Writing Macros

As you are working you will find that you have to type (and document) the same lines of code over and over. To simplify your task you may create macros. Macros are similar to the preprocessor in C or C++. When you assemble a source file, all macros are expanded. This can lead to large files when you are done compiling. Typically you will want to use procedures for blocks of code that are repeated often. However, for a block of code that is only used once in a program, but you need in every program you write, you may want to create a macro file like the following. The other time you may want to have macros is where you have written a procedure, but setting up the stack to pass the parameters is tedious. In this case, a macro with several parameters is probably your best route. I will give you an example of how to set up a macro with parameters to call a procedure that will print a string to the screen later on in the course. For now you can start to work on your own macros. Remember, the macros provided in the book are much to complex for this course, and may not be used in the projects. You may write your own versions of those in the books.

TITLE IO.MAC

;=======================================================================
; ExitProgram () -- Terminates current application
;       PreCond  --  None
;       PostCond --  Nothing (no registers modified)
;       Notes    --  Assumes nothing about our current segment
;=======================================================================
ExitProgram MACRO
    mov     ax, 4c00h               ;standard exit code
    int     21h
ENDM

So if we combine this with our hello world program we would write it as

TITLE hi_world.asm 
;=======================================================================
; Hello World application - prints "Hello World" to the screen
;=======================================================================

include io.mac

;Some Useful Constants
STDIN                   EQU         0
STDOUT                  EQU         1
STDERR                  EQU         2
LF                      EQU         0Ah
CR                      EQU         0Dh

_DATA SEGMENT USE16 BYTE PUBLIC 'DATA'
      myString DB "Hello World", CR, LF, "$"
_DATA ENDS

_TEXT SEGMENT USE16 WORD PUBLIC 'CODE'
   ASSUME cs:_TEXT, ds:_DATA

   Start:
      mov   ax, _DATA           ; set DS to point to our data segment
      mov   ds, ax

      mov   ah, 09h             ; function: print string $ terminated
      mov   dx, OFFSET myString ; pointer to string
      int   21h

      ExitProgram

_TEXT ENDS

_STACK SEGMENT USE16 STACK 'STACK'
      DW  100 dup(?)
_STACK ENDS

END Start

One last point to bring up here. If you are using labels (for jumping) in a macro, think about what is going to happen. The assembler blindly substitutes a copy of your macro code at each point you use it. If you have a label in your macro it is going to be copied with the same name each time. To avoid this problem look into the LOCAL directive (used within a macro or procedure) or the LOCALS keyword (applies to your entire file). Both of these typically use the @ symbol as the first character of their name. These can get really fancy (don't waste time on it), but you are welcome to use them.