ENTER. Pressing ENTER separates two numbers keyed in one after another. It does so by lifting the stack and copying the number in the display (X-register) into the Y-register. The next number entered then writes over the value in the X-register; there is no stack lift. The example below shows what happens as the stack is filled with the numbers 1, 2, 3, 4.
|----------> lost |--------> lost
T t--| |--------> z--| |------> y
Z z----| |------> y----| |----> x
Y y------| |----> x------| |--> 1
X x--------| |--> 1--------|--> 1
|
Keys: 1 ENTER
|--------> lost
T y-------------> y--| |------> x
Z x-------------> x----| |----> 1
Y 1-------------> 1------| |--> 2
X 1 |--> 2--------|--> 2
|
Keys: 2 ENTER
|--------> lost
T x-------------> x--| |------> 1
Z 1-------------> 1----| |----> 2
Y 2-------------> 2------| |--> 3
X 2 |--> 3--------|--> 3
|
Keys: 3 ENTER
T 1-------------> 1
Z 2-------------> 2
Y 3-------------> 3
X 3 |--> 4
|
Keys: 4
See "CALCULATOR FUNCTIONS AND THE STACK", this help.