Tetris-OS: An operating system that only plays Tetris
This is cool, but I think it's misleading to call it an operating system.  It's Tetris that runs without an operating system, which is to say, on bare metal, old school.  Writing programs that run on bare metal is a cool and worthwhile thing to do, but calling any such program an "operating system" is confusing.This probably sounds like I'm picking a nit, but I think it matters for the sake of newbies just starting to climb the learning curve and trying to understand what an "operating system" actually is.  IMHO, the defining characteristic of an operating system is that it provides an environment in which to run other programs, ones that are not part of the operating system, and which may not even exist at the time that the operating system starts to run.