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Curiously, the 29C and 19C hold the X register in continuous memory but not the Y, Z, T nor LASTX registers. I wonder why they didn't go for the whole shebang? Or why they didn't even bother holding the X register in the Spice "C" machines.

My guess is that all the stack registers in the Woodstocks are on the CPU (ACT) chip. But in the 29C and 19C they keep a copy of it in the CMOS chip as well. (Probably this was a design compromise between CMOS storage space cost and wanting to keep the whole stack in CMOS.) Upon power-up they copy back from the CMOS chip. In the Spice machines they went for low cost and consistency of design so gave up on this idea.

I never understood THAT choice at all. :-)


after Katie's wise words and your observation, I add the fact that the ACT chip, if containing stack registers, may not contain the LASTx. Both HP21 and HP37E have no LASTx access, maybe no LASTx register too. Are there others without LASTx?

Then we conclude (hopefully in a correct reasoning) that the LASTx register is in fact in RAM, not in the ACT chip, what explains the fact it is kept in the HP25. As it is not so practical, maybe a remanagement in the HP19C/29C exchanged their places. I agree with Katie but I believe that as RAM was an expensive resource at that time, they have simply used LASTx holding feature and rearranged addresses so users would take the advantage of having at least the X-register back.

Wrong reasoning? Maybe. This would not need a new internal ACT design, just new ROM coding with the X-register addressed in RAM and LASTx addressed inside ACT.

My 2¢ (now I know how to get it...)

Luiz C. Vieira - Brazil

The ACT and ROM chips in the 25 and 25C are identical. The only difference is the RAM chip and it's backup supply connection. Therefore the HP25 must save LASTX in the RAM chip and the stack in the ACT which is not backed up with the battery.


I love your 2¢ now that I know how to get it!


Hello, Trent;

thank you. If it is not for you, I'd never notice it.

You, guys, are gold!

Best regards.

Luiz C. Vieira - Brazil