HP25c continous memory

[teenix]

(02-16-2021 02:51 PM)Dave Britten Wrote:  Interesting, apparently I hadn't read that in very close detail before, as I didn't realize the 25C didn't detect power loss and start up with "Pr Error" like other models. It makes sense that they didn't have room to add error detection though, since the 25 ROM was pretty much packed full right to the last byte. What differences are there in the 25C ROM? Just the omission of the startup sequence that zeros out the registers and fills program memory with GTO 00?

No spare RAM either which is how the others detect a battery out reset, by storing a special value in RAM. For the 33C, the display information for "Pr Error", the 34C stores "EAE"

This is tested on power up, and if it is not there, as when batteries have been removed for more than that allowed, then the software does a full reset and clears all RAM registers and writes the special value to RAM.

In the case of the 33C, I'm not quite sure of the mechanism, as there is only supposed to be 16 RAM registers as far as I know, but this value is stored in RAM address 20. Maybe it is a special register in the processor chip so that normal RAM has full user functionality.

cheers

Tony