Circle is a CPlusPlus library for building "bare-metal" applications on the RaspberryPi.
https://github.com/rsta2/circle
In other words, it has all the bits you need to control the low level Raspi hardware from your C++ application.
Or you can build something like CrossTalk (the bare-metal SmallTalk) with it.
Wonder if anyone's done a bare-metal LispMachine with it?
(Thinking about FerretLanguage)
There's InterimOS
See also :