HP 35s HP 35s scientific calculator - User Guide - Page 50

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. T1 2 3 4 1 Z2 3 4 1 2 Y3 4 1 2 3 X 4  1  2  3  4 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 ÷ (13 × 8): 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