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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