Macros and Nested Macros

Macro can be defined as a group of repetitive instructions in a program that are codified only once but can be repeated n number of times.

Syntax

Macro_name MACRO argument1, argument2,…..argumentn

                Statement 1

                Statement 2

                Statement k

EndM

An actual argument can be any variable, immediate value or a register name.  They may also be duplicates of other names like labels, variables etc.

A macro call designed to generate assembly language instructions must be called in a code segment.

A macro call designed to generate data definitions must appear in a portion o f a program where it will not be considered as an instruction.

Lets take an example. A complex 8086 program contains many complicated procedures. Each time a procedure is called all the flags need to be stored in stack and then retrieved. Adding long pushing lines at the beginning of the procedures and poping lines at the end of the procedure. So instead of writing these lines repetitively in every procedure we create 2 macros. One for pushing and other for poping. Like this.

PUSH_ALL MACRO }  PUSH F }  PUSH AX }  PUSH BX }  PUSH CX }  PUSH DX }  PUSH BP }  PUSH SI }  PUSH DI }  PUSH DS }  PUSH ES }  PUSH SS ENDM

Similar to this one there will be POP_ALL MACRO.

Now calling macros inside a procedure.

RATE PROC FAR ASSUME CS:PROCEDURES, DS:PARAMETERS PUSH_ALL ; macro call ;initialize statements MOV AX, PARAMETERS MOV DS, AX RATE ENDP PROCEDURES ENDS

A macro definition can occur anywhere before END directive.

A macro definition cannot occur inside another macro. Usually all macros are placed at beginning of program before segment definition.

A macro definition is not assembled until macro is called. Each macro call is replaced by statements in the macro.

Passing parameters to a macro.

Simple example

MOV_NUMBERS MACRO NUMBER,SOURCE,DESTINATION MOV CX,NUMBER LEA SI, SOURCE LEA DI, DESTINATION CLD REP MOVSB; copy from SI to DI till counter 0 MOV_NUMBERS ENDM

Nested macros.

A macro calling another macro is called a nested macro. TASM and MASM can contain a macro that calls a previously defined macros. All macros should be defined before segment definition though their order can be anything. Like for example…

DISPLAY_CHAR  MACRO  CHAR                            PUSH  AX                            PUSH  DX                            MOV  AH , 02H                            MOV  DL ,  CHAR                            INT  21H                            POP  DX                            POP  AX ENDM CRLF  MACRO                       DIPLAY_CHAR  0DH                       DISPLAY_CHAR  0AH ENDM