Assembly - Constants

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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.