Algorithm for entering numbers?

[rea5245]

Hi,

I'm implementing an RPN calculator and I'm confused about the algorithm for entering numbers. It's weird, since I've been using HPs for decades, and yet I realize now I'm not sure how they work!

When you type in a number, it goes into X. When you press Enter, it gets copied into Y. When you start entering a new number, it overwrites what's in X.

But if you type in a number, do not hit Enter, but you hit 1/x, the X register holds the result. Then, when you start entering a new number, the result of 1/x gets pushed into Y before you overwrite X.

So sometimes when you start typing a number, it simply overwrites; other times, it pushes then overwrites.

How does an HP calc decide whether to push X or not?

Thank you,
Bob