assembly reference on mac code example

Example: how to use assembly on mac

global start
section .text
start:
  mov rax, 0x2000004 ; write
  mov rdi, 1 ; stdout
  mov rsi, msg
  mov rdx, msg.len
  syscall
  mov rax, 0x2000001 ; exit
  mov rdi, 0
  syscall
section .data
msg:    db      "Hello, world!", 10
.len:   equ     $ - msg