Roll-down key in HP-15C

[Steve Ross]

The roll-down (and roll-up) key on my HP-15C do not behave as I expect them to behave. Would someone please explain the following behavior to me.

Key     X-register   Comment

-----   ----------   ----------

0       0

Enter   0.00         Fill the stack with zeros.

Enter   0.00

Enter   0.00

1       1

Rv      0.00         Roll-down

2       2

Rv      0.00

R^      2.00         I expect to see "2.00".  So far, so good.

R^      0.00         I expect to see "1.00", not "0.00"!

                     Where did the "1.00" go?

I see the same behavior if I exchange the locations of the two roll-ups and the two roll-downs in the above listing.

Thanks in advance for your help,
-- Steve Ross

[Mark Hardman]

Think about it...

1 v 2 v ^ ^

- - - - - -

1 0 2 0 2 0

0 0 0 0 0 2

0 0 0 0 0 0

0 1 0 2 0 0

[Richard Berler]

When you roll down after entering "1", the "1" goes to level "z", 4th stack level. When you enter "2", shouldn't this lift everything a stack level up meaning that you are now losing what was in level 4, your "1"?

[Michael de Estrada]

As soon as you typed 2 in the x-register, the calculator did a stack lift and replicated the 0 in the z-register into the t-register, overwriting the 1 that had been there.

[Marcus von Cube, Germany]

You can disable the stack lift that occurs when typing the 2 if you precede it with a Clx.

[Steve Ross]

Thanks to all of Mark, Richard, Michael, and Marcus for your quick responses. I see that I didn't have a good understanding of "stack lift enabling" and that the stack lift did not actually "fire" until you key in the next digit. So I key in the first number, "Rv" enables the stack lifting behavior, and the second digit that I keyed in lifts the stack and causes the first number to be lost. Understood.

Thanks also to Marcus for his "CLx" work-around.

-- Steve

[Nick_S]

You will find a web-based simulation of the HP-15c that has both the calculator interface and fields with the stack-levels and registers so you can see what is going on under the hood as you type in an example.

http://hp15c.com/web/hp15c.html

Nick

Edited: 15 Oct 2013, 4:13 a.m.