CEPTIC is an acronym for Control and Execute Program for TI Calculators.
I started work on a completely independent operating system for the TI-83 Plus calculator. I wrote every line of it myself - a truly independent project. I was making progress, and then TI e-mailed me about something that I said on a forum. To shorten a story, other factors (lack of time and motivation) have caused me to discontinue the project.
The source code is available for educational purposes. Please understand that this is at your own risk, and I am not liable for whatever you do with it. I will not tell you how to transform the compiled output into something that can be put on the calculator - this left as an exercise for the reader :).
This version is 83+ SE only, but it shouldn't be too hard to modify to run on the regular 83+ as well. Just remove the extra ports, and modify a few of the flash pages (7Dh...1Dh, et cetera). I'm just happy that I had the ability and knowledge to actually create a working (though non-practical at current) operating system.