Explain Macro Expansion in System Programming

 

Objectives of macro expansion: 

• Definition 
• macro assembler 
• macro preprocessor
• key notions of macro expansion 
• flow of control during expansion 
• algorithm
• lexical substitution 
• 1) positional parameter 
• 2) keyword parameter

Definition: Macro Expansion can be performed  by using two kinds of language processors and when we use of macro name with a set of actual parameters is replaced by same code generated from its body .This is called macro expansion.

macro expansion can be performed two ways:

1. macro assembler 
2. macro pre-processor
3. Macro Assembler :  It performs expansion of each macro call in a program into a sequence of assembly language statements and also assembles the resultant assembly language program.
4. Macro pre-processor:  It only processes the macro call . Other statements are processes with the help of assembler a macro pre-processor merely performs expansion of macro in program.   It produces an assembly program in which a macro call has been replaced by statements that resulted form its expansion but statements that were not macro calls have been retained in their original from . This program can be assembled by using assembler.  

The macro preprocessor operates as follows: If  the first statement in the program input to its is macro header statement , it knows that one or more macro definitions exist in  the program. It processes and stores all macro definitions in its own data program as follow.

Key notations of Macro Expansion:      

We discuss the two key notions  used in implementing macro expansion before we present a scheme for it.

1. flow of control during expansion:  
2. Lexical substitution

1) Flow of Control During Expansion : 

1.  The default flow of control during macro expansion is sequential.
2.   Thus, unless the flow of control is altered by using a preprocessor statement , model statement  in a macro definition are visited sequentially starting with the statement following the macro prototype statement and ending with statement preceding the MEND statement.
3. It rules determines the order in which model statements are visited during  macro expansion.
4. Flow of control during macro expansion is implemented using a macro expansion counter.

Algorithm:  

Step 1 : MEC : = statement  number of the first model statement following the prototype statements in the definition of the called macro.

Step 2 :  while : - The statements is pointed to by MEC  is not a MEND statement

  1) If a model statements then  

• expand the statements through lexical substitution.
• MEC:= MEC+1;         

  2) else   

       MEC:= value specified in the preprocessor statement.

step 3: Exit from macro expansion.

Lexical Substitution :   

Lexical substitution is used to generate an assemble statement  from a  model statement  

and model statement   3 types: 

1. ordinary string - Which is any string other than a string type 2 or type 3 . 
2. The name of formal parameter: Which is preceded by the character '&'.
3. The name of preprocessor variable , which is also preceded by the character '&'.

During lexical substitution , an ordinary string in a model statement is retained in  original form.  A name formal parameter or preprocessor variable appearing in model statement is replaced with its value.  

The value of Formal parameter is the corresponding to the actual parameter string used in a macro call.  Where the rules of correspondence depend on what kind of parameter is. We will discuss two kinds of

 formal parameters. positional and keyword parameters.   

The value of a preprocessor variable is readily known to the preprocessor from its own data structures. 

1) positional parameters:    For positional formal parameters , the specification <parameter kind> of syntax rule is simply omitted. 

The positional formal parameters is written as follows: &<parameter name> and example &SAMPLE  where is sample is name of parameter. 

In a call on a macro using positional parameters , the <actual parameters specifications> is an ordinary string. 

2) keyword parameter : 

For keyword parameter , the specifications < parameter kind> is the string = in syntax rule . 

<The actual parameters specification > is written as <formal parameter name > = < ordinary string> .