Thank you! I think I see in the schematic where to grab those signals, I'll test.

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Are you sure that the bootloader has a built-in RAM test? Never heard of this before...

Still, the bootloader uses the upper RAM for its stack and variables (0xe000-0xffff). Coincidentally, this is also the upper bank of the video frame buffer - the only one used by the bootloader in order to reserve all RAM below 0xe000 for loading programs (hence the pixel "garbage" in the upper right corner of the bootloader grid screen)
I attached 3 data probes to a13-a15 and one to wr to see if the computer was trying to access the upper RAM (0xe000-0xffff).


At around +1000ms I hit the wwod and blk keys. It looks does two bursts of reads and writes to the 0xe000-0xffff region at around +1400 and +1700. Then a14 goes low and will stay low indefinitely.

At around +2700ms I hit blk and sbr, and it looks like it's trying to read that upper block again.