Solos rom question

[Hugo Holden]

I'm in the process of building a replica personality module for the SOL-20 using 4 MM5204's.

I have inspected the dump of a solos that came on a disc with a replacement personality module from ebay and also I have dumped another personality module's contents from another version of the module. These dumps match, indicating they are probably correct/normal.

However, I got a little surprise when I dumped the contents of my Sol's original module with the 4 MM5204's, it does not match, The final byte at C7FF which is an FA for the other dumps, is an 00 in my MM5204 module. All other bytes are a perfect match.

Can anyone suggest:

1) how this could have come about, if it is rom rot, would it go to FF or 00 for this IC ?
2) what would be the implication/result of that last byte of the Solos file being incorrect ?

 

[deramp5113]

That byte is unused. I’ve seen both 00h and FAh in SOLOS ROMs.

Mike

 

[Hugo Holden]

That byte is unused. I’ve seen both 00h and FAh in SOLOS ROMs.

Mike

Thanks Mike, it had me a little concerned, especially since when I got the MM5204 module the clear windows were not sealed to light and I had wondered if they might have partially erased over the years. So it is not a worry at all in this case.

Also I was pretty sure that if it was rom rot it would go to FF so it looked like the 00 had been deliberately programmed. Still, if it wasn't used one might have expected that byte to be either an 00 or an FF depending on the initial state of the type of rom before it was programmed. So the FA looked like it might have had some significance and I have not seen the disassembled program yet.