HP-25 Question

[Greg]

The HP-25 includes an unlabelled function, g sum, which can be entered into program memory and which seems, on emulators at least, to be NOP.

The emulators include some strange behaviour in how they emulate one of the microcode instructions. If the emulators are correct, the function does behave as NOP. If the microcode instruction behaves more as its name suggests, the function is more likely to behave like R/S.

Would someone with a physical HP-25 or HP-25C, be able to:
- clear program memory,
- key in three g Sum ("15 25" in PRGM mode) steps,
- switch to RUN,
- f CLEAR PRGM (to go to step 00),
- press R/S (to run the three steps),
- switch to PRGM again, and
- let me know what step it ends on please?

If it does behave as a NOP, it should go through all three and then do the GTO 00 at the end. In that case, it would end at step 00.

If it behaves like R/S instead, it should stop at step 01.

s1 is the "running" flag in the HP-25. This is to check whether the microcode for g sum clears s1 and stops as a result; or if - as the emulators suggest - s1 remains set.

I appreciate any help you can give me with this.

All the best,