HP 33s - Manual (Page 308)

B–4 User Memory and the Stack
Memory may inadvertently be cleared if the calculator is dropped or if power is
interrupted.
The Status of Stack Lift
The four stack registers are always present, and the stack always has a stack–lift
status
. That is to say, the stack lift is always enabled or disabled regarding its
behavior when the next number is placed in the X–register. (Refer to chapter 2,
"The Automatic Memory Stack.")
All functions except those in the following two lists will enable stack lift.
Disabling Operations
The four operations ENTER, Σ+, Σ–, and CLx disable stack lift. A number keyed in
after one of these disabling operations writes over the number currently in the
X–register. The Y–, Z– and T–registers remain unchanged.
In addition, when ‡ and b act like CLx, they also disable stack lift.
The INPUT function disables stack lift as it halts a program for prompting (so any
number you then enter writes over the X–register), but it enables stack lift when the
program resumes.
Neutral Operations
The following operations do not affect the status of stack lift: