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That code just sets A5 to the A5 World location for the current application. Typically it would be used for the application's variables, probably just something the compiler leaves in for the possibility that it will be used.

I'm not familiar with how C or Pascal might be doing it, but my guess would be it uses LINK to save the stack pointer to an address register. If you can get that stack pointer you should have a predictable location for the exception stack frame. It will be a bit further back however, as the...

The model for the Portable is the DataShow HR/MP. The DataShow 512C is a 512 color model that probably takes standard VGA.

Perhaps it is somehow getting shorted to ground at the power plug end. I've seen it happen with a bench supply that was not even outputting power yet, so somehow just physically plugging it in does it. I ended up installing a resettable fuse. It does this after the fuse is blown? With the fuse...

I usually just use Ghidra to do all my disassembly. Ghidra has a built in script to load labels quickly, so I format the ROM maps and load those in, as well as loading in labels for low memory globals and hardware addresses. Ghidra isn't perfect though (at least for this use case), I often...

Dissecting the stack from the start: 40800192 The return address from InitADB, meaning InitADB was started but never completed. 2000 Status Register 40806DD8 Program Counter 0064 Exception vector offset (Level 1 interrupt) The stack frame for a level 1 interrupt generated...

That looks correct. It looks like it started initializing ADB but never finished, and then during that it looks to be trying to service an interrupt on VIA1 (possibly from the ADB or RTC). It's still hard to tell what exactly is going wrong though, I may have to look at it some more.

Well I'm not too sure, if the program counter was truly at 0x22 then there's a good chance that generated the bus error, because that's in between two exception vectors (0x22 and 0x24) and since it likely interpreted that as move.l D6,-(A0) then if the contents of A0-4 was a bad location in...

That should be correct, but it seems like the right information isn't there. Typically what you should see is Program counter where the exception occurred - XXXXXXXX Processor status register - XXXX Additional info depending on the exception Instead you are getting: 408060D0 - SCC Interrupt...

If you're getting 0xF then you should be past SCSI initialization. Check the value stored at 0xC70 for the place where the exception occurred.

I did end up buying one of the 10.2" screens just to test it, here's what it looks like (poorly aligned and held in with Kapton tape):

There's a good chance the expansion chassis likely rewrote address access to PDS cards since the SE and Portable address spaces aren't quite the same. It's hard to say for sure because I've never seen one outside of a picture I have in a magazine. There are some "free" address spaces that...

The MacsBug disassembly is correct. Ignoring the extra bits, the instruction should compare D1 the contents of location A4+4. If they are equal, D2 is written to A4+4. If they are not, then A4+4 is written to D1. I wonder if A1 is being modified due to the way the silicon is designed, as if...

Yes, it looks like right when it is running the CAS instruction is where it is changing A1. I think the problem is some sort of either undefined or undocumented behavior because the second word of the CAS instruction is invalid. The first word is fine. The size is word, addressing mode is...

Well I tested it just now and it seems to pass that code without issue on real hardware at the very least.

Are you sure the Classic II has the ASC enabled in the gestalt? It doesn't have a full ASC, so if the gestalt doesn't have the ASC bit set then that code is unreachable.

It's hard to see on the picture, but it looks to me like you're not getting video D3 to the display. Check the path between that pin on the video connector to R171 on the back side, then to the video chip from R171.

Based on rough estimated measurements I think the 10.2" iPad LCD is missing just a few MM off the sides. I think the 10.5" would be the proper size, but there aren't any easy driver boards for that model that I can find (I'm guessing because the ProMotion display interface is more complex). I...

The video circuit should be mostly the same. They just added in the brightness control potentiometer and disconnected the contrast (the trace is still there but it no longer connects). Are you able to show which traces you are looking at? The traces under C11 are data/address lines along with...

An iPad LCD would be a cheap option, however I think the 10.2" LCD might just be a tiny bit too small, I think it's close though so hard to say for sure.