SECTION .data
hello db 'hello world', 10
hellolen equ $-hello
SECTION .text
global start
start: mov rax, 1
mov rdi, 1
mov rsi, qword hello
mov rdx, hellolen
syscall

Assemble, link and run it. Make sure no important app is running since your OS will probably crash!

yasm -f macho64 a.asm
ld a.o
./a.out