When the Assembler reads a statement in a source program calling a previously defined macro, it processes the call as described in the following paragraphs.
The symbol table is searched for the macro name. If it is not in the symbol table, an undefined symbol error message is issued.
The rest of the line is scanned for arguments. Any argument in the macro call is saved as a literal or null value in one of the 35 possible parameter fields. When the number of arguments in the call is less than the number of parameters used in the macro the argument, which have not been defined at invocation time are initialize with "" (empty string).
Starting with the line following the MACRO directive, each line of the macro body is saved and is associated with the named macro. Each line is retrieved in turn, with parameter designators replaced by argument strings or assembler-generated label strings.
Once the macro is expanded, the source lines are evaluated and object code is produced.