HP-41CY re-initialization ROM

[Jurgen Keller]

(11-03-2019 05:59 PM)hth Wrote:  

(11-03-2019 02:32 PM)Jurgen Keller Wrote:  It looks like I'm doing a monologue ;-)

Because I am so totally lost by all this.

Why does it not have a fail-safe mechanism to re-install its OS/control image (which is in RAM?) How did they even program it from the factory? And if the software is broken and without such fail-safe mechanism, how is it even possible to repair the software?

I guess I am mentally blocked from following your endeavours as I just cannot get it into my mind that this should be a problem in the first place...

I am probably missing something here.

W&W did for sure have such a fail-safe mechanism. Unfortunately, I didn't found any information about it. There is also a valued member of this forum who has developed such a re-initialization module in the past. However, he decided to not disclose that module for personal reasons and I fully respect his decision. Nevertheless, for personal curiosity, to get into MCODE-ing and to be able to restore my 41CY when needed, I wanted to solve the mystery.

The whole re-initialization process is not very complicated once you understand how the ENROMx instructions work in the 41CY. There is only one situation which caused me headaches:

• Assume that the 41CY "died" while bank #2 was active.
• Please note that after a ENROMx instruction is executed the next instruction is taken from the bank to which you switched to.
• It is possible to only enable odd or even pages of bank #2 but the pages of bank #1 can only be enabled together (ENROM1).
• Therefore you would end up with a situation as shown in my first picture: after the ENROM1 instruction the CPU would execute garbage which potentially crashes the calculator.
  

Not sure if that sheds some more light on it ...

After many partially working attempts the current version of my re-initialization module seems to work fine now. Hopefully, I can publish it soon.