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:

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 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.