HP HP-21 - How the HP-21 Works; The Stack; Initial Display

How the HP-21 Works
The Stack
Automatic storage of intermediate results is the reason that the
HP-21 slides so easily through the most complex equations.
And the key to automatic storage is the Hewlett-Packard auto-
matic memory stack.
Initial Display
When you first switch the calculator ON, the display shows
. This represents the contents of the display, or
“*X-register.”
Basically, numbers are stored and manipulated in the machine
‘“‘registers.”” Each number, no matter how simple (e.g., 0, 1,
or 5) or how complex (e.g., 3.141592654, -23.28362, or
2.87148907 x 10?7), occupies one entire register.
The displayed X-register, which is the only visible register,
is one of four registers inside the calculator that are positioned
to form the automatic memory stack. We label these registers
X, Y, Z,and T. They are ‘‘stacked’’ one on top of the other
with the displayed X-register on the bottom. When the calcu-
lator is switched ON, these four registers are cleared to 0.00.
Name Register
T-(top) | 0.00
Z 0.00
Y 0.00
X 0.00 Always displayed.
You can also place all 0’s in the stack registers by pressing
B (7). Do this now.
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