I just had a look at the code and it is indeed very compact. I haven't compiled or used it.

Looks like RISC-V 32-bit integer and multiply and atomic instr extension. Floating point supported when compiling via gcc or similar the example apps (not by the emulator itself but by the compiler emiting the required software functions to emulate the floating point operations instead).

I think it is very clever. Very compact instruction set, with the advantage of being supported by several compilers.

Wrapper over this other project which is the one implementing the instruction set itself: https://github.com/cnlohr/mini-rv32ima

Kudos to both projects.