The Undocumented 68030 Instruction That Saved the Mac Classic II

While debugging a Macintosh Classic II in MAME, the author discovered a bus error causing a 'Sad Mac' in 32-bit addressing mode. Reverse engineering revealed an out-of-bounds jump in the ROM, leading the CPU to execute an undocumented 68030 instruction. Surprisingly, this instruction corrected the value of register A1, preventing a crash and allowing the Classic II to boot successfully. The author validated this by repairing a vintage Classic II and modifying its ROM. This case highlights the power of emulators in uncovering hardware quirks and showcases how a software bug was inadvertently fixed by hardware.