(HP71B) ASM question

[brouhaha]

I forgot to mention, a C compiler actually exists for the HP Nut architecture:

https://www.calypsi.cc/

It's not open-source, but the binary build targeting Nut is available free-of-charge.

I have not tried it.

It won't help you with Saturn, though. In principle, if one had the source code (whcih unfortunately is not available), one might consider trying to retarget it to Saturn, since in many regards the architectures are similar.