The Free42 is a reimplementation of the HP-42S Scientific Programmable Calculator and the HP-82240 Printer.
This is a completely rewritten process that does not use any HP code. HP-42S does not require a ROM image.
Although it features significant improvements over the original calculator, the Free42 remains fully compatible with the HP-42S, both in terms of user interface and the ability to run programs written for the HP-42S.