w04AssemblyLanguageBasics.docx

w04AssemblyLanguageBasics.docx

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w04AssemblyLanguageBasics

Module4:

AssemblyLanguageBasics

Module4Overview

Thex86Architecture

Objectives

WhatYouWillLearn

Attheendofthissection,thestudentwillbeableto:

∙DiscussthebasicarchitectureoftheIntel®x86familyofmicroprocessors

∙DiscussRegistersandtheirfunctions

∙DiscussFlagsandhowtheyareused

Intelx86Architecture

Pentiumi-386isa32-bitCISC（ComplexInstructionSetComputer）processor,whereastheAPLHAisRISC-based（ReducedInstructionSetComputer）.No16-bitcodeiscoveredinthismodule,howevertheprinciplesaresimilar.

Theprocessorshaveanumberofgeneral-purposeregisters.Registersarespecial-purposememorylocationsthatarelocatedinthemicroprocessoritself.Accessingregistersismuchfasterthanaccessingexternalmemory.Someregistersaregeneralpurpose,whileothershavededicatedfunctions.

General-purposeregistersareusedtoholdvaluesforcomputationandstoreresultsinternally.Registersingeneralarebyte,work,orDWORDaddressable.

Theflagsregisteristheonlybitaddressableregister.Anyinstructionsaffecttheflagsregisterbasedontheresultsofacomputation.Forexample,afterasubtractioninstruction,thezeroflagissetiftheresultiszeroandclearediftheresultisnon-zero.

Assemblyinstructionstake0,1,or2argumentscalloperands.Anoperandcanbeanimmediatevalue,registervalue,orapointertoamemorylocation.ThegeneralsyntaxforIntelAssemblyisoneofthefollowing:

PrefixInstruction[operand1],[operand2];comment

Where:

Prefix

-Anaddresslabelorinstructionmodifier

Instruction

-Instructionbeingexecuted

Operand1

-Typicallythedestinationoperand,butitcanbethesourceoperand（oneargumentinstruction）,oritcanbeboththesourceanddestinationoperands（INCinstruction）

Operand2

-Sourceoperandfortwoargumentinstructions

SoforaMOVEAX,[12341234]instruction,thesourceoperandisthevalueataddress12341234andthedestinationoperationistheEAXregister.Thisinstructionwillmovethevaluestoredat[12341234]intotheEAXregister.

IntelCPURegisters

Intel386+processorscontainavarietyof32-bitregisters.Thei-386familyofprocessorsisknownasnon-orthogonal.Thismeanstheregisterandtheinstructionsetarenotcompletelyinterchangeable.Inotherwords,someinstructionscanonlybeusedwithcertainregisters.Forexample,INandOUTinstructionsarehardwiredtotheaccumulator（EAX）register,ECXisusedforcountingforloopinstructions,andEDIandESIareusedforindexing/stringinstructions.Otheraddressingmodescanonlybeappliedtocertainregisters.

Thei-386familyhassixgeneral-purposeregisters–EAX,EBX,ECX,EDX,EDI,ESI.Eachregisterbeginswiththeletter“E"for“Extended”toaddressthefull32-bitsinsteadofjust16.

Addressing

EAX,EBX,ECX,EDXregisterscanbeaddressedinthefollowingways:

AL

lower8bitsoftheEAXregister

AH

highorderbyteoflowerorderwordoftheEAXregister

AX

lower16bitsoftheEAXregister

EAX

thefull32bitsoftheregister

SimilarlysyntaxexistsforEBX,ECX,andEDX.

Note:

TheESIandEDIregistersarenotbyteaddressable.

RegisterTypes

General-PurposeRegisters

EAXisalsoreferredtoastheAccumulator.Thisregisterisusedthemostandcontainstheresultsofmanyinstructions.ItiscommonforcompiledcodethatreturnsavaluetodosousingtheEAXregister.

ECXspecializesincounting.

EBXandEDXaregeneral-purposeregistersandareusedforaddressingmemoryasapointer,usedasoperandsforlogicandarithmeticfunctions,andcontaintheresultsofinstructions.EAXandECXcanalsobeusedforthesefunctions.

IndexingRegisters

EDI,ESIaregeneral-purposeregistersthatspecializeinindexing.StringinstructionsuseEDIasthedestinationpointer,andESIforthesourcepointer.Therefore,inordertocopyablockofmemoryfromoneplacetoanother,ESIwouldbesetuptopointtothesourceblock,andEDIwouldbesetuptopointtothedestinationblock.ECXwouldbeloadedwiththenumberofbytestotransfer,thedirectionflagwouldbesetuptoincrementordecrement,andthenREPMOVSwouldcopythebytes.

StackRegisters

ESPandEBPareprimarilyusedforstackmanipulationandcontrol.ESPisthestackpointerandusedtopointtothecurrentstacklocation.TheEBPregisterisusedtopointtothestackframeforagivenroutine.Atroutineentry,theEBPregisteristypicallysavedonthestackandthensetequaltothevalueofthecurrentstackpointer.TheEBPisthenusedtoreferenceargumentsandlocalvariables.

FlagRegister

Theflagsregisterisacollectionofsingle-bitflags.Manyinstructionsaltertheflagstodescribetheresultoftheinstruction.Theseflagscanthenbetestedbyconditionaljumpinstructions.

InstructionPointer

EIPistheinstructionpointer.Itpointstothenextinstructiontobeexecuted.EIPismodifiedbyRET,RETI,JMP,CALL,andINTinstructions.ThisregisterisperhapsthemostimportantregisterintheCPUsinceitdirectstheCPUtothenextinstructiontoexecute.

Thex86Instructions

Objectives

WhatYouWillLearn

Aftercompletingthissection,thestudentwillbeableto:

∙RecognizethedifferenttypesofAssemblylanguageinstructions,memory,branch,conditional,stack,etc

∙DescribetheAssemblyinstructionsusedtocreatelocalvariables

∙DescribetheAssemblyinstructionstopassargumentsonthestack

Intelx86Instructions

Onthex86processor,instructionsarevariable-sized;sodisassemblingbackwardsisanexerciseinpatternmatching.Todisassemblebackwardsfromanaddress,youshouldstartdisassemblingatapointfurtherbackthanyoureallywanttogo,thenlookforwarduntiltheinstructionsstartmakingsense.Thefirstfewinstructionsmaynotmakeanysensebecauseyoumayhavestarteddisassemblinginthemiddleofaninstruction.Thereisapossibility,unfortunately,thatthedisassemblywillneversynchronizewiththeinstructionstreamandyouwillhavetotrydisassemblingatadifferentstartingpointuntilyoufindastartingpointthatworks.

InstructionNotation

Thegeneralnotationforinstructionsistoputthedestinationregisterontheleftandthesourceontheright.Theremightbesomeexceptionstothisrule.

PrefixInstruction[operand1],[operand2];comment

Arithmeticinstructionsaretypicallytwo-registerwiththesourceanddestinationregisterscombining.Theresultisstoredintothedestination.

Tosavespace,manyoftheinstructionsareexpressedincombinedform;whichmeansthatthefirstparametermustbearegister,butthesecondcanbearegisteroramemoryreferenceoranimmediatevalue.

MOVEAX,177

ADDEAX,EDX

Tosaveevenmorespace,instructionscanalsobeexpressedwiththefirstparameterasaregisteroramemoryreference,andthesecondcanbearegister,memoryreference,orimmediatevalue.

Unlessotherwisenoted,whenthisabbreviationisused,youcannotchoose"memory"forbothsourceanddestination

Operands

Assemblyinstructionseachtakeavariablenumberofargumentscallsoperands.Thisvariesonthetypeofinstructionandvariesfrom0to2operands.

ImplicitOperands

Implicitoperandsarespecifiedbytheinstructionitself.CLIandSTIareexamplesofinstructionsthatdonotspecifyanoperand.TheCLIandSTIsetandcleartheinterruptflagintheEFLAGSregister.CLIandSTIalwaysapplytothatbitonly.

RegisterOperands

Registeroperandsarewhentheinstructionreferencestheregisterforthesourceordestination.Someinstructionsreferenceasingleregister–whichcanbeeither,orboththesourceanddestination.

Example:

CALLEDI;EDIcontainsaddresstocall

MOVEAX,EBX;EAX<-EBXEAX=destinationEBX=source

INCEAX;EAX=EAX+1–EAX=sourceanddestination

ImmediateOperands

Immediateoperandsarepartoftheinstructionitself.Valuetobeusedfollowstheopcodeintheinstruction.Immediateallowstheusertoloadaconstantintoavariableorreferenceahard-codedorfixed-memorylocation.

Example:

MOVEAX,177;loadEAXwith177

MOVCL,0FFH;loadCLwith255

CALL12341234H;callroutineatlocation12341234H

I/OOperands

I/Ooperandsarenotusedinusermodeprograms.I/OinstructionsareusedtoreferencedevicesintheI/Omap.TheseincludemosthardwaredeviceslikePICS,serialports,parallelports,diskcontrollers,etc.

Example:

INAL,04H;readportlocation4intoALregister

OUT04H,AL;writeALregistertoportlocation4

MemoryReferenceOperands

Memoryreferenceoperandsareusedtoreferencememoryviaamultitudeofaddressingmodes.Thesesaddressingmodesincludedirectaddressing,basedaddressing,baseplusdisplacementaddressing,indexplusdisplacementaddressing,andbaseplusdisplacementplusindexaddressing.

DirectAddressing

Directaddressingiswheretheaddressitselfisusedasanoperand.Thisisusedtoaddressfixedlocationsinaprogram–aglobalvariableforinstance.

Example:

MOVAL,[12341234H]

INCDWORDPTR[12341234H]

BasedAddressing

Basedaddressingusesaregistertoholdtheaddress.Thisisusedtode-referenceapointertoavariable.

Example:

MOVAL,[ECX]

DECDWORDPTR[ESI]

BasePlusDisplacementAddressing

BasePlusDisplacementAddressingissimilartobasedaddressingexceptforanadditionaloffsetthatisaddedtothebaseaddress.Thisisusedtoaccessavariablecontainedinastructure.Thebase（register）pointstothestartofthestructureandtheoffsetallowthecorrectelementtobereferenced.Thisisbetterthanthepointerhavingtopointdirectlytotheelementsinceoftenmorethanonestructureisreferencedbyasec