Well, it's the reason separation of concerns exist. You don't have to know all of the requirement to begin writing something real world, depending on what you mean by "real world". If you're trying to rewrite a feature complete GCC from scratch you're going to have to start somewhere and you won't be able to write everything from the start. So you divide the work in pieces and focus on each individually. Start out dumping assembly. If you really want to custom write ELF files that's fine but no place to start.