HP HP-21 - Clearing; Arithmetic-How the Stack Does It

Clearing
To clear the display only, press [EB3. To clear the entire auto-
matic memory stack, including the displayed X-register (but
not the manual storage register—more about that later), press
I (cr]. (Notice that it isn’t necessary—although it may be
comforting—to clear the calculator when starting a new calcu-
lation.) To clear everything, including the manual storage
register, turn the HP-21 OFF, then ON.
Arithmetic—How the Stack Does It
In Hewlett-Packard calculators, arithmetic is performed by
first positioning the numbers in the stack the same way you
would on paper. For instance, if you wanted to add 34 and 21
you would write 34 on a piece of paper and then write 21 under-
neath it like this:
34
21
and rhen you’d add like this:
34
+21
55
Numbers are positioned the same way in the HP-21. Here’s
how it is done. (Clear the previous number entry first by
pressing J)
Press Display
34 34 is keyed into X.
34 is copied into Y.
21 21 writes over the 34 in X.