68kMLA Classic Interface

So the full range 0x40000000 to 0x403FFFFF might already be addressable. I could probably mock together a 4MB ROM file for you that has an embedded ROM disk driver in the first 2MB, and the ROM disk in the second 2MB. I won't really have a way to test it, but you could try burning it on your prototype whenever you get around to it. Want to give this a try?

Oh, wow... I was shortly going to post that I couldn't get my PowerBook to read past the 2MB boundary!

I see that on yours, 40200000 is displaying the same contents as 40000000 ... not sure what that means in practice but it seems to be looping back to the first 0-2MB in the 2-4MB part of the map.

What are you guys CPU board revisions ? AFAIK we still don't know the differences, might be one of them

Great info! I was trying to find a pinout but couldn't. You may have put together the best resource for this 🙂

With 20 address lines being exposed (A2-A21), that works out to be: (2^20 longs) * (4 bytes / long) = 4MB

And the fact that your most recent DM shows all 0xFF starting at 0x40200000 is very promising. I am guessing when you burned these chips, your programmer may have just kept everything at and above that address as 0xFF.

So I guess that does beg the question, why did weren't you able to DM 0x40200000 on the 550c ROM initially? You pulled the daughtercard and put a new one in with the 540c ROM, then when you swapped back to the 550c you were able to do it. Does your machine have a working PRAM battery? It would be interesting if you could recreate the problem somehow. I'll see if I can tweak with my 540c to have that same issue.

But nevertheless, I think we should give a 4MB ROM a test run. It might be a few days, but I'll spin it up and send it over to you.

Exciting!

I dumped the contents of one of the 2MiB 520/540 Mask ROM chips just now and the code repeats starting at 0x00100000; it's not blank like on my programmed chips. That is presumably why we're seeing the same code 'aliased' starting from 0x40200000 with the stock ROM.

And 2 megs of space isn't bad at all, especially with FC8 compression.

OK sharing here for all to see, inspect, and pull apart. This has the 550c ROM in the first 2MB with the edits above, and the driver embedded at offset 60080. Starting at the 2MB mark is the ROM disk. This is a bit of a hack. I found a System 7.5.0 boot floppy image, and decided to use that. The disk has a SCSI driver installed, which takes up unnecessary space for a ROM disk. But I added it as I wanted to test booting via BlueSCSI on my PB540c. (Booted without issue FYI)

We will build a better ROM disk with FC8 once we know that this is working OK. Didn't want to add FC8 right out of the gate. Also this is what I call driver v1.0. It uses some Toolbox routines to paint the splash screen. So hopefully those work OK during the early boot process.

@croissantking I hate for you to have to go through all the work of desoldering, burning, and resoldering if this is a bust. Fingers crossed, but in my experience things tend not to work the first time around 🙂

Well darn. I had a very similar issue on my LCIII when I was customizing that ROM. So in the driver I had to disable the splash screen then it booted OK.

Let me do some tinkering with this, and maybe we can give it another go around here soon.

Ready for your next update!

• 1_550c_MOD.ROM = Offset 1278 does not automatically open the ROMDISK driver.
• 1_550c_MOD.ROM = Offset 1706 does not treat the ROMDISK like a floppy.

OK here are two to try out. I reverted to an earlier version of the driver at offset 60080 that does not have a splash screen. Here is the difference between the two files:

• 1_550c_MOD.ROM = Offset 1278 does not automatically open the ROMDISK driver.
• 1_550c_MOD.ROM = Offset 1706 does not treat the ROMDISK like a floppy.

So in essence, 1_550c_MOD.ROM might be a bit 'safer' to test with first since the driver won't automatically load at startup. I can write up a small program that opens the driver if you can boot this into the Mac OS.

0_550c_MOD.ROM will automatically try to open the driver at boot. You can try R or RA to get to boot to the ROM disk.

Fingers crossed for round 2!

That's good news that it booted. Checksum patch worked. Do a DM 40200000. Does that show the start of the ROM disk? This disk starts with a SCSI driver that is really not needed at all.
View attachment 101237

Also run these two programs from Rob Braun. These are handy little programs to see what drivers are loaded, and which of those have assigned drive numbers. This way we can see if the ROMDISK driver was able to open.