HP 33s hp 33s_user's manual_English_E_HDPM20PIE56.pdf - Page 308

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: B-4 User Memory and the Stack