The invalid 68030 instruction that accidentally allowed the Mac Classic II to successfully boot up

A bug in the ROM for the Macintosh II was recently discovered that causes a crash when booting in 32-bit mode. Doug Brown discovered and documented the bug while playing with the MAME debugger. Why did it never show up before? It seems a quirk in Motorola's 68030 CPU inadvertently fixes it when executing an illegal instruction that shouldn't have been executed in the first place.

I was starting to believe something that sounded almost too crazy to be true: Apple had an out-of-bounds jump bug in the Classic II's ROM that should have caused a Sad Mac during boot, but they had no idea the bug was there because the 68030 was accidentally fixing the value of A1 by executing an undocumented instruction. How could I prove that my theory was correct?

By buying a Classic II and hacking the ROM in order to see exactly what is happening on hardware, of course!

Doug Brown

What follows is his process for investigating the room on emulated hardware, and then testing it on actual hardware.