zlacker

HP used to make their own custom CPUs for their calculators, with an architecture designed for BCD arithmetic using 56-bit and later 64-bit registers. In 2003 they switched their graphing calculators to ARM9 processors running an emulator of their old CPU architecture so that they didn't need to re-write the whole OS.

SwissMicros did something similar for HP's non-graphing calculators, recreating the keyboard layouts but using modern ARM processors that run emulators of the original HP calculators. Apparently HP's early calculators did not include copyright notices for their OS: https://nonpareil.brouhaha.com/microcode_copyright_status/