> I don't see a cbm.inc

You are right. I mixed it up with cbm.mac.

> And I strongly believe one definition (e.g. register layout) should only be in one place.

My suggestion was to keep them in one place in atari.inc and use that file for the 5200 too.

> I'm fine with renaming _xxxx.inc to atari_xxxx.inc

Okay, then let's keep the content layout as you propose it and have just the files renamed.

> but apart from this, what else don't you like?

I'm primarily suggesting to remove the .include directives from the .inc files. I'd rather like the  source files contain all necessary .include directives themselves.