How to: upgrade a 520/540 to a 550c ROM

[croissantking]

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?

I didn't see this part of your message – yes, sure go ahead! Although it's a bit fiddly, I dont mind soldering + desoldering a few times to get this right.

I'm still keen to understand why I'm not able to dump past the 2mb limit on my own machine...

 

[frontein1]

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.

Very interesting difference. Yes on mine, the ROM is essentially being aliased over again from 40200000 to 403FFFFF. But not on your machine. (Although to be fair I am not totally convinced if this is true aliasing or if this space is addressable. A good test would be that custom 4MB ROM and then dumping 0x40200000 to make sure that is different from 0x40000000.)

It might be interesting for you to issue this in the debugger: DM 40080870. What does that look like? Here is the one from my 540c (Just to rule out MacsBug I switched back to MicroBug and was still able to dump out the second 2MB area):

 

[croissantking]

I am very confused, Mr Frontein1!

I tried DM 40080870 and it spat out the same data as on yours, but DM 40200000 still told me it couldn't access that address.
So I pulled out another 540c (different machine) and it displays the aliased ROM at 40200000, just like yours.
Then I removed the CPU card from the first machine which has a 550c ROM, and installed another with the 520/540 ROM. It displayed the aliased data just like yours.
Then I put back the card with 550c ROM and now it is showing the actual contents of the ROM past the 2MB mark!