Algorithm for entering numbers?

[Thomas Okken]

As a general rule, when you start entering a number, or recall a number from a register, the stack lifts, i.e. X is pushed into Y, Y is pushed into Z, Z is pushed into T, and T is lost.

There are only a small number of exceptions to this rule. The stack is not pushed, and X is overwritten instead, after ENTER, CLX, Σ+, and Σ-. These operations are said to disable stack lift; all other functions enable it.

On the HP-42S, it's a bit more complicated, and the RPL machines are a different story altogether, but on almost all of the four-level HP calculators, this is all there is to it.