HP 9100A/B: Effects of AC power interruption?

[rickb]

I have tested this on both the 9100A and 9100B, with various different programs on both machines, and have always experienced that the programs resume right where they left off when the power was interrupted, without causing any errors or other problems with the programs.
That would imply that all of the state of the calculator is stored in the core memory, including various bits of information that are not accessible to the user. I don't know if this information is stored in an 'off limits' area in the core memory array that isn't accessible programmatically, but is accessible by the microcode. The calculator microcode must sequence the operations in such a way that the state information is maintained in such a way that if the operation of the machine is interrupted, it can be restarted where it left off. Pressing the [STOP] key has this effect, and I suspect that when the power is sensed to be dropping that the microcode essentially immediately executes the equivalent of the [STOP] key quickly enough that everything is static before the power drops off below operating levels. That is made possible by the large capacitors in the power supply. All supposition on my part...I haven't looked at the microcode to see what it does when the power drops, but I'm just guessing.