Assembly - Macros

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Writing a macro is another way of ensuring modular programming in assembly language.

  • A macro is a sequence of instructions, assigned by a name and could be used anywhere in the program.

  • In NASM, macros are defined with %macro and %endmacro directives.

  • The macro begins with the %macro directive and ends with the %endmacro directive.

The Syntax for macro definition:

%macro macro_name  number_of_params
<macro body>
%endmacro

Where, number_of_params specifies the number parameters, macro_name specifies the name of the macro.

The macro is invoked by using the macro name along with the necessary parameters. When you need to use some sequence of instructions many times in a program, you can put those instructions in a macro and use it instead of writing the instructions all the time.

For example, a very common need for programs is to write a string of characters in the screen. For displaying a string of characters, you need the following sequence of instructions:

mov	edx,len	    ;message length
mov	ecx,msg	    ;message to write
mov	ebx,1       ;file descriptor (stdout)
mov	eax,4       ;system call number (sys_write)
int	0x80        ;call kernel

We have observed that, some instructions like IMUL, IDIV, INT, etc., need some of the information to be stored in some particular registers and even returns values in some specific register(s). If the program was already using those registers for keeping important data, then the existing data from these registers should be saved in the stack and restored after the instruction is executed.

In the above example of displaying a character string also, the registers EAX, EBX, ECX and EDX have been used by the INT 80H function call. So, each time you need to display on screen, you need to save these registers on the stack, invoke INT 80H and then restore the original value of the registers from the stack. So, it could be useful to write two macros for saving and restoring data.

Example:

Following example shows defining and using macros:

; A macro with two parameters
; Implements the write system call
   %macro write_string 2 
      mov   eax, 4
      mov   ebx, 1
      mov   ecx, %1
      mov   edx, %2
      int   80h
   %endmacro
 
section	.text
    global _start            ;must be declared for using gcc
_start:    ;tell linker entry point
	write_string msg1, len1               
	write_string msg2, len2    
	write_string msg3, len3   
	mov eax,1          ;system call number (sys_exit)
	int 0x80           ;call kernel

section	.data
msg1 db	'Hello, programmers!',0xA,0xD 	
len1 equ $ - msg1			
msg2 db 'Welcome to the world of,', 0xA,0xD 
len2 equ $- msg2 
msg3 db 'Linux assembly programming! '
len3 equ $- msg3

When the above code is compiled and executed, it produces the following result:

Hello, programmers!
Welcome to the world of,
Linux assembly programming!


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