HP-41 damaged OS ROM recovering.


Hi all,

This may be of little interest, but some of you may take advantage of the technique described herein to revive a defeated Fullnut, or getting an OS-free one for M-code experimental purposes.

Saturday afternoon I picked up a non-working fullnut 41C from the post office. Symptoms were somewhat strange. The calculator itself works flawlessly... but, in ALPHA mode, almost every key I press shows a "CAT _" in the display, and even performed a real "CAT (1, 2 or 3)" if I selected it... without leaving ALPHA mode!!!

After some testing with Service ROM, the offending chip was easily identified as ROM-2. It was an "F" version ROM.

Apart from that the calculator was *really* mint. No scratches at all, no markings and no corrosion traces neither external nor internally.

I take the CPU board apart and remove the ROM-2 chip (lower leftmost), obviously no longer "ON" reaction at all...

Then I prepared a modified Clonix to fill the H'2000 page gap with the appropriate Nut2-F.ROM image. Yes, it works!

Next step (I bet you can figure it out) was removing the remaining two ROM chips and configure a new Clonix with the whole Nut OS ROM images for pages H'0000 to H'2000... Yes, it also works!!

Further configs allow me to "play" with different ROM set versions and test "those wonderfull 'bugs'" everyone of us have heard of... but only those fortunate owners on a 19xxAxxxxx have tried... up to now ;-))

I'm *really* far from knowing enough on M-code, so, for me, it isn't anything but a curiosity (and a means by wich I can get a ROM damaged Fullnut back to life). I think a skilled M-code programmer can get a lot of juice out of this "feature"... I may be wrong though.

If there's any interest please let me know.

Best wishes from Spain.


Edited: 26 Jan 2004, 9:15 a.m. after one or more responses were posted


Saludos Diego,

Cool hacking indeed, I see you're having fun exploring the uncharted waters of the 41's OS.

This reminds me of an article I read some years ago about the Aussies writing a take-over ROM (addressed on port 4) that would redirect the execution to an alternative OS... which (why not?) could reside on a Clonix. Such setup should of course also work on machines with their chips soldered.

This is also the mechanism used by the FORTH EPROMs, although I've never used them myself.

Coding the changes isn't a trivial task, however. If I only had the *time* required to dig out more about all this...



Hola Ángel,

Yes, H'4000 page is a suitable workaround to take over the HP-41, what you've noticed for sure is that the "Clonix OS" approach also frees those lower three pages, allowing even Bank-Switching... a nice 24k (48k if banks 3 & 4 are used) word space!!

Too much for a simple Hardware guy like me :-) and for most users I'd say...

Anyhow, whomever is willing to play with this little Black Beast, can now have a really wide field (theoretically up to 256k!!!) I still remember my first 8088 PC had only 64K :-o

As stated on my first post I don't really think this issue is for much interest nowadays but, I'm glad to know it can be done.

The side effect could be more interesting though: The possibility of bring back to life a non-working 41 due to faulty ROM(s) chip(s), and/or upgrade/retrograde OS versions at will.

Time will tell.

Best wishes.



MLDL2000 will support this as well! Actually, part of the design was done for exactly this type of use, should anyone wish to do so.


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