TOS was GEM + GEMDOS on 68k, which was really a kind of ... rewrite of CP/M 68k, but with DOS-ish additions. Many of the internals and source code look identical/similar to CP/M 68k.

(GEM itself had its genesis with Lee Jay Lorenzen moving from Xerox (in Dallas, not PARC) to DR. At Xerox he'd tried to float the idea of a Xerox Star-like interface on PC class hardware. He ended up doing that once he moved to DR, as GEM. It has Mac influences, for sure, but it was their own creation.)

http://www.cpm.z80.de/source.html

http://www.deltasoft.com/downloads-gemworld.htm

Kind of wonder how much of that DNA made it into DR-DOS and Concurrent DOS.