Depends on what you want to do. I know lots of people who use their smart phone for every thing they need to do and others who use an arm based tablet. They don't use it to get away from an x86, it's just the tool that does the job for them. My newest CNCs use an Arduino and any thing that can drip ...

The Atari had a 10 K os in rom. It was smart enough to load a program and provide some basic IO services. There is a port of CP/M to the 6502 http://www.z80.eu/dos65.html called DOS65.

I've got several cnc's and a few other machines running on printer ports, under dos. I've done the driver thing and got involved with a couple OS projects. Gotten around windows to lowlevel format usb drives to boot exe programs or other os's, but it gets to be a bigger thing when you get to video ...

I started to look at and play with worn out relay and air logic stuff in grade school and discovered the book 'we built our own computers' in 5th or 6th grade. I did an adder for 6th grade science fair that went over with a flop, no one knowing what it was or why it only added 0 and 1's.... Except ...

Wow, an impressive project. I haven't done any low level computer projects since the 70's and the S-100's, just controllers and things that plug into them or a computer port. I loved my Atari computers and the Amigas. I saw the Mycpu project and liked it, except the video, so I went looking around ...