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