HP HP-21 - Exchangingxandy

When you press[x]the first time, you calculate 1.15 x 1000. The
result (1150.00) is displayed in the X-register and a new copy of
the growth factor drops into the Y-register. Since a new copy of
the growth factor is duplicated from the T-register each time the
stack drops, you never have to reenter it.
Notice that performing a two-number operation such as [x]
causes the number in the T-register to be duplicated there each
time the stack is dropped. However, the [ key, since it ro-
tates the contents of the stack registers, cannot rewrite any
number, but merely shifts the numbers that are already in
the stack.
Exchanging x and y
Another key that manipulates the stack contents is the
(x exchange y) key. The BB key exchanges the contents of
the X- and Y-registers without affecting the Z-and T-registers.
For example:
The stack contents now look
Press
like
this.
1 2 3 4 T [ 100
z 2.00
Y 3.00
X 4. Display
And the stack contents
Press are changed to this.
T 1.00
Z 2.00
Y 4.00
X 3.00 Display
Pressing EBJ again will restore the numbers in the X- and Y-
registers to their original places. You can always verify the
position of the data in the stack by reviewing with the EXllkey.