Of HP fame is a bit missleading. HP calculators are programmed with the RPL Programming language,[1] that was both a high level user language and a low level system language for the 4-bit Saturn processor.
This is a free and portable reimplementation, with a few extensions (and some incompatibilities).
There also is a unrelated iPhone RPL reimplementation [2].
IMO the most comfortable way to program a calculator, i personally use a HP-28c with 680 KHz 4 bit cpu. I think it's crazy how much you can do with so little power.