Question about Stack and Roll

[Guenter Schink]

(07-01-2016 02:03 PM)Antlab Wrote:  Hi all.
I have a question for the RPN experts.
To refresh my RPN skills, I am re-reading the original manual of the HP-15C, the Owner's Handbook. I dont' have the original HP-15C, but several Android emulators.
The question is this:
Page 34 of the manual reports the simple example that fills the four registers with the digits 1, 2, 3 and 4.
The normal method is obviously:
1) 1
2) Enter
3) 2
4) Enter
5) 3
6) Enter
7) 4

At this point, with the Roll Down key is possible to inspect and verify the contents of the registers.

My problem is that if I use Roll Down to inspect the register contents after each Enter of the previous sequence (returning each time to the X register) I found at the end that the registers are filled with the sequence:
T: 2.0000
Z: 3.0000
Y: 3.0000
X: 4.0000

This surprises me, because I was expecting the sequence 1, 2, 3, 4.
Probably I am missing some subtlety of the RPN Roll behaviuor, could you please help me to understand what happens?
Thanks.

It's the same on the real 15c. When you enter "1" "Enter" "Rdn" you'll recognise that at the first "Rdn" the "1" blinks shortly and only the next "Rdn" will show another value. If you don't enter other values rather than continue pushing "Rdn" you'll see that "1" is twice on the stack.

And this happens consequently as you continue to enter the numbers 2,3,and 4 this way.
The two "1s" and one of the duplicate "2s" are pushed from the stack, leaving one remaining "2", the two "3s" and finally the "4".

Page 33 of the manual. "enter"copies the number in the display to the y-register and prepares the x-register (display) for another input. But when you don't enter a number but execute a function (e.g. sin, ln) this will be done on the x-register. "Rdn" also is such a function, therefore you end up with the y and x-register having the same content and actually the "Rdn" didn't do a roll down yet.

HTH, Günter

Edit: have a look at appendix B "Stack lift ..."