Constants - assembly language

Constants are also a type of variable that stores a memory or instruction location or a constant value. The EQU instruction is used to define constants in the nasm assembly language.

Content

• define Directive
• Assign Directive
• EQU Directive

Constants in NASM assembly language

There are several directives provided by NASM that define constants. EQU instruction, %assign instruction, %define instruction etc. to define constants.

In particular we will discuss three instructions

1. EQU
2. %assign
3. %define

EQU Directive

EQU Directive is used to define a constant.

Syntax of EQU

CONSTANT_NAME EQU expression

Example

TOTAL_STUDENTS equ 50

using a constant value

mov  ecx,  TOTAL_STUDENTS 
    cmp  eax,  TOTAL_STUDENTS

The operand of an EQU statement can be an expression - and can be written as such.

LENGTH equ 20
        WIDTH  equ 10
        AREA   equ length * width

The above code section will store and define Area as 200.

Example

The following example shows 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 'NASM assembly programming! '
    len3 equ $- msg3

above code result

 Hello, programmers!
    Welcome to the world of,
    NASM assembly programming!

%assign Directive

% The ASSIGN Directive can be used to define numeric constants like the EQU instruction. This directive allows redistribution. For example, you can define the constant total as −

syntax

%assign TOTAL 10

Later in the code, you can redefine it as -

%assign TOTAL 30

This command is case-sensitive.

The %define Directive

% The define directive allows both numeric and string constants to be defined. For example, you can define the constant PTR as -

Example

 %define PTR [EBP+4]

The above code replaces PTR with [EBP+4].

This directive also allows reallocation and is case-sensitive.

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Assembly - Addressing Modes

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