Interesting article Взлом карманного компьютера "Электроника МК-85" / Блог им. alemorf / HYPE "how to hack pocket computer MK-85" which can run Basic (programming language) programs only and switch it to user machine code.
This pocket PC (MK-series) is more like calculator. It can run Basic on it's tiny display, however every program must be typed from keyboard and there is no way to save them in case of power loss.
Author of article is going to attach input/output devices to this one (there is no standard way to do it) and this is first step on this way.