This is the good ol' BASIC-11 (not BASIC-PLUS) ported to the Zilog Z80 CPU. The port is accurate enough (down to the format of the tokenized program) so that programs "compiled" under e.g. RT-11 could be loaded and executed.
The included Makefile is intended to be used from within the RSX180 build tree, and produces both CP/M and RSX180 executables.