LastXY Register for WP34S

[Marcus von Cube]

I don't think that's easy.

The problem is that there is no U register, it would need to be emulated by L and probably I which is the imaginary part of complex LastX. We do not save Y to I in normal dyadic operations. We could do, of course, but that would break programs which are relying on the contents of I not being changed except by complex operations. And where to store Z and T when a complex dyadic operation is performed?

The next problem is that such an operation would cause an unusual behavior of RCL: It would not push the stack in a fashion we are used to but would push it up one level and then replace Y by some other saved value. That might be useful but it's not consistent behavior.

The next question arises in the interpretation of the keyboard input. How shall we distinguish between RCL U and RLC 2? An additional keystroke like ENTER would be required before the U.

The 31S has an undo feature which restores the complete working memory to the state it had before the last operation. This is only possible because the working memory of the 31S is way below the 2K RAM we have available. Implementing such a feature is just impossible on the 34S.