The Key; One-Number Functions And The Stack; Two-Number Functions And The Stack - HP 12C Platinum User Manual

Appendix A: RPN and the Stack 235
The Key
When d (roll down) is pressed the number in each
stack register is copied into the register below, and the
number formerly in the X-register is copied into the
T-register.
Pressing d four times successively displays the numbers in the Y-, Z-, and
T-registers and returns the numbers to their original registers.

One-Number Functions and the Stack

One-number mathematics and number-alteration functions — y, r, °, >,
¡, e, B, Ñ, and T — use only the number in the displayed X-register.
When the key is pressed, the function is performed upon the number in the
X-register, and the answer is then placed into the X-register. The stack does not lift,
so the number formerly in the X-register does not get copied into the Y-register; but
this number is copied into the LAST X register. The numbers in the Y-, Z-, and
T-registers are not affected when a one number function is performed.

Two-Number Functions and the Stack

Two-number functions — +, -, §, z, q, b, à, and Z — use the
numbers in both the X- and the Y-registers.