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Would the HP-15C Limited Edition Firmware Age Away?
03-22-2018, 09:15 AM
Post: #32
RE: Would the HP-15C Limited Edition Firmware Age Away?
Hi Mike,

The calculators loop around from the final step back to step 1 and continue. That's why you can find a LBL that is before you and why, in some cases, they had "rapid reverse branching". It was "rapid" (VERY rapid actually) because it just adjusted the user program counter value rather than looking all the way through memory (here to end, then start to here) for a LBL.

It is interesting to watch the HP-65 as it actually gets to step 00 and executes the "top of memory marker" instruction. It doesn't do anything but the microcode goes through the motions as if it were a real instruction. I haven't noticed how the other models treat the transition. It just was notable when I was investigating what you were seeing.

I think, and I don't have anything immediately to hand to back this up, that Eric and Jacques Laporte were working on the HP-65 for nonpareil. Eric seems to have moved on (site hasn't been updated for a while). The HP-65 seems to have brought more than its fair share of mysteries to untangle and I'm sure that both of them must have felt it, on many occasions, to be more effort that it was worth. The whole card reader, 600 bit circular buffer, and embedded pointers is a real nightmare as I'm sure Tony will agree. It's not at all intuitive today; but it was probably best practice at the time. I suspect Eric got lots of the others going in the meantime. The card reader added a lot of complexity. I do know that Jacques did eventually get it to work in Java as Francois Roulet's one ( is based on that. Mine derives from Francois's. There have been a few tweaks and we've both contributed to each other.

- Greg from
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RE: Would the HP-15C Limited Edition Firmware Age Away? - Greg - 03-22-2018 09:15 AM

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