write.nasm

; nasm -D OpenBSD -f elf64 exit.nasm -o exit.o && ld.bfd -nopie  -e _start exit.o -o exit && ./exit ; echo $?

%ifdef  OpenBSD
section .note.openbsd.ident
        align   2
        dd      8,4,1
        db      "OpenBSD",0
        dd      0
        align   2

%define SYS_EXIT        1
%define SYS_WRITE       4
%endif


global _start

section .text

_start:
        mov rax, SYS_WRITE
        mov rsi, hello_world    ; *buf
        mov rdx, length         ; nbytes
        mov rdi, 0x1            ; fd
        syscall


        mov rax, SYS_EXIT
        mov rdi, 42
        syscall


section .data
        hello_world: db 'Hello World', 0xa
        length: equ $-hello_world