; Just a hello world with nasm syntax as opposed to MASM

SYS_WRITE equ 1
SYS_EXIT equ 60
stdio equ 1

global _start ; all program begin exec at _start or main
; _start is ok if you don't intend to call functions from 
; libraries otherwise go with main

section .data
    goodbye: db "Goodbye cruel world!", 0x0a, 0

section .text
_start:
    mov rax, SYS_WRITE ; in rax goes this syscall by number
    ; follow fastcall as defined in syscall manpage
    mov rdi, stdio ; int fd at startup you have file 0-2 open
    mov rsi, goodbye ; char *buf
    mov rdx, 22 ; int count always account for NULL terminator
    syscall ; this instruction on amd64, not int 0x80
    mov r10, rax ; move return value into a spare register
    mov rax, SYS_EXIT 
normal: 
    xor rdi, rdi ; exit int status of 0 all is well
    cmp r10, 22
    jne error
    syscall
error:
    mov rdi, r10 ; exit status non zero for how much we did write
    syscall