothsignedandunsignedintegers. cdq O/S/Z/A/P/C Convertsthe%eaxwordintothedouble-wordconsistingof%edx:%eaxwithsignextension.Theqsigniesthatitisaquad-word.It'sactuallyadouble-word,butit'scalledaquad-wordbecauseoftheterminologyusedinthe16-bitdays.Thisisusuallyusedbeforeissuinganidivlinstruction. cmpl I/R/M,R/M O/S/Z/A/P/C Comparestwointegers.Itdoesthisbysubtractingtherstoperandfromthesecond.Itdiscardstheresults,butsetstheagsaccordingly.Usuallyusedbeforeaconditionaljump. decl R/M O/S/Z/A/P Decrementstheregisterormemorylocation.Usedecbtodecrementabyteinsteadofaword. divl R/M O/S/Z/A/P Performsunsigneddivision.Dividesthecontentsofthedouble-wordcontainedinthecombined%edx:%eaxregistersbythevalueintheregisterormemorylocationspecied.The%eaxregistercontainstheresultingquotient,andthe%edxregistercontainstheresultingremainder.Ifthequotientistoolargetotin%eax,ittriggersatype0interrupt. idivl R/M O/S/Z/A/P 260 PAGE 267 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags Performssigneddivision.Operatesjustlikedivlabove. imull R/M/I,R O/S/Z/A/P/C Performssignedmultiplicationandstorestheresultinthesecondoperand.Ifthesecondoperandisleftout,itisassumedtobe%eax,andthefullresultisstoredinthedouble-word%edx:%eax. incl R/M O/S/Z/A/P Incrementsthegivenregisterormemorylocation.Useincbtoincrementabyteinsteadofaword. mull R/M/I,R O/S/Z/A/P/C Performunsignedmultiplication.Samerulesasapplytoimull. negl R/M O/S/Z/A/P/C Negatesgivesthetwo'scomplementinversionofthegivenregisterormemorylocation. sbbl I/R/M,R/M O/S/Z/A/P/C Subtractwithborrowing.Thisisusedinthesamewaythatadcis,exceptforsubtraction.Normallyonlysublisused. subl I/R/M,R/M O/S/Z/A/P/C Subtractthetwooperands.Thissubtractstherstoperandfromthesecond,andstorestheresultinthesecondoperand.Thisinstructioncanbeusedonbothsignedandunsignednumbers. LogicInstructionsTheseinstructionsoperateonmemoryasbitsinsteadofwords.TableB-3.LogicInstructions Instruction Operands AffectedFlags 261 PAGE 268 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags andl I/R/M,R/M O/S/Z/P/C Performsalogicalandofthecontentsofthetwooperands,andstorestheresultinthesecondoperand.Setstheoverowandcarryagstofalse. notl R/M Performsalogicalnotoneachbitintheoperand.Alsoknownasaone'scomplement. orl I/R/M,R/M O/S/Z/A/P/C Performsalogicalorbetweenthetwooperands,andstorestheresultinthesecondoperand.Setstheoverowandcarryagstofalse. rcll I/%cl,R/M O/C Rotatesthegivenlocation'sbitstotheleftthenumberoftimesintherstoperand,whichiseitheranimmediate-modevalueortheregister%cl.Thecarryagisincludedintherotation,makingituse33bitsinsteadof32.Alsosetstheoverowag. rcrl I/%cl,R/M O/C Sameasabove,butrotatesright. roll I/%cl,R/M O/C Rotatebitstotheleft.Itsetstheoverowandcarryags,butdoesnotcountthecarryagaspartoftherotation.Thenumberofbitstorolliseitherspeciedinimmediatemodeoriscontainedinthe%clregister. rorl I/%cl,R/M O/C Sameasabove,butrotatesright. sall I/%cl,R/M C Arithmeticshiftleft.Thesignbitisshiftedouttothecarryag,andazerobitisplacedintheleastsignicantbit.Otherbitsaresimplyshiftedtotheleft.Thisisthesameastheregularshiftleft.Thenumberofbitstoshiftiseitherspeciedinimmediatemodeoriscontainedinthe%clregister. sarl I/%cl,R/M C 262 PAGE 269 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags Arithmeticshiftright.Theleastsignicantbitisshiftedouttothecarryag.Thesignbitisshiftedin,andkeptasthesignbit.Otherbitsaresimplyshiftedtotheright.Thenumberofbitstoshiftiseitherspeciedinimmediatemodeoriscontainedinthe%clregister. shll I/%cl,R/M C Logicalshiftleft.Thisshiftsallbitstotheleftsignbitisnottreatedspecially.Theleftmostbitispushedtothecarryag.Thenumberofbitstoshiftiseitherspeciedinimmediatemodeoriscontainedinthe%clregister. shrl I/%cl,R/M C Logicalshiftright.Thisshiftsallbitsintheregistertotherightsignbitisnottreatedspecially.Therightmostbitispushedtothecarryag.Thenumberofbitstoshiftiseitherspeciedinimmediatemodeoriscontainedinthe%clregister. testl I/R/M,R/M O/S/Z/A/P/C Doesalogicalandofbothoperandsanddiscardstheresults,butsetstheagsaccordingly. xorl I/R/M,R/M O/S/Z/A/P/C Doesanexclusiveoronthetwooperands,andstorestheresultinthesecondoperand.Setstheoverowandcarryagstofalse. FlowControlInstructionsTheseinstructionsmayaltertheowoftheprogram.TableB-4.FlowControlInstructions Instruction Operands AffectedFlags call destinationaddress O/S/Z/A/C 263 PAGE 270 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags Thispusheswhatwouldbethenextvaluefor%eipontothestack,andjumpstothedestinationaddress.Usedforfunctioncalls.Alternatively,thedestinationaddresscanbeanasteriskfollowedbyaregisterforanindirectfunctioncall.Forexample,call*%eaxwillcallthefunctionattheaddressin%eax. int I O/S/Z/A/C Causesaninterruptofthegivennumber.Thisisusuallyusedforsystemcallsandotherkernelinterfaces. Jcc destinationaddress O/S/Z/A/C 264 PAGE 271 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags Conditionalbranch.ccistheconditioncode.Jumpstothegivenaddressiftheconditioncodeistruesetfromthepreviousinstruction,probablyacomparison.Otherwise,goestothenextinstruction.Theconditioncodesare: [n]a[e]-aboveunsignedgreaterthan.Anncanbeaddedfor"not"andanecanbeaddedfor"orequalto" [n]b[e]-belowunsignedlessthan [n]e-equalto [n]z-zero [n]g[e]-greaterthansignedcomparison [n]l[e]-lessthansignedcomparison [n]c-carryagset [n]o-overowagset [p]p-parityagset [n]s-signagset ecxz-%ecxiszero jmp destinationaddress O/S/Z/A/C Anunconditionaljump.Thissimplysets%eiptothedestinationaddress.Alternatively,thedestinationaddresscanbeanasteriskfollowedbyaregisterforanindirectjump.Forexample,jmp*%eaxwilljumptotheaddressin%eax. 265 PAGE 272 AppendixB.Commonx86Instructions Instruction Operands AffectedFlags ret O/S/Z/A/C Popsavalueoffofthestackandthensets%eiptothatvalue.Usedtoreturnfromfunctioncalls. AssemblerDirectivesTheseareinstructionstotheassemblerandlinker,insteadofinstructionstotheprocessor.Theseareusedtohelptheassemblerputyourcodetogetherproperly,andmakeiteasiertouse.TableB-5.AssemblerDirectives Directive Operands .ascii QUOTEDSTRING Takesthegivenquotedstringandconvertsitintobytedata. .byte VALUES Takesacomma-separatedlistofvaluesandinsertsthemrightthereintheprogramasdata. .endr Endsarepeatingsectiondenedwith.rept. .equ LABEL,VALUE Setsthegivenlabelequivalenttothegivenvalue.Thevaluecanbeanumber,acharacter,oranconstantexpressionthatevaluatestoaanumberorcharacter.Fromthatpointon,useofthelabelwillbesubstitutedforthegivenvalue. .globl LABEL Setsthegivenlabelasglobal,meaningthatitcanbeusedfromseparately-compiledobjectles. .include FILE 266 PAGE 273 AppendixB.Commonx86Instructions Directive Operands Includesthegivenlejustasifitweretypedinrightthere. .lcomm SYMBOL,SIZE Thisisusedinthe.bsssectiontospecifystoragethatshouldbeallocatedwhentheprogramisexecuted.Denesthesymbolwiththeaddresswherethestoragewillbelocated,andmakessurethatitisthegivennumberofbyteslong. .long VALUES Takesasequenceofnumbersseparatedbycommas,andinsertsthosenumbersas4-bytewordsrightwheretheyareintheprogram. .rept COUNT Repeatseverythingbetweenthisdirectiveandthe.endrdirectivesthenumberoftimesspecied. .section SECTIONNAME Switchesthesectionthatisbeingworkedon.Commonsectionsinclude.textforcode,.datafordataembeddedintheprogramitself,and.bssforuninitializedglobaldata. .type SYMBOL,@function Tellsthelinkerthatthegivensymbolisafunction. DifferencesinOtherSyntaxesandTerminologyThesyntaxforassemblylanguageusedinthisbookisknownattheAT&Tsyntax.ItistheonesupportedbytheGNUtoolchainthatcomesstandardwitheveryLinuxdistribution.However,theofcialsyntaxforx86assemblylanguageknownastheIntelsyntaxisdifferent.Itisthesameassemblylanguageforthesameplatform,butitlooksdifferent.Someofthedifferencesinclude: InIntelsyntax,theoperandsofinstructionsareoftenreversed.Thedestinationoperandislistedbeforethesourceoperand. 267 PAGE 274 AppendixB.Commonx86Instructions InIntelsyntax,registersarenotprexedwiththepercentsign%. InIntelsyntax,adollar-sign$isnotrequiredtodoimmediate-modeaddressing.Instead,non-immediateaddressingisaccomplishedbysurroundingtheaddresswithbrackets[]. InIntelsyntax,theinstructionnamedoesnotincludethesizeofdatabeingmoved.Ifthatisambiguous,itisexplicitlystatedasBYTE,WORD,orDWORDimmediatelyaftertheinstructionname. ThewaythatmemoryaddressesarerepresentedinIntelassemblylanguageismuchdifferentshownbelow. Becausethex86processorlineoriginallystartedoutasa16-bitprocessor,mostliteratureaboutx86processorsrefertowordsas16-bitvalues,andcall32-bitvaluesdoublewords.However,weusetheterm"word"torefertothestandardregistersizeonaprocessor,whichis32bitsonanx86processor.Thesyntaxalsokeepsthisnamingconvention-DWORDstandsfor"doubleword"inIntelsyntaxandisusedforstandard-sizedregisters,whichwewouldcallsimplya"word". Intelassemblylanguagehastheabilitytoaddressmemoryasasegment/offsetpair.WedonotmentionthisbecauseLinuxdoesnotsupportsegmentedmemory,andisthereforeirrelevanttonormalLinuxprogramming.Otherdifferencesexist,buttheyaresmallincomparison.Toshowsomeofthedifferences,considerthefollowinginstruction:movl%eax,8%ebx,%edi,4InIntelsyntax,thiswouldbewrittenas:mov[8+%ebx+1*edi],eaxThememoryreferenceisabiteasiertoreadthanit'sAT&Tcounterpartbecauseitspellsoutexactlyhowtheaddresswillbecomputed.However,buttheorderofoperandsinIntelsyntaxcanbeconfusing. 268 PAGE 275 AppendixB.Commonx86Instructions WheretoGoforMoreInformationIntelhasasetofcomprehensiveguidestotheirprocessors.Theseareavailableathttp://www.intel.com/design/pentium/manuals/NotethatalloftheseusetheIntelsyntax,nottheAT&Tsyntax.ThemostimportantonesaretheirIA-32IntelArchitectureSoftwareDeveloper'sManualinitsthreevolumes:: Volume1:SystemProgrammingGuidehttp://developer.intel.com/design/pentium4/manuals/245470.htm Volume2:InstructionSetReferencehttp://developer.intel.com/design/pentium4/manuals/245471.htm Volume3:SystemProgrammingGuidehttp://developer.intel.com/design/pentium4/manuals/245472.htmInaddition,youcanndalotofinformationinthemanualfortheGNUassembler,availableonlineathttp://www.gnu.org/software/binutils/manual/gas-2.9.1/as.html.Similarly,themanualfortheGNUlinkerisavailableonlineathttp://www.gnu.org/software/binutils/manual/ld-2.9.1/ld.html. 269 PAGE 276 AppendixB.Commonx86Instructions 270 PAGE 277 AppendixC.ImportantSystemCallsThesearesomeofthemoreimportantsystemcallstousewhendealingwithLinux.Formostcases,however,itisbesttouselibraryfunctionsratherthandirectsystemcalls,becausethesystemcallsweredesignedtobeminimalisticwhilethelibraryfunctionsweredesignedtobeeasytoprogramwith.ForinformationabouttheLinuxClibrary,seethemanualathttp://www.gnu.org/software/libc/manual/Rememberthat%eaxholdsthesystemcallnumbers,andthatthereturnvaluesanderrorcodesarealsostoredin%eax.TableC-1.ImportantLinuxSystemCalls %eax Name %ebx %ecx %edx Notes 1 exit returnvalueint Exitstheprogram 3 read lede-scriptor bufferstart buffersizeint Readsintothegivenbuffer 4 write lede-scriptor bufferstart buffersizeint Writesthebuffertotheledescriptor 5 open null-terminatedlename optionlist permissionmode Opensthegivenle.Returnstheledescriptororanerrornumber. 6 close lede-scriptor Closesthegiveledescriptor 271 PAGE 278 AppendixC.ImportantSystemCalls %eax Name %ebx %ecx %edx Notes 12 chdir null-terminateddirectoryname Changesthecurrentdirectoryofyourprogram. 19 lseek lede-scriptor offset mode Repositionswhereyouareinthegivenle.Themodecalledthe"whence"shouldbe0forabsolutepositioning,and1forrelativepositioning. 20 getpid ReturnstheprocessIDofthecurrentprocess. 39 mkdir null-terminateddirectoryname permissionmode Createsthegivendirectory.Assumesalldirectoriesleadinguptoitalreadyexist. 40 rmdir null-terminateddirectoryname Removesthegivendirectory. 41 dup lede-scriptor Returnsanewledescriptorthatworksjustliketheexistingledescriptor. 42 pipe pipearray Createstwoledescriptors,wherewritingononeproducesdatatoreadontheotherandvice-versa.%ebxisapointertotwowordsofstoragetoholdtheledescriptors. 272 PAGE 279 AppendixC.ImportantSystemCalls %eax Name %ebx %ecx %edx Notes 45 brk newsystembreak Setsthesystembreaki.e.-theendofthedatasection.Ifthesystembreakis0,itsimplyreturnsthecurrentsystembreak. 54 ioctl lede-scriptor request arguments Thisisusedtosetparametersondeviceles.It'sactualusagevariesbasedonthetypeofleordeviceyourdescriptorreferences. Amorecompletelistingofsystemcalls,alongwithadditionalinformationisavailableathttp://www.lxhp.in-berlin.de/lhpsyscal.htmlYoucanalsogetmoreinformationaboutasystemcallbytypinginman2SYSCALLNAMEwhichwillreturnyoutheinformationaboutthesystemcallfromsection2oftheUNIXmanual.However,thisreferstotheusageofthesystemcallfromtheCprogramminglanguage,andmayormaynotbedirectlyhelpful.ForinformationonhowsystemcallsareimplementedonLinux,seetheLinuxKernel2.4Internalssectiononhowsystemcallsareimplementedathttp://www.faqs.org/docs/kernel_2_4/lki-2.html#ss2.11 273 PAGE 280 AppendixC.ImportantSystemCalls 274 PAGE 281 AppendixD.TableofASCIICodesTousethistable,simplyndthecharacterorescapethatyouwantthecodefor,andaddthenumberontheleftandthetop.TableD-1.TableofASCIIcodesindecimal +0 +1 +2 +3 +4 +5 +6 +7 0 NUL SOH STX ETX EOT ENQ ACK BEL 8 BS HT LF VT FF CR SO SI 16 DLE DC1 DC2 DC3 DC4 NAK SYN ETB 24 CAN EM SUB ESC FS GS RS US 32 # $ % & 40 + / 48 0 1 2 3 4 5 6 7 56 8 9 : ; < = > ? 64 @ A B C D E F G 72 H I J K L M N O 80 P Q R S T U V W 88 X Y Z [ ] ^ 96 ` a b c d e f g 104 h i j k l m n o 112 p q r s t u v w 120 x y z { | } ~ DEL ASCIIisactuallybeingphasedoutinfavorofaninternationalstandardknownasUnicode,whichallowsyoutodisplayanycharacterfromanyknownwritingsystemintheworld.Asyoumayhavenoticed,ASCIIonlyhassupportforEnglishcharacters.Unicodeismuchmorecomplicated,however,becauseitrequiresmorethanonebytetoencodeasinglecharacter.Thereareseveral 275 PAGE 282 AppendixD.TableofASCIICodes differentmethodsforencodingUnicodecharacters.ThemostcommonisUTF-8andUTF-32.UTF-8issomewhatbackwards-compatiblewithASCIIitisstoredthesameforEnglishcharacters,butexpandsintomultiplebyteforinternationalcharacters.UTF-32simplyrequiresfourbytesforeachcharacterratherthanone.WindowsusesUTF-16,whichisavariable-lengthencodingwhichrequiresatleast2bytespercharacter,soitisnotbackwards-compatiblewithASCII.Agoodtutorialoninternationalizationissues,fonts,andUnicodeisavailableinagreatArticlebyJoeSpolsky,called"TheAbsoluteMinimumEverySoftwareDeveloperAbsolutely,PositivelyMustKnowAboutUnicodeandCharacterSetsNoExcuses!",availableonlineathttp://www.joelonsoftware.com/articles/Unicode.html 276 PAGE 283 AppendixE.CIdiomsinAssemblyLanguageThisappendixisforCprogrammerslearningassemblylanguage.ItismeanttogiveageneralideaabouthowCconstructscanbeimplementedinassemblylanguage.IfStatementInC,anifstatementconsistsofthreeparts-thecondition,thetruebranch,andthefalsebranch.However,sinceassemblylanguageisnotablockstructuredlanguage,youhavetoworkalittletoimplementtheblock-likenatureofC.Forexample,lookatthefollowingCcode:ifa==b{/*TrueBranchCodeHere*/}else{/*FalseBranchCodeHere*/}/*AtThisPoint,Reconverge*/Inassemblylanguage,thiscanberenderedas:#Moveaandbintoregistersforcomparisonmovla,%eaxmovlb,%ebx#Comparecmpl%eax,%ebx#IfTrue,gototruebranchjetrue_branch 277 PAGE 284 AppendixE.CIdiomsinAssemblyLanguage false_branch:#Thislabelisunnecessary,#onlyherefordocumentation#FalseBranchCodeHere#Jumptorecovergencepointjmpreconvergetrue_branch:#TrueBranchCodeHerereconverge:#BothbranchesrecovergetothispointAsyoucansee,sinceassemblylanguageislinear,theblockshavetojumparoundeachother.Recovergenceishandledbytheprogrammer,notthesystem.Acasestatementiswrittenjustlikeasequenceofifstatements.FunctionCallAfunctioncallinassemblylanguagesimplyrequirespushingtheargumentstothefunctionontothestackinreverseorder,andissuingacallinstruction.Aftercalling,theargumentsarethenpoppedbackoffofthestack.Forexample,considertheCcode:printf"Thenumberis%d",88;Inassemblylanguage,thiswouldberenderedas:.section.datatext_string:.ascii"Thenumberis%d" 278 PAGE 285 AppendixE.CIdiomsinAssemblyLanguage .section.textpushl$88pushl$text_stringcallprintfpopl%eaxpopl%eax#%eaxisjustadummyvariable,#nothingisactuallybeingdone#withthevalue.Youcanalso#directlyre-adjust%esptothe#properlocation.VariablesandAssignmentGlobalandstaticvariablesaredeclaredusing.dataor.bssentries.Localvariablesaredeclaredbyreservingspaceonthestackatthebeginningofthefunction.Thisspaceisgivenbackattheendofthefunction.Interestingly,globalvariablesareaccesseddifferentlythanlocalvariablesinassemblylanguage.Globalvariablesareaccessedusingdirectaddressing,whilelocalvariablesareaccessedusingbasepointeraddressing.Forexample,considerthefollowingCcode:intmy_global_var;intfoo{intmy_local_var;my_local_var=1;my_global_var=2;return0;} 279 PAGE 286 AppendixE.CIdiomsinAssemblyLanguage Thiswouldberenderedinassemblylanguageas:.section.data.lcommmy_global_var,4.typefoo,@functionfoo:pushl%ebp#Saveoldbasepointermovl%esp,$ebp#makestackpointerbasepointersubl$4,%esp#Makeroomformy_local_var.equmy_local_var,-4#Cannowusemy_local_varto#findthelocalvariablemovl$1,my_local_var%ebpmovl$2,my_global_varmovl%ebp,%esp#Cleanupfunctionandreturnpopl%ebpretWhatmaynotbeobviousisthataccessingtheglobalvariabletakesfewermachinecyclesthanaccessingtheglobalvariable.However,thatmaynotmatterbecausethestackismorelikelytobeinphysicalmemoryinsteadofswapthantheglobalvariableis.AlsonotethatintheCprogramminglanguage,afterthecompilerloadsavalueintoaregister,thatvaluewilllikelystayinthatregisteruntilthatregisterisneededforsomethingelse.Itmayalsomoveregisters.Forexample,ifyouhaveavariablefoo,itmaystartonthestack,butthecompilerwilleventuallymoveitintoregistersforprocessing.Iftherearen'tmanyvariablesinuse,thevaluemaysimplystayintheregisteruntilitisneededagain.Otherwise,whenthatregisterisneededforsomethingelse,thevalue,ifit'schanged,iscopiedbacktoitscorrespondingmemorylocation.InC,youcanusethekeywordvolatiletomakesureallmodicationsandreferencestothevariablearedonetothememory 280 PAGE 287 AppendixE.CIdiomsinAssemblyLanguage locationitself,ratherthanaregistercopyofit,incaseotherprocesses,threads,orhardwaremaybemodifyingthevaluewhileyourfunctionisrunning.LoopsLoopsworkalotlikeifstatementsinassemblylanguage-theblocksareformedbyjumpingaround.InC,awhileloopconsistsofaloopbody,andatesttodeterminewhetherornotitistimetoexittheloop.Aforloopisexactlythesame,withoptionalinitializationandcounter-incrementsections.Thesecansimplybemovedaroundtomakeawhileloop.InC,awhilelooplookslikethis:whilea PAGE 288 AppendixE.CIdiomsinAssemblyLanguage Thex86assemblylanguagehassomedirectsupportforloopingaswell.The%ecxregistercanbeusedasacounterthatendswithzero.Theloopinstructionwilldecrement%ecxandjumptoaspeciedaddressunless%ecxiszero.Forexample,ifyouwantedtoexecuteastatement100times,youwoulddothisinC:fori=0;i<100;i++{/*Doprocesshere*/}Inassemblylanguageitwouldbewrittenlikethis:loop_initialize:movl$100,%ecxloop_begin:##DoProcessHere##Decrement%ecxandloopsifnotzerolooploop_beginrest_of_program:#ContinuesontohereOnethingtonoticeisthattheloopinstructionrequiresyoutobecountingbackwardstozero.Ifyouneedtocountforwardsoruseanotherendingnumber,youshouldusetheloopformwhichdoesnotincludetheloopinstruction.Forreallytightloopsofcharacterstringoperations,thereisalsotherepinstruction,butwewillleavelearningaboutthatasanexercisetothereader.StructsStructsaresimplydescriptionsofmemoryblocks.Forexample,inCyoucansay: 282 PAGE 289 AppendixE.CIdiomsinAssemblyLanguage structperson{charfirstname[40];charlastname[40];intage;};Thisdoesn'tdoanythingbyitself,exceptgiveyouwaysofintelligentlyusing84bytesofdata.Youcandobasicallythesamethingusing.equdirectivesinassemblylanguage.Likethis:.equPERSON_SIZE,84.equPERSON_FIRSTNAME_OFFSET,0.equPERSON_LASTNAME_OFFSET,40.equPERSON_AGE_OFFSET,80Whenyoudeclareavariableofthistype,allyouaredoingisreserving84bytesofspace.So,ifyouhavethisinC:voidfoo{structpersonp;/*Dostuffhere*/}Inassemblylanguageyouwouldhave:foo:#Standardheaderbeginningpushl%ebpmovl%esp,%ebp#Reserveourlocalvariablesubl$PERSON_SIZE,%esp#Thisisthevariable'soffsetfrom%ebp 283 PAGE 290 AppendixE.CIdiomsinAssemblyLanguage .equP_VAR,0-PERSON_SIZE#DoStuffHere#Standardfunctionendingmovl%ebp,%esppopl%ebpretToaccessstructuremembers,youjusthavetousebasepointeraddressingwiththeoffsetsdenedabove.Forexample,inCyoucouldsettheperson'sagelikethis:p.age=30;Inassemblylanguageitwouldlooklikethis:movl$30,P_VAR+PERSON_AGE_OFFSET%ebpPointersPointersareveryeasy.Remember,pointersaresimplytheaddressthatavalueresidesat.Let'sstartbytakingalookatglobalvariables.Forexample:intglobal_data=30;Inassemblylanguage,thiswouldbe:.section.dataglobal_data:.long30TakingtheaddressofthisdatainC:a=&global_data; 284 PAGE 291 AppendixE.CIdiomsinAssemblyLanguage Takingtheaddressofthisdatainassemblylanguage:movl$global_data,%eaxYousee,withassemblylanguage,youarealmostalwaysaccessingmemorythroughpointers.That'swhatdirectaddressingis.Togetthepointeritself,youjusthavetogowithimmediatemodeaddressing.Localvariablesarealittlemoredifcult,butnotmuch.HereishowyoutaketheaddressofalocalvariableinC:voidfoo{inta;int*b;a=30;b=&a;*b=44;}Thesamecodeinassemblylanguage:foo:#Standardopeningpushl%ebpmovl%esp,%ebp#Reservetwowordsofmemorysubl$8,$esp.equA_VAR,-4.equB_VAR,-8#a=30 285 PAGE 292 AppendixE.CIdiomsinAssemblyLanguage movl$30,A_VAR%ebp#b=&amovl$A_VAR,B_VAR%ebpaddl%ebp,B_VAR%ebp#*b=30movlB_VAR%ebp,%eaxmovl$30,%eax#Standardclosingmovl%ebp,%esppopl%ebpretAsyoucansee,totaketheaddressofalocalvariable,theaddresshastobecomputedthesamewaythecomputercomputestheaddressesinbasepointeraddressing.Thereisaneasierway-theprocessorprovidestheinstructionleal,whichstandsfor"loadeffectiveaddress".Thisletsthecomputercomputetheaddress,andthenloaditwhereveryouwant.So,wecouldjustsay:#b=&alealA_VAR%ebp,%eaxmovl%eax,B_VAR%ebpIt'sthesamenumberoflines,butalittlecleaner.Then,tousethisvalue,yousimplyhavetomoveittoageneral-purposeregisteranduseindirectaddressing,asshownintheexampleabove.GettingGCCtoHelpOneofthenicethingsaboutGCCisit'sabilitytospitoutassemblylanguagecode.ToconvertaClanguageletoassembly,youcansimplydo: 286 PAGE 293 AppendixE.CIdiomsinAssemblyLanguage gcc-Sfile.cTheoutputwillbeinfile.s.It'snotthemostreadableoutput-mostofthevariablenameshavebeenremovedandreplacedeitherwithnumericstacklocationsorreferencestoautomatically-generatedlabels.Tostartwith,youprobablywanttoturnoffoptimizationswith-O0sothattheassemblylanguageoutputwillfollowyoursourcecodebetter.SomethingelseyoumightnoticeisthatGCCreservesmorestackspaceforlocalvariablesthanwedo,andthenAND's%esp1Thisistoincreasememoryandcacheefciencybydouble-wordaligningvariables.Finally,attheendoffunctions,weusuallydothefollowinginstructionstocleanupthestackbeforeissuingaretinstruction:movl%ebp,%esppopl%ebpHowever,GCCoutputwillusuallyjustincludetheinstructionleave.Thisinstructionissimplythecombinationoftheabovetwoinstructions.Wedonotuseleaveinthistextbecausewewanttobeclearaboutexactlywhatishappeningattheprocessorlevel.IencourageyoutotakeaCprogramyouhavewrittenandcompileittoassemblylanguageandtracethelogic.Then,addinoptimizationsandtryagain.Seehowthecompilerchosetorearrangeyourprogramtobemoreoptimized,andtrytogureoutwhyitchosethearrangementandinstructionsitdid. 1. NotethatdifferentversionsofGCCdothisdifferently. 287 PAGE 294 AppendixE.CIdiomsinAssemblyLanguage 288 PAGE 295 AppendixF.UsingtheGDBDebuggerBythetimeyoureadthisappendix,youwilllikelyhavewrittenatleastoneprogramwithanerrorinit.Inassemblylanguage,evenminorerrorsusuallyhaveresultssuchasthewholeprogramcrashingwithasegmentationfaulterror.Inmostprogramminglanguages,youcansimplyprintoutthevaluesinyourvariablesasyougoalong,andusethatoutputtondoutwhereyouwentwrong.Inassemblylanguage,callingoutputfunctionsisnotsoeasy.Therefore,toaidindeterminingthesourceoferrors,youmustuseasourcedebugger.Adebuggerisaprogramthathelpsyoundbugsbysteppingthroughtheprogramonestepatatime,lettingyouexaminememoryandregistercontentsalongtheway.Asourcedebuggerisadebuggerthatallowsyoutotiethedebuggingoperationdirectlytothesourcecodeofaprogram.Thismeansthatthedebuggerallowsyoutolookatthesourcecodeasyoutypeditin-completewithsymbols,labels,andcomments.ThedebuggerwewillbelookingatisGDB-theGNUDebugger.ThisapplicationispresentonalmostallGNU/Linuxdistributions.Itcandebugprogramsinmultipleprogramminglanguages,includingassemblylanguage.AnExampleDebuggingSessionThebestwaytoexplainhowadebuggerworksisbyusingit.Theprogramwewillbeusingthedebuggeronisthemaximumprogramusedin Chapter3 .Let'ssaythatyouenteredtheprogramperfectly,exceptthatyouleftouttheline:incl%ediWhenyouruntheprogram,itjustgoesinaninniteloop-itneverexits.Todeterminethecause,youneedtoruntheprogramunderGDB.However,todothis,youneedtohavetheassemblerincludedebugginginformationintheexecutable.Allyouneedtodotoenablethisistoaddthe--gstabsoptiontotheascommand.So,youwouldassembleitlikethis: 289 PAGE 296 AppendixF.UsingtheGDBDebugger as--gstabsmaximum.s-omaximum.oLinkingwouldbethesameasnormal."stabs"isthedebuggingformatusedbyGDB.Now,toruntheprogramunderthedebugger,youwouldtypeingdb./maximum.Besurethatthesourcelesareinthecurrentdirectory.Theoutputshouldlooksimilartothis:GNUgdbRedHatLinux.2.1-4Copyright2002FreeSoftwareFoundation,Inc.GDBisfreesoftware,coveredbytheGNUGeneralPublicLicense,andyouarewelcometochangeitand/ordistributecopiesofitundercertainconditions.Type"showcopying"toseetheconditions.ThereisabsolutelynowarrantyforGDB.Type"showwarranty"fordetails.ThisGDBwasconfiguredas"i386-redhat-linux"...gdbDependingonwhichversionofGDByouarerunning,thisoutputmayvaryslightly.Atthispoint,theprogramisloaded,butisnotrunningyet.Thedebuggeriswaitingyourcommand.Torunyourprogram,justtypeinrun.Thiswillnotreturn,becausetheprogramisrunninginaninniteloop.Tostoptheprogram,hitcontrol-c.Thescreenwillthensaythis:Startingprogram:/home/johnnyb/maximumProgramreceivedsignalSIGINT,Interrupt.start_loopatmaximum.s:3434movldata_items,%edi,4,%eaxCurrentlanguage:auto;currentlyasmgdbThistellsyouthattheprogramwasinterruptedbytheSIGINTsignalfromyourcontrol-c,andwaswithinthesectionlabelledstart_loop,andwasexecutingonline34whenitstopped.Itgivesyouthecodethatitisabouttoexecute. 290 PAGE 297 AppendixF.UsingtheGDBDebugger Dependingonexactlywhenyouhitcontrol-c,itmayhavestoppedonadifferentlineoradifferentinstructionthantheexample.Oneofthebestwaystondbugsinaprogramistofollowtheowoftheprogramtoseewhereitisbranchingincorrectly.Tofollowtheowofthisprogram,keeponenteringstepifor"stepinstruction",whichwillcausethecomputertoexecuteoneinstructionatatime.Ifyoudothisseveraltimes,youroutputwilllooksomethinglikethis:gdbstepi35cmpl%ebx,%eaxgdbstepi36jlestart_loopgdbstepi32cmpl$0,%eaxgdbstepi33jeloop_exitgdbstepi34movldata_items,%edi,4,%eaxgdbstepi35cmpl%ebx,%eaxgdbstepi36jlestart_loopgdbstep32cmpl$0,%eaxAsyoucantell,ithaslooped.Ingeneral,thisisgood,sincewewroteittoloop.However,theproblemisthatitisneverstopping.Therefore,tondoutwhattheproblemis,let'slookatthepointinourcodewhereweshouldbeexittingtheloop:cmpl$0,%eaxjeloop_exitBasically,itischeckingtoseeif%eaxhitszero.Ifso,itshouldexittheloop.Thereareseveralthingstocheckhere.Firstofall,youmayhaveleftthispieceout 291 PAGE 298 AppendixF.UsingtheGDBDebugger altogether.Itisnotuncommonforaprogrammertoforgettoincludeawaytoexitaloop.However,thisisnotthecasehere.Second,youshouldmakesurethatloop_exitactuallyisoutsidetheloop.Ifweputthelabelinthewrongplace,strangethingswouldhappen.However,again,thisisnotthecase.Neitherofthosepotentialproblemsaretheculprit.So,thenextoptionisthatperhaps%eaxhasthewrongvalue.TherearetwowaystocheckthecontentsofregisterinGDB.Therstoneisthecommandinforegister.Thiswilldisplaythecontentsofallregistersinhexadecimal.However,weareonlyinterestedin%eaxatthispoint.Tojustdisplay%eaxwecandoprint/$eaxtoprintitinhexadecimal,ordoprint/d$eaxtoprintitindecimal.NoticethatinGDB,registersareprexedwithdollarsignsratherthanpercentsigns.Yourscreenshouldhavethisonit:gdbprint/d$eax$1=3gdbThismeansthattheresultofyourrstinquiryis3.Everyinquiryyoumakewillbeassignedanumberprexedwithadollarsign.Now,ifyoulookbackintothecode,youwillndthat3istherstnumberinthelistofnumberstosearchthrough.Ifyoustepthroughtheloopafewmoretimes,youwillndthatineveryloopiteration%eaxhasthenumber3.Thisisnotwhatshouldbehappening.%eaxshouldgotothenextvalueinthelistineveryiteration.Okay,nowweknowthat%eaxisbeingloadedwiththesamevalueoverandoveragain.Let'ssearchtoseewhere%eaxisbeingloadedfrom.Thelineofcodeisthis:movldata_items,%edi,4,%eaxSo,stepuntilthislineofcodeisreadytoexecute.Now,thiscodedependsontwovalues-data_itemsand%edi.data_itemsisasymbol,andthereforeconstant.It'sagoodideatocheckyoursourcecodetomakesurethelabelisin 292 PAGE 299 AppendixF.UsingtheGDBDebugger frontoftherightdata,butinourcaseitis.Therefore,weneedtolookat%edi.So,weneedtoprintitout.Itwilllooklikethis:gdbprint/d$edi$2=0gdbThisindicatesthat%ediissettozero,whichiswhyitkeepsonloadingtherstelementofthearray.Thisshouldcauseyoutoaskyourselftwoquestions-whatisthepurposeof%edi,andhowshoulditsvaluebechanged?Toanswertherstquestion,wejustneedtolookinthecomments.%ediisholdingthecurrentindexofdata_items.Sinceoursearchisasequentialsearchthroughthelistofnumbersindata_items,itwouldmakesensethat%edishouldbeincrementedwitheveryloopiteration.Scanningthecode,thereisnocodewhichalters%ediatall.Therefore,weshouldaddalinetoincrement%ediatthebeginningofeveryloopiteration.Thishappenstobeexactlythelinewetossedoutatthebeginning.Assembling,linking,andrunningtheprogramagainwillshowthatitnowworkscorrectly.HopefullythisexerciseprovidedsomeinsightintousingGDBtohelpyounderrorsinyourprograms.BreakpointsandOtherGDBFeaturesTheprogramweenteredinthelastsectionhadaninniteloop,andcouldbeeasilystoppedusingcontrol-c.Otherprogramsmaysimplyabortornishwitherrors.Inthesecases,control-cdoesn'thelp,becausebythetimeyoupresscontrol-c,theprogramisalreadynished.Toxthis,youneedtosetbreakpoints.Abreakpointisaplaceinthesourcecodethatyouhavemarkedtoindicatetothedebuggerthatitshouldstoptheprogramwhenithitsthatpoint.Tosetbreakpointsyouhavetosetthemupbeforeyouruntheprogram.Beforeissuingtheruncommand,youcansetupbreakpointsusingthebreakcommand. 293 PAGE 300 AppendixF.UsingtheGDBDebugger Forexample,tobreakonline27,issuethecommandbreak27.Then,whentheprogramcrossesline27,itwillstoprunning,andprintoutthecurrentlineandinstruction.Youcanthenstepthroughtheprogramfromthatpointandexamineregistersandmemory.Tolookatthelinesandlinenumbersofyourprogram,youcansimplyusethecommandl.Thiswillprintoutyourprogramwithlinenumbersascreenatatime.Whendealingwithfunctions,youcanalsobreakonthefunctionnames.Forexample,inthefactorialprogramin Chapter4 ,wecouldsetabreakpointforthefactorialfunctionbytypinginbreakfactorial.Thiswillcausethedebuggertobreakimmediatelyafterthefunctioncallandthefunctionsetupitskipsthepushingof%ebpandthecopyingof%esp.Whensteppingthroughcode,youoftendon'twanttohavetostepthrougheveryinstructionofeveryfunction.Well-testedfunctionsareusuallyawasteoftimetostepthroughexceptonrareoccasion.Therefore,ifyouusethenexticommandinsteadofthestepicommand,GDBwillwaituntilcompletionofthefunctionbeforegoingon.Otherwise,withstepi,GDBwouldstepyouthrougheveryinstructionwithineverycalledfunction. WarningOneproblemthatGDBhasiswithhandlinginterrupts.OftentimesGDBwillmisstheinstructionthatimmediatelyfollowsaninterrupt.Theinstructionisactuallyexecuted,butGDBdoesn'tstepthroughit.Thisshouldnotbeaproblem-justbeawarethatitmayhappen. GDBQuick-ReferenceThisquick-referencetableiscopyright2002RobertM.Dondero,Jr.,andisusedbypermissioninthisbook.Parameterslistedinbracketsareoptional. 294 PAGE 301 AppendixF.UsingtheGDBDebugger TableF-1.CommonGDBDebuggingCommands Miscellaneous quit ExitGDB help[cmd] Printdescriptionofdebuggercommandcmd.Withoutcmd,printsalistoftopics. directory[dir1][dir2]... Adddirectoriesdir1,dir2,etc.tothelistofdirectoriessearchedforsourceles. RunningtheProgram run[arg1][arg2]... Runtheprogramwithcommandlineargumentsarg1,arg2,etc. setargsarg1[arg2]... Settheprogram'scommand-lineargumentstoarg1,arg2,etc. showargs Printtheprogram'scommand-linearguments. UsingBreakpoints infobreakpoints Printalistofallbreakpointsandtheirnumbersbreakpointnumbersareusedforotherbreakpointcommands. breaklinenum Setabreakpointatlinenumberlinenum. break*addr Setabreakpointatmemoryaddressaddr. breakfn Setabreakpointatthebeginningoffunctionfn. conditionbpnumexpr Breakatbreakpointbpnumonlyifexpressionexprisnon-zero. 295 PAGE 302 AppendixF.UsingtheGDBDebugger UsingBreakpoints command[bpnum]cmd1[cmd2]... Executecommandscmd1,cmd2,etc.wheneverbreakpointbpnumorthecurrentbreakpointishit. continue Continueexecutingtheprogram. kill Stopexecutingtheprogram. delete[bpnum1][bpnum2]... Deletebreakpointsbpnum1,bpnum2,etc.,orallbreakpointsifnonespecied. clear*addr Clearthebreakpointatmemoryaddressaddr. clear[fn] Clearthebreakpointatfunctionfn,orthecurrentbreakpoint. clearlinenum Clearthebreakpointatlinenumberlinenum. disable[bpnum1][bpnum2]... Disablebreakpointsbpnum1,bpnum2,etc.,orallbreakpointsifnonespecied. enable[bpnum1][bpnum2]... Enablebreakpointsbpnum1,bpnum2,etc.,orallbreakpointsifnonespecied. SteppingthroughtheProgram nexti "Stepover"thenextinstructiondoesn'tfollowfunctioncalls. stepi "Stepinto"thenextinstructionfollowsfunctioncalls. nish "Stepout"ofthecurrentfunction. ExaminingRegistersandMemory inforegisters Printthecontentsofallregisters. 296 PAGE 303 AppendixF.UsingtheGDBDebugger ExaminingRegistersandMemory print/f$reg Printthecontentsofregisterregusingformatf.Theformatcanbexhexadecimal,uunsigneddecimal,ooctal,aaddress,ccharacter,orfoatingpoint. x/rsfaddr Printthecontentsofmemoryaddressaddrusingrepeatcountr,sizes,andformatf.Repeatcountdefaultsto1ifnotspecied.Sizecanbebbyte,hhalfword,wword,orgdoubleword.Sizedefaultstowordifnotspecied.Formatisthesameasforprint,withtheadditionsofsstringandiinstruction. infodisplay Showsanumberedlistofexpressionssetuptodisplayautomaticallyateachbreak. display/f$reg Ateachbreak,printthecontentsofregisterregusingformatf. display/siaddr Ateachbreak,printthecontentsofmemoryaddressaddrusingsizessameoptionsasforthexcommand. display/ssaddr Ateachbreak,printthestringofsizesthatbeginsinmemoryaddressaddr. undisplaydisplaynum Removedisplaynumfromthedisplaylist. ExaminingtheCallStack where Printthecallstack. backtrace Printthecallstack. 297 PAGE 304 AppendixF.UsingtheGDBDebugger ExaminingtheCallStack frame Printthetopofthecallstack. up Movethecontexttowardthebottomofthecallstack. down Movethecontexttowardthetopofthecallstack. 298 PAGE 305 AppendixG.DocumentHistory 12/17/2002-Version0.5-InitialpostingofbookunderGNUFDL 07/18/2003-Version0.6-AddedASCIIappendix,nishedthediscussionoftheCPUintheMemorychapter,reworkedexercisesintoanewformat,correctedseveralerrors.ThankstoHaraldKorneliussenforthemanysuggestionsandtheASCIItable. 01/11/2004-Version0.7-AddedCtranslationappendix,addedthebeginningsofanappendixofx86instructions,addedthebeginningsofaGDBappendix,nishedouttheleschapter,nishedoutthecountingchapter,addedarecordschapter,createdasourceleofcommonlinuxdenitions,correctedseveralerrors,andlotsofotherxes 01/22/2004-Version0.8-FinishedGDBappendix,mostlynishedw/appendixofx86instructions,addedsectiononplanningprograms,addedlotsofreviewquestions,andgoteverythingtoacompleted,initialdraftstate. 01/29/2004-Version0.9-Lotsofedittingofallchapters.Madecodemoreconsistentandmadeexplanationsclearer.Addedsomeillustrations. 01/31/2004-Version1.0-Rewrotechapter9.Addedfullindex.Lotsofminorcorrections. 299 PAGE 306 AppendixG.DocumentHistory 300 PAGE 307 AppendixH.GNUFreeDocumentationLicense0.PREAMBLEThepurposeofthisLicenseistomakeamanual,textbook,orotherwrittendocumentfreeinthesenseoffreedom:toassureeveryonetheeffectivefreedomtocopyandredistributeit,withorwithoutmodifyingit,eithercommerciallyornoncommercially.Secondarily,thisLicensepreservesfortheauthorandpublisherawaytogetcreditfortheirwork,whilenotbeingconsideredresponsibleformodicationsmadebyothers.ThisLicenseisakindofcopyleft,whichmeansthatderivativeworksofthedocumentmustthemselvesbefreeinthesamesense.ItcomplementstheGNUGeneralPublicLicense,whichisacopyleftlicensedesignedforfreesoftware.WehavedesignedthisLicenseinordertouseitformanualsforfreesoftware,becausefreesoftwareneedsfreedocumentation:afreeprogramshouldcomewithmanualsprovidingthesamefreedomsthatthesoftwaredoes.ButthisLicenseisnotlimitedtosoftwaremanuals;itcanbeusedforanytextualwork,regardlessofsubjectmatterorwhetheritispublishedasaprintedbook.WerecommendthisLicenseprincipallyforworkswhosepurposeisinstructionorreference.1.APPLICABILITYANDDEFINITIONSThisLicenseappliestoanymanualorotherworkthatcontainsanoticeplacedbythecopyrightholdersayingitcanbedistributedunderthetermsofthisLicense.TheDocument,below,referstoanysuchmanualorwork.Anymemberofthepublicisalicensee,andisaddressedasyou.AModiedVersionoftheDocumentmeansanyworkcontainingtheDocumentoraportionofit,eithercopiedverbatim,orwithmodicationsand/ortranslatedintoanotherlanguage.ASecondarySectionisanamedappendixorafront-mattersectionoftheDocumentthatdealsexclusivelywiththerelationshipofthepublishersorauthorsoftheDocumenttotheDocument'soverallsubjectortorelatedmattersand 301 PAGE 308 AppendixH.GNUFreeDocumentationLicense containsnothingthatcouldfalldirectlywithinthatoverallsubject.Forexample,iftheDocumentisinpartatextbookofmathematics,aSecondarySectionmaynotexplainanymathematics.Therelationshipcouldbeamatterofhistoricalconnectionwiththesubjectorwithrelatedmatters,oroflegal,commercial,philosophical,ethicalorpoliticalpositionregardingthem.TheInvariantSectionsarecertainSecondarySectionswhosetitlesaredesignated,asbeingthoseofInvariantSections,inthenoticethatsaysthattheDocumentisreleasedunderthisLicense.TheCoverTextsarecertainshortpassagesoftextthatarelisted,asFront-CoverTextsorBack-CoverTexts,inthenoticethatsaysthattheDocumentisreleasedunderthisLicense.ATransparentcopyoftheDocumentmeansamachine-readablecopy,representedinaformatwhosespecicationisavailabletothegeneralpublic,whosecontentscanbeviewedandediteddirectlyandstraightforwardlywithgenerictexteditorsorforimagescomposedofpixelsgenericpaintprogramsorfordrawingssomewidelyavailabledrawingeditor,andthatissuitableforinputtotextformattersorforautomatictranslationtoavarietyofformatssuitableforinputtotextformatters.AcopymadeinanotherwiseTransparentleformatwhosemarkuphasbeendesignedtothwartordiscouragesubsequentmodicationbyreadersisnotTransparent.AcopythatisnotTransparentiscalledOpaque.ExamplesofsuitableformatsforTransparentcopiesincludeplainASCIIwithoutmarkup,Texinfoinputformat,LaTeXinputformat,SGMLorXMLusingapubliclyavailableDTD,andstandard-conformingsimpleHTMLdesignedforhumanmodication.OpaqueformatsincludePostScript,PDF,proprietaryformatsthatcanbereadandeditedonlybyproprietarywordprocessors,SGMLorXMLforwhichtheDTDand/orprocessingtoolsarenotgenerallyavailable,andthemachine-generatedHTMLproducedbysomewordprocessorsforoutputpurposesonly.TheTitlePagemeans,foraprintedbook,thetitlepageitself,plussuchfollowingpagesasareneededtohold,legibly,thematerialthisLicenserequiresto 302 PAGE 309 AppendixH.GNUFreeDocumentationLicense appearinthetitlepage.Forworksinformatswhichdonothaveanytitlepageassuch,TitlePagemeansthetextnearthemostprominentappearanceofthework'stitle,precedingthebeginningofthebodyofthetext.2.VERBATIMCOPYINGYoumaycopyanddistributetheDocumentinanymedium,eithercommerciallyornoncommercially,providedthatthisLicense,thecopyrightnotices,andthelicensenoticesayingthisLicenseappliestotheDocumentarereproducedinallcopies,andthatyouaddnootherconditionswhatsoevertothoseofthisLicense.Youmaynotusetechnicalmeasurestoobstructorcontrolthereadingorfurthercopyingofthecopiesyoumakeordistribute.However,youmayacceptcompensationinexchangeforcopies.Ifyoudistributealargeenoughnumberofcopiesyoumustalsofollowtheconditionsinsection3.Youmayalsolendcopies,underthesameconditionsstatedabove,andyoumaypubliclydisplaycopies.3.COPYINGINQUANTITYIfyoupublishprintedcopiesoftheDocumentnumberingmorethan100,andtheDocument'slicensenoticerequiresCoverTexts,youmustenclosethecopiesincoversthatcarry,clearlyandlegibly,alltheseCoverTexts:Front-CoverTextsonthefrontcover,andBack-CoverTextsonthebackcover.Bothcoversmustalsoclearlyandlegiblyidentifyyouasthepublisherofthesecopies.Thefrontcovermustpresentthefulltitlewithallwordsofthetitleequallyprominentandvisible.Youmayaddothermaterialonthecoversinaddition.Copyingwithchangeslimitedtothecovers,aslongastheypreservethetitleoftheDocumentandsatisfytheseconditions,canbetreatedasverbatimcopyinginotherrespects.Iftherequiredtextsforeithercoveraretoovoluminoustotlegibly,youshouldputtherstoneslistedasmanyastreasonablyontheactualcover,andcontinuetherestontoadjacentpages.IfyoupublishordistributeOpaquecopiesoftheDocumentnumberingmorethan100,youmusteitherincludeamachine-readableTransparentcopyalongwith 303 PAGE 310 AppendixH.GNUFreeDocumentationLicense eachOpaquecopy,orstateinorwitheachOpaquecopyapublicly-accessiblecomputer-networklocationcontainingacompleteTransparentcopyoftheDocument,freeofaddedmaterial,whichthegeneralnetwork-usingpublichasaccesstodownloadanonymouslyatnochargeusingpublic-standardnetworkprotocols.Ifyouusethelatteroption,youmusttakereasonablyprudentsteps,whenyoubegindistributionofOpaquecopiesinquantity,toensurethatthisTransparentcopywillremainthusaccessibleatthestatedlocationuntilatleastoneyearafterthelasttimeyoudistributeanOpaquecopydirectlyorthroughyouragentsorretailersofthateditiontothepublic.Itisrequested,butnotrequired,thatyoucontacttheauthorsoftheDocumentwellbeforeredistributinganylargenumberofcopies,togivethemachancetoprovideyouwithanupdatedversionoftheDocument.4.MODIFICATIONSYoumaycopyanddistributeaModiedVersionoftheDocumentundertheconditionsofsections2and3above,providedthatyoureleasetheModiedVersionunderpreciselythisLicense,withtheModiedVersionllingtheroleoftheDocument,thuslicensingdistributionandmodicationoftheModiedVersiontowhoeverpossessesacopyofit.Inaddition,youmustdothesethingsintheModiedVersion: A.UseintheTitlePageandonthecovers,ifanyatitledistinctfromthatoftheDocument,andfromthoseofpreviousversionswhichshould,iftherewereany,belistedintheHistorysectionoftheDocument.Youmayusethesametitleasapreviousversioniftheoriginalpublisherofthatversiongivespermission. B.ListontheTitlePage,asauthors,oneormorepersonsorentitiesresponsibleforauthorshipofthemodicationsintheModiedVersion,togetherwithatleastveoftheprincipalauthorsoftheDocumentallofitsprincipalauthors,ifithaslessthanve. 304 PAGE 311 AppendixH.GNUFreeDocumentationLicense C.StateontheTitlePagethenameofthepublisheroftheModiedVersion,asthepublisher. D.PreserveallthecopyrightnoticesoftheDocument. E.Addanappropriatecopyrightnoticeforyourmodicationsadjacenttotheothercopyrightnotices. F.Include,immediatelyafterthecopyrightnotices,alicensenoticegivingthepublicpermissiontousetheModiedVersionunderthetermsofthisLicense,intheformshownintheAddendumbelow. G.PreserveinthatlicensenoticethefulllistsofInvariantSectionsandrequiredCoverTextsgivenintheDocument'slicensenotice. H.IncludeanunalteredcopyofthisLicense. I.PreservethesectionentitledHistory,anditstitle,andaddtoitanitemstatingatleastthetitle,year,newauthors,andpublisheroftheModiedVersionasgivenontheTitlePage.IfthereisnosectionentitledHistoryintheDocument,createonestatingthetitle,year,authors,andpublisheroftheDocumentasgivenonitsTitlePage,thenaddanitemdescribingtheModiedVersionasstatedintheprevioussentence. J.Preservethenetworklocation,ifany,givenintheDocumentforpublicaccesstoaTransparentcopyoftheDocument,andlikewisethenetworklocationsgivenintheDocumentforpreviousversionsitwasbasedon.ThesemaybeplacedintheHistorysection.YoumayomitanetworklocationforaworkthatwaspublishedatleastfouryearsbeforetheDocumentitself,oriftheoriginalpublisheroftheversionitreferstogivespermission. K.InanysectionentitledAcknowledgementsorDedications,preservethesection'stitle,andpreserveinthesectionallthesubstanceandtoneofeachofthecontributoracknowledgementsand/ordedicationsgiventherein. L.PreservealltheInvariantSectionsoftheDocument,unalteredintheirtextandintheirtitles.Sectionnumbersortheequivalentarenotconsideredpartof 305 PAGE 312 AppendixH.GNUFreeDocumentationLicense thesectiontitles. M.DeleteanysectionentitledEndorsements.SuchasectionmaynotbeincludedintheModiedVersion. N.DonotretitleanyexistingsectionasEndorsementsortoconictintitlewithanyInvariantSection.IftheModiedVersionincludesnewfront-mattersectionsorappendicesthatqualifyasSecondarySectionsandcontainnomaterialcopiedfromtheDocument,youmayatyouroptiondesignatesomeorallofthesesectionsasinvariant.Todothis,addtheirtitlestothelistofInvariantSectionsintheModiedVersion'slicensenotice.Thesetitlesmustbedistinctfromanyothersectiontitles.YoumayaddasectionentitledEndorsements,provideditcontainsnothingbutendorsementsofyourModiedVersionbyvariousparties--forexample,statementsofpeerrevieworthatthetexthasbeenapprovedbyanorganizationastheauthoritativedenitionofastandard.YoumayaddapassageofuptovewordsasaFront-CoverText,andapassageofupto25wordsasaBack-CoverText,totheendofthelistofCoverTextsintheModiedVersion.OnlyonepassageofFront-CoverTextandoneofBack-CoverTextmaybeaddedbyorthrougharrangementsmadebyanyoneentity.IftheDocumentalreadyincludesacovertextforthesamecover,previouslyaddedbyyouorbyarrangementmadebythesameentityyouareactingonbehalfof,youmaynotaddanother;butyoumayreplacetheoldone,onexplicitpermissionfromthepreviouspublisherthataddedtheoldone.TheauthorsandpublishersoftheDocumentdonotbythisLicensegivepermissiontousetheirnamesforpublicityforortoassertorimplyendorsementofanyModiedVersion.5.COMBININGDOCUMENTSYoumaycombinetheDocumentwithotherdocumentsreleasedunderthisLicense,underthetermsdenedinsection4aboveformodiedversions,providedthatyouincludeinthecombinationalloftheInvariantSectionsofallof 306 PAGE 313 AppendixH.GNUFreeDocumentationLicense theoriginaldocuments,unmodied,andlistthemallasInvariantSectionsofyourcombinedworkinitslicensenotice.ThecombinedworkneedonlycontainonecopyofthisLicense,andmultipleidenticalInvariantSectionsmaybereplacedwithasinglecopy.IftherearemultipleInvariantSectionswiththesamenamebutdifferentcontents,makethetitleofeachsuchsectionuniquebyaddingattheendofit,inparentheses,thenameoftheoriginalauthororpublisherofthatsectionifknown,orelseauniquenumber.MakethesameadjustmenttothesectiontitlesinthelistofInvariantSectionsinthelicensenoticeofthecombinedwork.Inthecombination,youmustcombineanysectionsentitledHistoryinthevariousoriginaldocuments,formingonesectionentitledHistory;likewisecombineanysectionsentitledAcknowledgements,andanysectionsentitledDedications.YoumustdeleteallsectionsentitledEndorsements.6.COLLECTIONSOFDOCUMENTSYoumaymakeacollectionconsistingoftheDocumentandotherdocumentsreleasedunderthisLicense,andreplacetheindividualcopiesofthisLicenseinthevariousdocumentswithasinglecopythatisincludedinthecollection,providedthatyoufollowtherulesofthisLicenseforverbatimcopyingofeachofthedocumentsinallotherrespects.Youmayextractasingledocumentfromsuchacollection,anddispbibuteitindividuallyunderthisLicense,providedyouinsertacopyofthisLicenseintotheextracteddocument,andfollowthisLicenseinallotherrespectsregardingverbatimcopyingofthatdocument.7.AGGREGATIONWITHINDEPENDENTWORKSAcompilationoftheDocumentoritsderivativeswithotherseparateandindependentdocumentsorworks,inoronavolumeofastorageordistributionmedium,doesnotasawholecountasaModiedVersionoftheDocument,providednocompilationcopyrightisclaimedforthecompilation.Suchacompilationiscalledanaggregate,andthisLicensedoesnotapplytotheother 307 PAGE 314 AppendixH.GNUFreeDocumentationLicense self-containedworksthuscompiledwiththeDocument,onaccountoftheirbeingthuscompiled,iftheyarenotthemselvesderivativeworksoftheDocument.IftheCoverTextrequirementofsection3isapplicabletothesecopiesoftheDocument,theniftheDocumentislessthanonequarteroftheentireaggregate,theDocument'sCoverTextsmaybeplacedoncoversthatsurroundonlytheDocumentwithintheaggregate.Otherwisetheymustappearoncoversaroundthewholeaggregate.8.TRANSLATIONTranslationisconsideredakindofmodication,soyoumaydistributetranslationsoftheDocumentunderthetermsofsection4.ReplacingInvariantSectionswithtranslationsrequiresspecialpermissionfromtheircopyrightholders,butyoumayincludetranslationsofsomeorallInvariantSectionsinadditiontotheoriginalversionsoftheseInvariantSections.YoumayincludeatranslationofthisLicenseprovidedthatyoualsoincludetheoriginalEnglishversionofthisLicense.IncaseofadisagreementbetweenthetranslationandtheoriginalEnglishversionofthisLicense,theoriginalEnglishversionwillprevail.9.TERMINATIONYoumaynotcopy,modify,sublicense,ordistributetheDocumentexceptasexpresslyprovidedforunderthisLicense.Anyotherattempttocopy,modify,sublicenseordistributetheDocumentisvoid,andwillautomaticallyterminateyourrightsunderthisLicense.However,partieswhohavereceivedcopies,orrights,fromyouunderthisLicensewillnothavetheirlicensesterminatedsolongassuchpartiesremaininfullcompliance.10.FUTUREREVISIONSOFTHISLICENSETheFreeSoftwareFoundationmaypublishnew,revisedversionsoftheGNUFreeDocumentationLicensefromtimetotime.Suchnewversionswillbesimilarinspirittothepresentversion,butmaydifferindetailtoaddressnewproblemsorconcerns.Seehttp://www.gnu.org/copyleft/.EachversionoftheLicenseisgivenadistinguishingversionnumber.Ifthe 308 PAGE 315 AppendixH.GNUFreeDocumentationLicense DocumentspeciesthataparticularnumberedversionofthisLicenseoranylaterversionappliestoit,youhavetheoptionoffollowingthetermsandconditionseitherofthatspeciedversionorofanylaterversionthathasbeenpublishednotasadraftbytheFreeSoftwareFoundation.IftheDocumentdoesnotspecifyaversionnumberofthisLicense,youmaychooseanyversioneverpublishednotasadraftbytheFreeSoftwareFoundation.AddendumTousethisLicenseinadocumentyouhavewritten,includeacopyoftheLicenseinthedocumentandputthefollowingcopyrightandlicensenoticesjustafterthetitlepage:CopyrightYEARYOURNAME.Permissionisgrantedtocopy,distributeand/ormodifythisdocumentunderthetermsoftheGNUFreeDocumentationLicense,Version1.1oranylaterversionpublishedbytheFreeSoftwareFoundation;withtheInvariantSectionsbeingLISTTHEIRTITLES,withtheFront-CoverTextsbeingLIST,andwiththeBack-CoverTextsbeingLIST.AcopyofthelicenseisincludedinthesectionentitledGNUFreeDocumentationLicense.IfyouhavenoInvariantSections,writewithnoInvariantSectionsinsteadofsayingwhichonesareinvariant.IfyouhavenoFront-CoverTexts,writenoFront-CoverTextsinsteadofFront-CoverTextsbeingLIST;likewiseforBack-CoverTexts.Ifyourdocumentcontainsnontrivialexamplesofprogramcode,werecommendreleasingtheseexamplesinparallelunderyourchoiceoffreesoftwarelicense,suchastheGNUGeneralPublicLicense,topermittheiruseinfreesoftware. 309 PAGE 316 AppendixH.GNUFreeDocumentationLicense 310 PAGE 317 AppendixI.PersonalDedicationTherearesomanypeopleIcouldthank.IwillnameherebutafewofthepeoplewhohavebroughtmetowhereIamtoday.Themanyfamilymembers,SundaySchoolteachers,youthpastors,schoolteachers,friends,andotherrelationshipsthatGodhasbroughtintomylifetoleadme,helpme,andteachmearetoomanytocount.Thisbookisdedicatedtoyouall.Therearesomepeople,however,thatIwouldliketothankspecically.Firstofall,IwanttothankthemembersoftheVineyardChristianFellowshipChurchinChampaign,Illinoisforeverythingthatyouhavedonetohelpmeandmyfamilyinourtimesofcrisis.It'sbeenalongtimesinceI'veseenorheardfromanyofyou,butIthinkaboutyoualways.Youhavebeensuchablessingtome,mywife,andDaniel,andIcouldneverthankyouenoughforshowingusChrist'slovewhenweneededitmost.IthankGodeverytimeIthinkofyou-IthankHimforbringingyoualltousinourdeepesttimesofneed.EvenoutinthemiddleofIllinoiswithnofriendsoffamily,GodshowedthatHewasstillwatchingafterus.ThankyouforbeingHishandsonEarth.Specically,I'dliketothankJoeandRhonda,PamandDell,andHerschelandVicki.Thereweremany,manyothers,too-somanypeoplehelpedusthatitwouldbeimpossibletolistthemall.Ialsowanttothankmyparents,whogavemetheexampleofperserveranceandstrengthinhardtimes.Yourexamplehashelpedmebeagoodfathertomychildren,andagoodhusbandtomywife.Ialsowanttothankmywife,whoevenfromwhenwerststarteddatingencouragedmetoseekGodineverything.Thankyouforyoursupportinwritingthisbook,andmoreimportantly,foryoursupportinbeingobedienttoGod.IalsowanttothankstheLittleLightHouseschool.Myentirefamilyiscontinuallyblessedbythehelpyougivetoourson.IalsowanttothankJoeandD.A.Thankyoufortakingachanceonmeinministry.BeingabletobeapartofGod'sministryagainhashelpedmeinsomanyways. 311 PAGE 318 AppendixI.PersonalDedication YouallhavegivenmethestrengthIneededtowritethisbookoverthelastfewyears.Withoutyoursupport,Iwouldhavebeentoooverwhelmedbypersonalcrisestoeventhinkaboutanythingmorethangettingthroughaday,muchlessputtingthisbooktogether.Youhaveallbeenagreatblessingtome,andIwillkeepyouinmyprayersalways. 312 PAGE 319 Index$?, 22 28 %ah, 44 %al, 44 %ax, 44 %eax, 25 41 57 68 75 91 115 118 125 135 259 271 %ebp, 25 55 62 173 294 %ebx, 25 41 68 75 91 271 %ecx, 25 75 91 263 271 282 %edi, 25 %edx, 25 75 91 259 271 %eags, 25 39 257 258 %eip, 25 54 %esi, 25 %esp, 25 53 149 173 287 294 *, 135 -dynamic-linker, 132 ..., 135 ./, 21 .ascii, 34 266 .bss, 77 149 .byte, 34 77 266 .data, 77 103 149 .endr, 100 266 .equ, 80 111 266 283 .globl, 24 35 104 266 .include, 104 266 .int, 34 .lcomm, 78 266 .long, 33 266 .rept, 100 266 .section, 23 149 266 .text, 23 77 149 .type, 62 69 266 /etc/ld.so.conf, 134 143 /lib, 140 142 /lib/ld-linux.so.2, 134 /usr/include, 217 /usr/lib, 140 142 /usr/local/include, 217 0x80, 27 80 @functions, 62 34 n, 34 t, 35 _start, 23 35 ABI, 58 adcl, 259 addl, 24 259 address, 11 111 284 addressingmodes, 15 41 228 alignedmemory, 228 AND, 188 andl, 261 ApplicationBinaryInterface, 58 argv, 92 arithmeticandlogicunit, 9 array, 139 as, 21 ASCII, 11 89 assemble, 20 assembler, 24 111 313 PAGE 320 assemblerdirectives, 23 AssemblyLanguage, 6 20 213 assert, 120 AT&Tsyntax, 267 auxiliarycarryag, 258 backtrace, 294 basecase, 64 basepointer, 57 basepointeraddressingmode, 16 43 54 69 96 228 279 284 basepointerregister, 55 baseten, 184 basetwo, 182 184 big-endian, 202 binary, 184 199 binarydigit, 186 binarynumber, 194 binaryoperations, 186 binaryoperators, 192 bits, 186 187 201 blockstructuredlanguage, 277 booleanalgebra, 192 booleanoperators, 192 branchprediction, 10 break, 293 294 breakpoints, 293 brk, 155 167 271 buffer, 88 buffers, 76 bytes, 11 44 186 198 200 201 Clanguagecallingconvention, 54 Cprogramminglanguage, 52 135 215 273 277 cache, 227 cachehierarchies, 10 caching, 226 call, 52 54 62 67 263 278 callingconvention, 58 callingconventions, 52 58 callinginterface, 135 carryag, 196 258 casestatement, 278 cdq, 259 char, 135 chdir, 271 clear, 294 close, 76 271 cmpl, 39 196 259 command, 294 command-line, 79 91 comments, 22 compilers, 213 computerarchitecture, 7 computermemory, 11 condition, 277 294 conditionaljump, 30 196 constants, 88 96 104 111 contextswitch, 174 continue, 294 coprocessors, 10 cornercases, 119 CPU, 7 9 currentbreak, 150 166 314 PAGE 321 databus, 9 datasection, 23 33 database, 95 decimal, 184 199 decl, 63 70 259 delete, 294 destinationoperand, 43 digit, 184 directaddressingmode, 15 26 42 78 139 228 279 directory, 294 disable, 294 display, 294 divl, 259 DLLs, 130 documentation, 223 double, 139 down, 294 drivers, 121 dup, 271 dynamiclinker, 132 134 dynamiclinking, 133 dynamicmemoryallocation, 156 dynamic-linklibraries, 130 dynamically-linked, 133 echo, 22 28 edgecases, 119 effectiveaddress, 286 ELF, 143 enable, 294 errorchecking, 126 errorcode, 121 errorconditions, 117 118 errormessages, 121 122 exit, 26 133 271 exitstatuscode, 22 27 35 exponent, 197 false, 190 falsebranch, 277 fclose, 141 fetch-executecycle, 9 fgets, 141 elds, 95 ledescriptors, 75 78 les, 75 79 nish, 294 ags, 194 257 oat, 138 owcontrol, 30 39 215 fopen, 141 fprintf, 141 fputs, 141 frame, 294 functioncall, 59 126 278 functionparameters, 55 89 functions, 49 54 64 129 135 194 GCC, 4 286 GDB, 290 general-purposeregisters, 9 25 getpid, 271 globaloptimizations, 225 229 globalvariables, 51 57 279 GNOME, 239 GNU/Linux, 3 315 PAGE 322 gprof, 225 GUI, 247 GUIbuilder, 255 heap, 156 166 help, 294 hexadecimal, 27 199 high-levellanguages, 6 218 idivl, 25 259 ifstatement, 277 immediatemodeaddressing, 15 26 43 56 111 139 228 285 imull, 24 63 259 incl, 40 70 259 index, 36 indexregister, 15 37 42 indexedaddressingmode, 15 37 42 indexedindirectaddressingmode, 90 228 indirectaddressingmode, 15 42 54 286 inniteloop, 31 info, 294 infodisplay, 294 inforegister, 292 inforegisters, 294 inlinefunctions, 227 instruction, 126 instructiondecoder, 9 instructionpointer, 13 54 int, 27 135 201 263 Intelsyntax, 267 interpreter, 214 interrupts, 27 294 ioctl, 271 Jcc, 263 jmp, 62 196 263 kernel, 4 kernelmode, 174 kill, 294 Knoppix, 4 labels, 24 33 50 80 Larry-Boy, 27 ld, 21 ldd, 134 LD_LIBRARY_PATH, 134 143 leal, 258 286 leave, 287 link, 20 linker, 21 104 Linux, 4 27 little-endian, 201 localoptimizations, 225 localvariables, 51 55 63 64 279 285 localityofreference, 226 logicaloperations, 188 long, 138 longlong, 138 loop, 38 282 291 loops, 31 281 lseek, 271 MachineLanguage, 5 macros, 227 mantissa, 197 mapping, 152 masking, 193 316 PAGE 323 memoizing, 226 memory, 7 201 memoryaddress, 41 memorymanager, 156 memorypages, 154 microcodetranslation, 10 mkdir, 271 movb, 44 258 movl, 24 36 258 mull, 259 multiplier, 15 37 42 negativenumbers, 197 negl, 259 newline, 34 nexti, 294 294 NOT, 188 198 notl, 261 null, 100 nullcharacter, 91 135 nullcharacters, 104 105 objdump, 140 objectle, 21 octal, 91 199 offset, 16 offsets, 96 one'scomplement, 261 open, 75 91 194 271 operands, 24 optimization, 223 OR, 188 orl, 261 out-of-orderexecution, 10 overowag, 257 O_APPEND, 195 O_CREAT, 195 O_RDWR, 194 O_TRUNC, 195 O_WRONLY, 194 pad, 104 padding, 100 pages, 154 parallelization, 229 parameter, 69 parameters, 26 49 50 54 67 135 parityag, 258 Perl, 218 permissions, 75 91 199 persistance, 75 persistent, 95 physicaladdress, 152 physicalmemory, 152 227 280 pipe, 271 pipelining, 10 pipes, 79 pointer, 53 63 pointers, 12 14 284 popl, 53 258 precision, 197 preprocessor, 216 primitivefunctions, 50 primitives, 50 print, 292 294 printf, 133 135 proler, 224 317 PAGE 324 programcounter, 9 programstatusregister, 195 programming, 1 prototype, 135 prototypes, 138 pseudo-operations, 23 pushl, 53 67 258 Python, 219 QTDesigner, 255 quit, 294 rcll, 261 rcrl, 261 read, 76 92 271 records, 95 recoverypoints, 122 recursive, 64 register, 63 190 203 registeraddressingmode, 15 43 registers, 10 12 27 36 58 186 201 227 regularles, 79 rep, 282 residentsetsize, 155 ret, 52 57 263 287 returnaddress, 51 54 89 returnvalue, 52 58 68 returnvalues, 69 rmdir, 271 robust, 117 117 119 roll, 261 rorl, 261 rotate, 192 run, 290 294 sall, 261 sarl, 199 261 sbbl, 259 set, 294 sharedlibraries, 129 132 sharedlibrary, 142 sharedobjects, 130 shift, 192 shifting, 193 shll, 261 short, 138 show, 294 shrl, 199 261 SIGINT, 290 sign, 197 signextension, 199 signag, 257 signed, 199 signednumbers, 198 skeletoncode, 129 sourcecode, 20 sourcele, 21 sourceoperand, 43 specialles, 79 specialregister, 55 special-purposeregister, 13 special-purposeregisters, 10 25 stack, 53 stackframe, 55 65 stackmemory, 53 stackpointer, 55 318 PAGE 325 stackregister, 53 standarderror, 79 standardinput, 78 standardoutput, 79 statelessfunctions, 229 staticvariables, 51 57 279 statically-linked, 133 statuscode, 28 118 statusregister, 39 STDERR, 79 STDIN, 78 stdio.h, 216 STDOUT, 79 stepi, 291 294 294 strcmp, 141 strdup, 141 strlen, 105 141 struct, 139 structs, 282 structureddata, 95 stub, 121 subl, 24 259 superscalarprocessors, 10 swapdeath, 155 swapping, 154 switches, 187 symbol, 23 50 symbols, 133 systembreak, 150 systemcall, 26 41 126 194 systemcalls, 26 50 273 tab, 35 testing, 118 testl, 261 textsection, 23 true, 190 truebranch, 277 two'scomplement, 198 259 typedef, 140 unconditionaljump, 30 196 undisplay, 294 UNIXmanual, 273 unsigned, 139 199 unstructureddata, 95 up, 294 usermode, 174 variables, 35 virtualaddress, 152 virtualmemory, 152 155 volatile, 280 VonNeumannarchitecture, 7 9 where, 294 whileloop, 281 Win32, 255 word, 12 44 200 write, 76 93 271 x, 294 xchgl, 258 XOR, 188 xorl, 261 zeroag, 258 319 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| MILLISECOND | CLASS.METHOD | MESSAGE |
|---|---|---|
| 0 | sobekcm_page_globals.constructor | |
| 0 | sobekcm_page_globals.constructor | Application State validated or built |
| 0 | sobekcm_database.verify_item_lookup_object | |
| 0 | sobekcm_page_globals.constructor | Navigation Object created from URI query string |
| 0 | sobekcm_database.verify_item_lookup_object | |
| 0 | sobekcm_page_globals.display_item | Retrieving item or group information |
| 0 | sobekcm_page_globals.get_entire_collection_hierarchy | Retrieving hierarchy information |
| 0 | sobekcm_assistant.get_entire_collection_hierarchy | |
| 0 | cached_data_manager.retrieve_item_aggregation | |
| 0 | cached_data_manager.retrieve_item_aggregation | Found item aggregation on local cache |
| 0 | item_aggregation_builder.get_item_aggregation | Found 'all' item aggregation in cache |
| 0 | system.web.ui.page.page_load (ufdc.page_load) | |
| 0 | sobekcm_page_globals.constructor.on_page_load | |
| 0 | html_echo_mainwriter.add_style_references | Adding style references to HTML |
| 0 | html_echo_mainwriter.add_text_to_page | Reading the text from the file and echoing back to the output stream |
| 97 | html_echo_mainwriter.add_text_to_page | Finished reading and writing the file |