There's KnightOS: https://knightos.org/

KnightOS will run on any upgradable Texas Instruments calculator based on the z80 CPU. This includes the TI-73, TI-83+, TI-83+ Silver Edition, TI-84+, TI-84+ Silver Edition, and TI-84+ Color Silver Edition. You can also run it on the French variations of these calculators, or in an emulator.