Ask HN: Base code that can run indefinitely but accept new code?

I have this thought about having a tiny, low power processor as a "base" for state and it's powered by a battery source that will make it last a while and also when you charge it, no interruption, TL;DR it won't die.

But then, it can add on more code without stopping the base code. Is that possible/what would you use(language) it sounds low level.

Conceptually it sounds impossible, how can you interface with an unknown endpoint(counts).