Slide S4-01


Chapter 4  -  MACRO PROCESSORS


Function
   macro expansion; substitute macro bodies for macro invocations



             | source program |
             | with macro     |
             | invocations    |

                    |
                    |
               macro processor
                    |
                    |


             | source program |
             | with macros    |
             | expanded       |

                    |
                    |
                assembler
                    |
                    |
                   ...



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                                                            Slide S4-02


Two-Pass Macro Processor

   Pass 1
        process macro definitions

   Pass 2 
        expand macro invocations

   Problem
        would not allow the body of one macro to contain
        definitions of other macros


                       PMAC    MACRO    ...

                       C1MAC   MACRO    ...
                               ...
                               MEND

                       C2MAC   MACRO    ...
                               ...
                               MEND

                               ...
                               MEND

                               ...




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                                                            Slide S4-03


One-Pass Macro Processor

   . will process macro definitions and expand macro invocations
     at the same time;  will alternate between macro definition
     and macro expansion

   . the definition of a macro must come before any of its
     invocations



Macro Processor Data Structures

definition table    DEFTAB   DEFTBL

   - macro definitions are stored in this table

   - references to parameters are converted to a positional 
     notation for efficiency in substituting arguments



                                                            Slide S4-04


name table     NAMTAB   NAMTBL

   - macro names are stored in this table along with pointers
     to the beginning and end of definitions in DEFTAB;
     serves as an index to DEFTAB


argument table    ARGTAB   ARGTBL

   - used during the expansion of macro invocations

   - arguments in a macro invocation are stored in this table



Macro Processor Logic

     pages 184-185 of text


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                                                            Slide S4-05


Section 4.2

Machine-Independent Macro Processor Features


Generation of Unique Labels

              MAC    MACRO    ...
                     ...
              LBL    ...
                     ...
                     MEND

        first macro expansion
            AALBL

        second macro expansion
            ABLBL

        ...



Conditional Macro Expansion

     IF ( Boolean expression )
          ...
     ELSE
          ...
     ENDIF

   - - - - - - - - - -

     WHILE ( Boolean expression )
          ...
     ENDW 



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                                                            Slide S4-06


Section 4.3

Macro Processor Design Options


Recursive Macro Expansion

     Problems with previous macro processor

        - when a macro invokes another macro
                 ARGTAB is overwritten

        - when the second macro finishes
                 EXPANDING is set to FALSE


     Solution

        - if macro processor is written in a high-level language
          that supports recursion, then there won't be problems
          since the compiler saves the previous values of
          variables on recursive calls

        - if macro processor must be written in a programming 
          language that does not support recursion, then the
          system programmer must save the data values (this can
          be done using a stack)





                                                            Slide S4-07


General-Purpose Macro Processors

     not dependent on any particular programming language;
     can be used with a variety of different languages

   Problems

      - problems related to different programming languages
     
           . comments
           . facilities for grouping
           . tokens (identifiers, operators, ...)

      - syntax used for macro definitions and macro invocations



Macro Processing within Language Translators

     previous macro processor: pre-processor

     alternative: combining the macro processing functions
         with the language translator

                . line-by-line macro processor

                . integrated macro processor



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