Hello world in GNU Assembler (GAS)

hello.s

# -------------------------------------------------------------
#
# Using Linux System calls for 64-bit
# to run:
#   gcc -c hello.s && ld hello.o && ./a.out
# o
#   gcc -nostdlib hello.s && ./a.out
#
# --------------------------------------------------------------

.global _start

.text

_start:
    # write (1, msj, 13)
    mov $1, %rax            # system call 1 is write
    mov $1, %rdi            # file handler 1 is stdout
    mov $message, %rsi      # address of string to output
    mov $13, %rdx           # number of bytes
    syscall

    # exit(0)
    mov $60, %rax           # system call 60 is exit
    xor %rdi, %rdi          # we want to return code 0
    syscall

message:
    .ascii "Hello, world\n"