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An invalid 68030 instruction accidentally allowed the Mac Classic II to boot

This is the story of how Apple made a mistake in the ROM of the Macintosh Classic II that probably should have prevented it from booting, but instead, miraculously, its Motorola MC68030 CPU accidentally prevented a crash and saved the day by executing an undefined instruction. I’ve been playing around with MAME a lot lately.

This was very similar to what happened to Adrian’s Digital Basement on his SE/30 board a few months ago, but lucky for me, my accidental short didn’t involve 12 volts and fry a bunch of chips! The Classic II has a cathode-ray tube for its screen, which scares the crap out of me, so I opted to use a different solution in order to run the logic board by itself without any dangerous voltages. These results motivated me to make a couple more hacked ROM images to run on hardware in order to glean the values of all of the CPU’s data and address registers immediately before and after the CAS instruction.

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