LastXY Register for WP34S

[PANAMATIK]

From my point of view, I cannot easily agree to the statement "Implementing such a feature is just impossible on the 34S"

I do know the value of consistency in programming. But an Undo feature imo would not break rules, but just add a new one. Lets get into it. The solution would be to generate 2 new registers like L and K, perhaps named LX,LY, not attached to any letters on the keyboard. The LastXY Undo register would consume just a few RAM bytes in the physical processor memory, about 16 bytes or double with complex numbers. If you dont have left this amount, then you could double purpose or sacrifice 2 to 4 general purpose registers. Before every dyadic instruction the operands could be stored into it, thats easy. By recalling it, there would indeed happen a new kind of stack lift. But this is just a reverse copy back into the X and Y registers. It should not considered as a stack operation at all. The problem with the ambiguity of RCL U and RCL 2 could be solved by a new Sequence, RCL CLx for example, which is afaik not yet occupied.

But again, I know that from the programming side there must be considered some more things. However: not impossible!

Addendum: I just recognized, that your 'impossibility' statement refers to storing the whole memory. Sorry! So still there is your statement "I dont think thats easy!"