Check http://arduboy.ried.cl
The whole emulator was written by http://home.citycable.ch/pierrefleur/Jacques-Laporte/HP35_Arduino.htm, this is merely an adaptation for the Arduboy form factor. Please check rom.h
credits for the original license text
To use an alternative version of the ROM with the exp(ln(2.02))=2
bug, hold button B when booting. http://www.jacques-laporte.org/HP3520bug.htm
In this fork, I redrew the theme and rotated the orientation to fit using the right hand control buttons