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Not quite 4k, but MicroPython runs on 16K of RAM.

As I understand it, that's a very cut back version and doesn't include the compiler/interpreter so you can't just hook up to a UART and start reprogramming. I could be wrong, so please feel free to correct my statement if it's completely off the wall.

What I want is something that I can hook up to the UART serial port, and start typing interactive commands into it to debug why my system is hung/stuck/etc. It needs to live in about 4K of RAM (and I can trade quite a bit of flash to do that).

16K of RAM is too much. Something like a Nordic nRF52811 only has 24K of RAM, and a lot of that gets used for the Bluetooth stack.

Sounds like you might want to explore Forth a bit. Punyforth on the ESP8266 is a convenient way to try get started hands-on, Jonesforth if you want to learn how you could go about implementing one.

On bigger devices (PC/SBC/...) with an OS, Tcl/Tk is a wonderfully effective programming tool!

...While checking the nRF52811 product specification to learn more about this chip, I found out that it contains a Cortex M4 CPU. You can research some of the existing Forths for these processors (myself I find a tethered Forth convenient to work on CPUs like this).

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