64-bit system call "Hello, world"

.text
.global main

.func main
main:
    mov $len, %edx         # message length
    lea msg(%rip), %rsi    # message to write (RIP relative addressing)
    mov $1, %edi           # file descriptor (stdout)
    mov %edi, %eax         # system call number (sys_write)
    syscall

    ret

.data
msg: .ascii "Hello, world!\n"
len = . - msg              # length of the msg string