Exchanging The X- And Y-Registers In The Stack - HP 35s User Manual

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
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.
T 1
Z 2
Y 3

X 4
Exchanging the X– and Y–Registers in the Stack
Another key that manipulates the stack contents is
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
 
Press
The keystrokes to calculate this expression from left–to–right are:

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.
2-4
RPN: The Automatic Memory Stack
2
3
4

1
(13
.
.

except that it "rolls" the stack
3
4
1

2

(x exchange y). This key
8):
4
1
2

3
1
2
3
4