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HP -28S Manual Page 299

Advanced scientific calculator.
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Stack Memory. A dynamic stack has the advantage that you can use
as many levels as you need for any calculation, without worrying
about losing objects
Hoff
the top" as you enter new ones. This also has
the disadvantage that you can tie up a significant amount of memory
with old objects, if you leave them on the stack after you are finished
with a calculation. With the HP-28S, you should get in the habit of
discarding unneeded objects from the stack.
DROP Versus CLX. In fixed-stack calculators, CLX means Hreplace
the contents of the X-register with 0, and disable stack liW (see be-
low). Its primary purpose is to throwaway an old number, prior to
replacing it with a new one-but you can also use it as a means to
enter O. On the HP-28S, CLX is replaced by DROP, which does what
its name implies-it drops the object in level 1 from the stack, and the
rest of the stack drops down to fill in. No extraneous 0 is entered.
Similarly, CLEAR drops all objects from the stack, instead of replacing
them with zeros as does its fixed-stack counterpart CLST (CLEAR
STACK).
Stack-Lift Disable and ENTER
Certain commands on fixed-stack calculators (ENTER t, CLX,
~
+,
~
-) exhibit a peculiar feature called
stack-lift disable.
That is, after
any of these commands is executed, the next number entered onto the
stack replaces the current contents of the X-register, rather than push-
ing it into the Y -register. This feature is entirely absent on the HP-
28S. New objects entered onto the stack
always
push the previous
stack objects up to higher levels.
The X-register and ENTER on fixed-stack calculators play dual roles
that are derived more from the single-line display of the calculators
than from the stack structure. The X-register acts as an input register
as well as an ordinary stack register-when you key in a number, the
digits are created in the X-register, until a non-digit key terminates
entry. The
I
ENTERt
I
key is provided for separating two consecutive
number entries. But in addition to terminating digit entry, the
I
ENTERt
I
key also copies the contents of the X-register into Y, and dis-
ables stack lift.
298
B: Notes for HP RPN Calculator Users

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