Free42 is a re-implementation of the HP-42S Scientific Programmable Calculator and HP-82240 Printer.
It is a complete rewrite, not using any HP code. It does not require an HP-42S ROM image.
Even though it contains significant enhancements compared to the original calculator, Free42 remains fully compatible with the HP-42S, both in its user interface and in its ability to run programs written for the HP-42S.