Hello World Assembly Language

HELLO.asm

; compiling on x86-64
; nasm -f elf64 hello.asm -o hello.o
; ld -s -o hello hello.o

section .text
    global _start

_start:
    mov eax,4                   ; system call ID: sys_write
    mov ebx,1                   ; file descriptor for standard output
    mov ecx,string              ; string address
    mov edx,length              ; string length
    int 0x80                    ; system call
    mov eax,1                   ; system call ID: sys_exit
    mov ebx,0                   ; exit code 0: no error
    int 0x80                    ; system call

section .data
string: db 'Hello, world', 0x0A ; output string
length: equ 13                  ; lenght of string