NeoCal for Palm OS User Guide
NeoCal uses several registers for performing calculations with the most commonly used labeled x, y, and last x. The x register contains the number shown in the display, while the y register is paired with the x register when two values are required for a calculation or returned from a calculation. The last x register stores the value of the x register before the last numeric operation was performed.
As an example, let's add the numbers 12 and 15:
Many keys are labeled with x () or x and y () referring to the x and y registers. The general rule is to enter the y value first, then the x value. Also,
The RPN input method also recognizes two more registers, z and t, which are discussed in RPN Tutorial.