Introduction. Chapter 4 Macro Processors -- Basic Macro Processor Functions. Macro expansion. Basic macro processor functions

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1 Introduction Chapter 4 Macro Processors -- Basic Macro Processor Functions Amacroinstruction(macro)isanotational convenience for the programmer Itallowstheprogrammertowriteshorthandversionofaprogram (module programming) Themacroprocessorreplaceseachmacro instruction with the corresponding group of source language statements(expanding) Normally,itperformsnoanalysisofthetextithandles Itdoesnotconcernthemeaningoftheinvolvedstatementsduring macro expansion Thedesignofamacroprocessorgenerallyis machine independent! Basic macro processor functions Macro expansion Twonewassemblerdirectivesareusedinmacro definition MACRO:identifythebeginningofamacrodefinition MEND:identifytheendofamacrodefinition Prototypeforthemacro Eachparameterbeginswith & name MACRO parameters : body : MEND Body:thestatementsthatwillbegeneratedastheexpansionofthe macro Source M1 MACRO &D1,&D2 STA &D1 STB &D2 MEND M1 DATA1, DATA2 M1 DATA4, DATA3 Expanded source STA DATA1 STB DATA2 STA DATA4 STB DATA3

2 Example of macro definition Figure 41, pp 178 Macro invocation Amacroinvocationstatement(amacrocall)givesthe name of the macro instruction being invoked and the arguments to be used in expanding the macro macro_name p1, p2, Differencebetweenmacrocallandprocedurecall Macrocall:statementsofthemacrobodyareexpandedeachtimethe macro is invoked Procedurecall:statementsofthesubroutineappearonlyone, regardless of how many times the subroutine is called Question Howdoesaprogrammerdecidetousemacrocallsorprocedurecalls? From the viewpoint of a programmer From the viewpoint of the CPU Exchange the values of two variables Pass by Reference void exchange(int a, int b) { int temp; temp = a; a = b; b = temp; void exchange(int *p1, int *p2) { int temp; temp = *p1; *p1 = *p2; *p2 = temp; main() { int i=1, j=3; printf("before - %d %d\n", i, j); exchange(i, j); printf("after - %d %d\n", i, j); What s the result? main() { int i=1, j=3; printf("before - %d %d\n", i, j); exchange(&i, &j); printf("after - %d %d\n", i, j);

3 12 Lines of Assembly Code Swap two variables by macro Subroutine EXCH EXCH STA TEMP LDA TEMP RSUB P1 RESW 1 P2 RESW 1 TEMP RESW 1 MAIN LDA #1 STA I LDA #3 STA J Call a subroutine LDA #I STA P1 LDA #J STA P2 JSUB EXCH I RESW 1 J RESW 1 END MAIN #define swap(i,j) { int temp; temp=i; i=j; j=temp; main() { int i=1, j=3; printf("before - %d %d\n", i, j); swap(i,j); printf("after - %d %d\n", i, j); 6LinesofAssemblyCode MAIN LDA #1 STA I LDA #3 STA J Invoke a macro LDA I STA TEMP LDA J STA I LDA TEMP STA J I RESW 1 J RESW 1 TEMP RESW 1 END MAIN Macro expansion Eachmacroinvocationstatementwillbeexpandedinto thestatementsthatformthebodyofthemacro Argumentsfromthemacroinvocationaresubstitutedfor the parameters in the macro prototype(according to their positions) Inthedefinitionofmacro:parameter Inthemacroinvocation:argument Commentlineswithinthemacrobodywillbedeleted Macroinvocationstatementitselfhasbeenincludedasa comment line Thelabelonthemacroinvocationstatementhasbeen retainedasalabelonthefirststatementgeneratedinthe macro expansion Wecanuseamacroinstructioninexactlythesamewayasanassembler language mnemonic

4 Example of macro invocation Figure 41, pp 178 Example of macro expansion Figure 42, pp 179 Example of macro expansion Figure 42, pp 179 Example of macro expansion Figure 42, pp 179

5 Nolabelinthemacrobody Problemofthelabelinthebodyofmacro: Ifthesamemacroisexpandedmultipletimesatdifferentplacesin the program Therewillbeduplicatelabels,whichwillbetreatedaserrorsby the assembler Solutions: Donotuselabelsinthebodyofmacro ExplicitlyusePC-relativeaddressinginstead Ex, in RDBUFF and WRBUFF macros, JEQ *+11 JLT *-14 It is inconvenient and error-prone Thewayofavoidingsucherror-pronemethodwillbediscussedin Section 422 Two-pass macro processor Youmaydesignatwo-passmacroprocessor Pass1: Process all macro definitions Pass2: Expand all macro invocation statements However,one-passmaybeenough Becauseallmacroswouldhavetobedefinedduringthe first pass before any macro invocations were expanded The definition of a macro must appear before any statements that invoke that macro Moreover,thebodyofonemacrocancontaindefinitionsof other macros Example of recursive macro definition Figure 43, pp182 Example of recursive macro definition Figure 43, pp182 MACROS(forSIC) ContainsthedefinitionsofRDBUFFandWRBUFF written in SIC instructions MACROX(forSIC/XE) ContainsthedefinitionsofRDBUFFandWRBUFF written in SIC/XE instructions

6 Example of macro definitions AprogramthatistoberunonSICsystem could invoke MACROS whereas a program to berunonsic/xecaninvokemacrox However,definingMACROSorMACROX does not define RDBUFF and WRBUFF These definitions are processed only when aninvocationofmacrosormacroxis expanded One-pass macro processor Aone-passmacroprocessorthatalternate between macro definition and macro expansioninarecursivewayisableto handle recursive macro definition Restriction Thedefinitionofamacromustappearinthesource program before any statements that invoke that macro Thisrestrictiondoesnotcreateanyrealinconvenience Data structures for one-pass macro processor Data structures DEFTAB(definitiontable) Storesthemacrodefinitionincludingmacroprototypeandmacrobody Commentlinesareomitted Referencestothemacroinstructionparametersareconvertedtoa positional notation for efficiency in substituting arguments NAMTAB Storesmacronames ServesasanindextoDEFTAB Pointers to the beginning and the end of the macro definition (DEFTAB) ARGTAB Storestheargumentsofmacroinvocationaccordingtotheirpositionsin the argument list Asthemacroisexpanded,argumentsfromARGTABaresubstitutedfor the corresponding parameters in the macro body

7 Algorithm MAIN program - Iterations of GETLINE PROCESSLINE Procedure GETLINE If EXPANDING then getthenextlinetobeprocessedfromdeftab Else readnextlinefrominputfile Procedure PROCESSLINE DEFINE EXPAND Output source line Algorithm Figure 45, pp 184 Procedure EXPAND SetuptheargumentvaluesinARGTAB Expand a macro invocation statement(like in MAIN procedure) - Iterations of GETLINE PROCESSLINE Procedure DEFINE Make appropriate entries in DEFTAB and NAMTAB Algorithm Figure 45, pp 185 Algorithm Figure 45, pp 185

8 Handling nested macro definition Comparison of Macro Processors Design InDEFINEprocedure WhenamacrodefinitionisbeingenteredintoDEFTAB,thenormal approach is to continue until an MEND directive is reached ThiswouldnotworkfornestedmacrodefinitionbecausethefirstMEND encountered in the inner macro will terminate the whole macro definition process Tosolvethisproblem,acounterLEVELisusedtokeeptrackofthelevel of macro definitions Increase LEVEL by 1 each time a MACRO directive is read Decrease LEVEL by 1 each time a MEND directive is read A MEND terminates the whole macro definition process when LEVEL reaches 0 This process is very much like matching left and right parentheses when scanning an arithmetic expression One-passalgorithm Everymacromustbedefinedbeforeitiscalled One-passprocessorcanalternatebetweenmacro definition and macro expansion Nestedmacrodefinitionsareallowedbutnestedcalls are not Two-passalgorithm Pass1:Recognizemacrodefinitions Pass2:Recognizemacrocalls Nestedmacrodefinitionsarenotallowed

UNIT-IV: MACRO PROCESSOR

UNIT-IV: MACRO PROCESSOR A Macro represents a commonly used group of statements in the source programming language. A macro instruction (macro) is a notational convenience for the programmer o It allows