Section 3
##### Automatic Memory Stack and ALPHA Register
This section covers the detailed operation of the automatic memory stack and the ALPHA register. If you wish to learn how the stack and ALPHA register work, and how you can take advantage of some of the more powerful features of the HP-41C, we suggest that you work through this section. Otherwise, you may wish to skip this section for now and continue with section 4, Using HP-41C functions.
###### The Automatic Memory Stack
Automatic storage of intermediate results is the reason that HP-41C makes solution of even the most complex equations simple. The automatic storage is made possible by the Hewlett-Packard memory ‘‘stack.’’
Here is what the registers of the automatic memory stack look like:
 The T 0 Automatic Z 0 Memory Stack Y 0 X 0 (Displayed)
When you are in normal mode, that is not in PRGM, USER, or ALPHA mode, numbers that appear in the display are the same as the contents of the X-register in the calculator.
Each register in the stack holds a 10-digit number and its 2-digit exponent of 10. ALPHA characters and their relationship to the stack are covered later. For now, let’s work with just numbers.
Basically, numbers are stored and manipulated in the HP-41C ‘‘registers.’’ Each number, no matter how few digits (e.g., 0, 1 ,5) or many (e.g., 3.14159265, –15.78352, or 1.7588028 × 1011), occupies one entire register. We label these registers X, Y, Z, and T. They are ‘‘stacked,’’ like shelves, one on top of the other, with the X-register on the bottom and the T-register on top.
The contents of these registers, as well as all other information in the HP-41C, are maintained by the calculator’s Continuous Memory. Even when the HP-41C is turned off, the values stored in the stack registers are all ‘‘remembered’’ by the calculator.
When you execute a function, the result is always placed in the X-register (the display). So when you compute the reciprocal of 5...
 Keystrokes Display 5 1/x 0.2000
 39