Wow, I didn't know until today that anyone had ever written an OS for the TI-89/TI-92 calculators.
ticacl.org link: https://www.ticalc.org/archives/files/fileinfo/319/31951.html
And someone else has made one too! Called Punix: http://punix-os.blogspot.com/
The author of Punix has also published a skeleton OS for anyone else who wants to play with it: https://www.cemetech.net/forum/viewtopic.php?t=7345&start=0