 section .text
 global main ;must be declared for using gcc
main: ;tell linker entry point
;create the file
 mov eax, 8
 mov ebx, file_name
 mov ecx, 0777 ;read, write and execute by all
 int 0x80 ;call kernel
 mov [fd_out], byte eax

; write into the file
 mov edx,len ;number of bytes
 mov ecx, msg ;message to write
 mov ebx, [fd_out] ;file descriptor
 mov eax,4 ;system call number (sys_write)
 int 0x80 ;call kernel
 ; close the file
 mov eax, 6
 mov ebx, [fd_out]

; write the message indicating end of file write
 mov eax, 4
 mov ebx, 1
 mov ecx, msg_done
 mov edx, len_done
 int 0x80

;open the file for reading
 mov eax, 5
 mov ebx, file_name
 mov ecx, 0 ;for read only access
 mov edx, 0777 ;read, write and execute by all
 int 0x80
 mov [fd_in], byte eax

;read from file
 mov eax, 3
 mov ebx, [fd_in]
 mov ecx, info
 mov edx, 26
 int 0x80

; close the file
 mov eax, 6
 mov ebx, [fd_in]

; print the info
 mov eax, 4
 mov ebx, 1
 mov ecx, info
 mov edx, 26
 int 0x80

 mov eax,1 ;system call number (sys_exit)
 
 int 0x80 ;call kernel
section .data
file_name db 'myfile.txt'
msg db 'Welcome to Tutorials Point'
len equ $-msg
msg_done db 'Written to file', 0xa
len_done equ $-msg_done
section .bss
fd_out resb 1
fd_in resb 1
info resb 26

created 3 years ago

Assembly Online Compiler

Write, Run & Share Assembly code online using OneCompiler's Assembly online compiler for free. It's one of the robust, feature-rich online compilers for Assembly language. Getting started with the OneCompiler's Assembly compiler is simple and pretty fast. The editor shows sample boilerplate code when you choose language as Assembly and start coding.

About Assembly

Assembly language(asm) is a low-level programming language, where the language instructions will be more similar to machine code instructions.

Every assembler may have it's own assembly language designed for a specific computers or an operating system.

Assembly language requires less execution time and memory. It is more helful for direct hardware manipulation, real-time critical applications. It is used in device drivers, low-level embedded systems etc.

Syntax help

Assembly language usually consists of three sections,

Data section

To initialize variables and constants, buffer size these values doesn't change at runtime.
bss section

To declare variables
text section

_start specifies the starting of this section where the actually code is written.

Variables

There are various define directives to allocate space for variables for both initialized and uninitialized data.

1. To allocate storage space to Initialized data

Syntax

variable-name    define-directive    initial-value

Define Directive	Description	Allocated Space
DB	Define Byte	1 byte
DW	Define Word	2 bytes
DD	Define Doubleword	4 bytes
DQ	Define Quadword	8 bytes
DT	Define Ten Bytes	10 bytes

2. To allocate storage space to un-initialized data

Define Directive	Description
RESB	Reserve a Byte
RESW	Reserve a Word
RESD	Reserve a Doubleword
RESQ	Reserve a Quadword
REST	Reserve a Ten Bytes

Constants

Constants can be defined using

1. equ

To define numeric constants

CONSTANT_NAME EQU regular-exp or value

2. %assign

To define numeric constants.

%assign constant_name value

3. %define

To define numeric or string constants.

%define constant_name value

Loops

Loops are used to iterate a set of statements for a specific number of times.

mov ECX,n
L1:
;<loop body>
loop L1

where n specifies the no of times loops should iterate.

Procedures

Procedure is a sub-routine which contains set of statements. Usually procedures are written when multiple calls are required to same set of statements which increases re-usuability and modularity.

procedure_name:
   ;procedure body
   ret