HP 35s Scientific Calculator - Manual (Page 50)

2-4 RPN: The Automatic Memory Stack
What was in the X–register rotates into the T–register, the contents of the T–register
rotate into the Z–register, etc. Notice that only the contents of the registers are rolled
— the registers themselves maintain their positions, and only the X– and Y–register's
contents are displayed.
R (Roll Up)
The  (roll up) key has a similar function to  except that it "rolls" the stack
contents upward, one register at a time.
The contents of the X–register rotate into the Y–register; what was in the T–register
rotates into the X–register, and so on.
Exchanging the X– and Y–Registers in the Stack
Another key that manipulates the stack contents is  (x exchange y). This key
swaps the contents of the X– and Y–registers without affecting the rest of the stack.
Pressing
 twice restores the original order of the X– and Y–register contents.
The  function is used primarily to swap the order of numbers in a calculation.
For example, one way to calculate 9 ч (13 Ч 8):
Press   .
The keystrokes to calculate this expression from left–to–right are:
 .
T 1 2 3 4 1
Z 2 3 4 1 2
Y 3 4 1 2 3
X 4  1  2  3  4
Note Understand that there are no more than four numbers in the stack
at any given time – the contents of the T-register (the top register)
will be lost whenever a fifth number is entered.