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In this guide, we will discuss Constants in Assembly programming Language. 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 −

CONSTANT_NAME EQU expression

For example,

TOTAL_STUDENTS equ 50

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

mov  ecx,  TOTAL_STUDENTS 
cmp  eax,  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.

Example

The following example illustrates the use of the EQU directive −

SYS_EXIT  equ 1
SYS_WRITE equ 4
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.

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