The display when the SST key is held down includes the step number, so checking program flow and branching is easy. The HP-25 contains a number of functions that make programming simpler. Eight comparisons allow the program to react depending on the data in the calculation stack. When encountered in a program, the calculator stops for a second, displays the most recent result, and then continues the program. This is useful when programming iterative functions because one can watch the function converge or diverge.
Key phrases go into numbered locations in memory, overwriting the previous contents.
Branching in the program is to the step number of a phrase. This is in contrast to the HP-65 type of programming. In the HP-65, keycodes shift around in the unnumbered memory as steps are inserted and deleted, and branching goes to label keycodes contained in the memory. The HP-25 merges keystrokes into key phrases using a microcoded finite state machine. The machine carefully checks for undefined key sequences.