there is nothing, and i mean nothing, in us copyright law and case law that makes emulation legal. none. not a single thing there are related cases and exceptions but you need to know what both are

related cases: oracle v google (it is ok to copy an API, but can’t use owner’s source code) and various wine/reactOS cases that pertain to reverse engineering exceptions: there are a handful of exceptions - general purpose computing - personal backups that you yourself do