A more mature s/w emulation of many HP classic calulators is at:
https://nonpareil.brouhaha.com/
I routinely use it's HP-41CV on my desktop. To me the Hp-41 represented a breakthrough in programmability.
The RPN user interface of these calulators was also highly superiour entry mode. Sort of the Beta of the Beta VHS shakedown. The superiour solution that lost to a larger marketing budget...
Many engineering students of all disciplines used HPs during the 1980-2000s. At some point, the HP 48 was everywhere in campus.
"* The original C version running on Z80 @ 4MHz was really slow! (maybe 25% of HP-25 speed)
* I started to re-write the emulator in Z80 assembly
* would love to finish this someday, but...
* Then I noticed that CMOS Z80 are available rated up to 20MHz
* I ended up running the Z80 at 16MHz, which results in an emulated speed within about 10% of the original HP-25. Not bad"
The other interesting thing is that this uses the 40-pin DIP version of the Z80, which will be discontinued on June 14, 2024: https://news.ycombinator.com/item?id=40083885