An "OS" that just boots and prints hello world would be on the order of tens to hundreds of lines depending how fancy you want to be and how much you want to le

An "OS" that just boots and prints hello world would be on the order of tens to hundreds of lines depending how fancy you want to be and how much you want to lean on legacy bios support still being a thing. There are a lot of tutorials out there that'll walk you through quite a bit more than that in a weekend.