Chris Gore: Programming: HP RPN Calculators
I've spent a lot of time using HP calculators, especially the RPN ones. My first HP calculator was the HP 38G, which is a bit of a weird one because it was basically just an algebraic version of the HP 48G. That was eventually replaced with an actual HP 48G and that is when I really learned to love reverse Polish notation, RPN.
RPN is a postfix notation instead of the normal infix notation. That is, you don't enter 3+4 to get 7, instead you represent that set of operations as 3 4 +. This may seem stupid at first but it has it's advantages, and it becomes quite natural eventually.
I don't use calculators at all these days. Generally if I need to do math I will do it at a desktop computer now, but at the time I spent a good deal of time making a library of additional functions for the HP calculator. The last version I made dates to 2004 and was tested on the HP 49G+. I don't know if it will work on the newer HP 50g, I don't currently own one of those. I do intend to get one sometime soon and make it work there, so keep posted.
Feel free to download it: additional_functions-2.07.zip. I hope it is useful to you. Just note that the contact information in the documentation is a bit out of date. This website, http://www.cgore.com, is the best way to find current contact information for me. I hope to keep it up as long as websites are a thing that people bother with.