The x and y–registers are in the display, Clearing the x–register – HP 33s User Manual
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RPN: The Automatic Memory Stack
T
0.0000
"Oldest" number
Z
Y
X
Displayed
0.0000
0.0000
0.0000
Displayed
The most "recent" number is in the X–register: this is the number you see in the
second line of the display.
In programming, the stack is used to perform calculations, to temporarily store
intermediate results, to pass stored data (variables) among programs and
subroutines, to accept input, and to deliver output.
The X and Y–Registers are in the Display
The X and Y–Registers are what you see except when a menu, a message, or a
program line is being displayed. You might have noticed that several function
names include an x or y.
This is no coincidence: these letters refer to the X– and Y–registers. For example,
{
raises ten to the power of the number in the X–register.
Clearing the X–Register
Pressing
{ c
{º} always clears the X–register to zero; it is also used to
program this instruction. The
key, in contrast, is context–sensitive. It either
clears or cancels the current display, depending on the situation: it acts like
{
c
{º} only when the X–register is displayed.
b
also acts like
{ c
{º} when the X–register is displayed and digit entry is terminated (no cursor
present). It cancels other displays: menus, labeled numbers, messages, equation
entry, and program entry.