section .text
    global _main ; must be declared for linker (gcc)
_main:           ; tell linker entry point
    mov rdx, len ; message length
    mov rcx, msg ; message to write
    mov rbx, 1   ; file descriptor (stdout)
    mov rax, 4   ; system call number (sys_write)
    syscall

    mov rdx, 9   ; message length
    mov rcx, s2  ; message to write
    mov rbx, 1   ; file descriptor (stdout)
    mov rax, 4   ; system call number (sys_write)
    syscall
    mov rax, 1   ; system call number (sys_exit)
    syscall

section  .data
msg: db 'Displaying 9 stars',0xa ; a message
len: equ $ - msg                 ; length of message
s2: times 9 db '*'

$ nasm -f macho64 ninestars.asm
$ ld -macosx_version_min 10.14 -lSystem -o ninestars ninestars.o
$ ./ninestars
Bus error: 10