Downtown Doug Brown » The invalid 68030 instruction that accidentally allowed the Mac Classic II to successfully boot up
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. If you haven’t heard of MAME, it’s an emulator that is known best for its support of many arcade games. It’s so much more than that, though! It is also arguably the most complete emulator of 68000-based Mac models, thanks in large part to Arbee‘s incredible efforts. I will admit that I’ve used MAME to play a game or two of Teenage Mutant Ninja Turtles: Turtles in Time, but my main use for it is Mac emulation.
Here’s how this adventure begins. I had been fixing some issues in MAME with the command + power key combination that invokes the debugger, and decided to see if the keystroke also worked on the Classic II. Even though this Mac model has a physical interrupt button on the side, it also has an “Egret” 68HC05 microcontroller for handling the keyboard and mouse (among other things) that should be able to detect the keypress and signal a non-maskable interrupt to the main CPU. I believe the Egret disables this keystroke by default, but MacsBug contains code that sends the command to enable it.
