Some success with the 4MB #Psion 3mx!

I've been able to write directly to a page of RAM in the upper 2MB of memory, using #EDisAsm. I modified one of the memory-dumping assembly routines to basically go in the opposite direction. It writes 16 bytes to bank 0x6000, page 0x20. Then I dumped that to a file.

It worked! There's no corruption, and I can't see it mirrored anywhere. Using this basic test, we can pretty safely say that ASIC9MX is able to address the extra RAM (like the SDK says), and that the second chip is soldered correctly.

However, this doesn't explain why #EPOC16 doesn't like it. To recap, although EPOC16 reports that it has 4MB RAM, the ramdrive (M:) is inaccessible - apps report that the "media is corrupted", and the OS says it's unformatted. Any attempt to format the ramdrive fails silently.

The current guess is that the ramdrive "driver" can't handle more than 2MB RAM. It might be just the formatting routine, but it could be the ramdrive filesystem can't handle it. However, there might be a hard limit on pointers. This could be a bigger issue: the ramdrive in EPOC16 resizes dynamically, so the ramdrive "driver" and the filesystem's pointers would need to be able to handle the bigger filesystem size.

Unfortunately, this filesystem is undocumented - Psion never expected anyone to need to fiddle with it. Fortunately, EDisAsm can dump all memory, so it would be possible to analyse it. Dump a freshly booted 2MB 3mx, add a file, dump it again, delete a file, dump it again...

If it turns out that this is the case, it might be necessary to create a custom EPOC16 ROM. This would require a few things: working out how to modify the driver (and anything else) successfully, putting together a new EPOC16 image with the be driver, and a 3mx that's been modded to take flash. Bearing in mind that no EPOC16 source has been unearthed.

In conclusion, definitely progress, but there's still a long way to go.

#retrocomputing