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 discovered an undocumented MC68030 instruction that performs a read-modify-write bus cycle and also changes the value of the A1 register.

This newly-discovered instruction turns out to be the glue that’s accidentally holding the Classic II together. Without this instruction modifying A1, the Classic II can’t boot. I’m confident that it was a mistake and not something intentional. A totally understandable mistake, at that. If the pesky 68030 hadn’t been hiding the bug from Apple’s ROM developers, there is no doubt they would have caught it before the Classic II shipped.

After all that, what’s the lesson we can learn from this story? I guess it’s that emulators can teach us new things about hardware that we never would have thought to look into! I bet this bug in the ROM would have gone undiscovered for all eternity if not for MAME providing emulation of the Classic II, which isn’t a particularly notable machine compared to more popular compact Macs like the SE/30 and Color Classic.”

See the complete story via Downtown Doug Brown