legOS · brickOS

A replacement firmware that runs native C and C++ on the RCX's Hitachi H8 instead of LEGO's bytecode interpreter. Much faster, much more memory, and direct hardware access. Markus Noga wrote it as legOS; the community kept it going as brickOS.

• Flashed in place of LEGO firmware; toolchain = an H8 GCC cross-compiler + a downloader over the IR tower.
• Ben Jackson's "VLL Transmit for legOS" shows an RCX emitting VLL under legOS to a Micro Scout / Code Pilot.
• Sibling alt-runtimes: pbForth (Forth), leJOS (Java).