Question about Stack and Roll

[wojtek]

(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 is correct (if you press Roll Down after each Enter). I believe all HP rpn calculators behave the same way. Make a quick test and clear the stack (press "clear" and Enter 4 times) then press: 1, Enter , 2. You will get the stack with: 0,0,1,2 (from T to X). But if you press Enter after 2 as well and then will inspect the stack it will contain: 0,1,2,2 (withs X register duplicated in Y). This is because of convention applied by HP that Enter moves up the stack and sets some flag what makes the next number entered replace the duplicated X register. If you press Roll down the flag is cleared and the duplicated value remains in X and Y registers