[WP 34s] emulator ENTER key behaviour

[emece67]

This is only an educated guess.

To implement an RPL machine you need an stack and also a separate register where to store the digits as you type them prior to pushing ENTER.

To implement an RPN machine you only need an stack, as the digits you type are directly entered into the X register. When you push ENTER such digits are copied to Y and, as the automatic stack lift is disabled, new digits will go into X (overwriting the old ones, but not missing them, as they are also in Y). If instead of pushing ENTER you push any operation, the digits you have just typed (in X) are computed with those on Y.

Thus, in an RPN machine you do not need a separate register to hold the digits you type prior to pushing ENTER, the bottom of the stack serves that purpose.

I suppose that, when memory was scarce, such behaviour was advantageous.