HP HP-21 - Page 29

Press [[X3 twice more . . . and the stack shifts . . .
. . . through this . . . . . . back to the start again .
T [0.00 T 10.00
Z 70.00 Z 0.00
Y 31432 Y 0.00
X 10.00 Display X 31432 Display.
Once again the number 314.32 is in the displayed X-register.
Now that you know how the stack is rotated, you can use the
XA key to review the contents of the stack at any time. Always
remember, though, that it takes four presses of the [l key to
return the contents to their original registers.
Now ensure that the original number, 314.32, is again in the
displayed X-register. In order to key in a second number, you
must first tell the calculator that you are finished writing the
first number.
Pressing the [[I@ key told the calculator that you were finished
writing the number 314.32, and you could key in a new number
right now. But in actual practice, as you already know, you use
the key to separate numbers.
So to see how the key normally works, press [§% and
then key the number 314.32 back into the X-register. Then press
to change the contents of the registers . . .
from this: to this:
T 10.00 T 10.00
Z 10.00 Z 0.00
Y 10.00 Yy 314.32
X [314.32 Display. X |314.32 Display.
As you can see, the number in the displayed X-register is copied
into Y. (The numbers in Y and Z have also been transferred to
Z and T, respectively, and the number in T has been lost off the
top of the stack. But this will be more apparent when we have
different numbers in all four registers.)