A free app for Android, by Thomas Okken.

The HP-42S calculator and the HP-82240 printer have been updated in Free42. It is an entirely new program that uses no HP code and doesn’t need an HP-42S ROM image. This application was created using open source software. The GNU General Public License, version 2 governs the release of the app’s executables and source code.

Hewlett Packard first offered the programmable RPN Scientific handheld calculator, the HP-42S, in 1988. It offers sophisticated features ideal for use in computer science, mathematics, linear algebra, and other fields. The menu mechanism of the calculator was extremely complex. The up/down keys could be used to navigate multiple line menus.