Roll-down key in HP-15C



#8

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


#9

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
#10

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"?

#11

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.

#12

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


#13

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


#14

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.


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