Some time ago I wrote an implementation of Forth that runs on the TI-84+ and TI-83+ calculators directly inspired by Jonesforth.[0] It runs under TI-OS as well, but the amount of available space is somewhat limited. On another implementation I wrote[1] you have full access to the calculator hardware and memory from Forth.

[0] https://github.com/siraben/ti84-forth

[1] https://github.com/siraben/zkeme80