Assembly - Constants


There are several directives provided by NASM that define constants. We have already used the EQU directive in previous chapters. We will particularly discuss three directives:

  • EQU

  • %assign

  • %define

The EQU Directive

The EQU directive is used for defining constants. The syntax of the EQU directive is as follows:


For example,


You can then use this constant value in your code, like:

mov  ecx,  TOTAL_STUDENTS 

The operand of an EQU statement can be an expression:

LENGTH equ 20
WIDTH  equ 10
AREA   equ length * width

Above code segment would define AREA as 200.


The following example illustrates the use of the EQU directive:

SYS_EXIT  equ 1
STDIN     equ 0
STDOUT    equ 1
section	 .text
   global _start    ;must be declared for using gcc
_start:   ;tell linker entry point
	mov eax, SYS_WRITE         
   	mov ebx, STDOUT         
   	mov ecx, msg1         
    	mov edx, len1 
    	int 0x80                
	mov eax, SYS_WRITE         
   	mov ebx, STDOUT         
   	mov ecx, msg2         
    	mov edx, len2 
    	int 0x80 
	mov eax, SYS_WRITE         
   	mov ebx, STDOUT         
   	mov ecx, msg3         
    	mov edx, len3 
    	int 0x80
        mov eax,SYS_EXIT    ;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!

The %assign Directive

The %assign directive can be used to define numeric constants like the EQU directive. This directive allows redefinition. For example, you may define the constant TOTAL as:

%assign TOTAL 10

Later in the code you can redefine it as:

%assign  TOTAL  20

This directive is case-sensitive.

The %define Directive

The %define directive allows defining both numeric and string constants. This directive is similar to the #define in C. For example, you may define the constant PTR as:

%define PTR [EBP+4]

The above code replaces PTR by [EBP+4].

This directive also allows redefinition and it is case-sensitive.