Emulate a 1977 vintage Texas Instruments TI-58/58C/59 calculator. This emulator is programmable, lets you load/save programs, includes a partial implementation of the original Master Library module. Sources on GitHub at https://github.com/TurboGit/ti5x_android include a tools for building your own program/module files.
ti5x does aim for 100% authenticity even if not possible in all cases. Yet, it supports many undocumented features
from the original calculator like the HIR registers and the /DSZ nn 51/ (decrement no-jump) instructions.
With the current version it is possible to play the famous Treasure-Island and 3D-Tic-Tac-Toe games. And many original programs are running without modification.