there is an interesting project made with Rust - an RPN-calculator, could be compiled for DM42 hardware:
Large RPN stack. The RPN stack is not limited to a fixed number of registers.
Accurate decimal floating point. Uses 128-bit decimal floating point with 34 digits of decimal precision.
Exact rationals with large integers. Integers and rational numbers are kept in an exact rational representation. Results can be displayed as exact fractions when possible, and the implementation never guesses the closest fraction.
Unit conversions. Supports conversions between common units. Utilizes the rational representation for unit conversions so that converting between them leads to no loss of precision.
RPN stack undo. Operations performed on the RPN stack can be undone with a deep undo buffer.
Works on dedicated calculator hardware. Designed to run on the excellent DM42 from Swiss Micros. This project has been optimized for highly efficient LCD rendering and memory usage to create a very responsive interface. This project is not an emulation and iterative improvement of the HP42 design like the default firmware, but is rather a ground-up modern redesign targeting the DM42 hardware.