You can access a built-in diagnostic "game" or secret menu through a specific key combination. 5 Games That You Can Play On Your Calculator
If you truly want to code games, look into the Casio fx-CG50 (Prizm) . It supports MicroPython and has a massive community of actual game developers!
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It is crucial to note that no true code execution or firmware modification is involved. Claims of “running Doom” or “playing Tetris” on the fx-991ES Plus are false or refer to a different model (the fx-9860GII, which is programmable). Repacks for the fx-991ES Plus are simply instruction sets, not software. There is no copyright violation, as no proprietary code is redistributed. The only risk is user frustration—many repacks are poorly tested, and entering a long exploit string only to receive a Syntax ERROR or Math ERROR is common.