Does a hobby OS need to be useful?

(If you want to communicate with other computers) You probably do want to speak IP, but if you want to do all the work, you don't need to use BSD sockets.

POSIX and other such things will let you use more of other people's software... but that's a choice.

You can also do a lot of reuse if you just want to focus on some parts. There's no need to write a bootloader unless you want to. You can pull in lwIP for the IP stack. You can do a lot of interesting (I think) things with barely any userland if you want.

If it's supposed to be a general purpose OS, you end up looking a lot like existing general purpose OSes though, cause it's hard to build everything and the closer you get to existing OSes, the more existing software you can leverage.

If you only want to run a subset of existing software, you might not need to cover that many of the syscalls from whatever OS it was targetted at. My hobby OS runs one specific FreeBSD executable, and it doesn't take that many syscalls to do it.

> Imagine we discover software from another planet that would not even work with 8-bit bytes, for instance.

8-bit bytes didn't become fully dominant until maybe the 1970s. It's a bit hard to find software older than that, but it's around somewhere.