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Undocumented Instruction Enables Macintosh Classic II Boot Success

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🛠️ Undocumented instruction saves Apple’s Macintosh Classic II from boot failure. An investigation into the Macintosh Classic II’s boot process revealed that an accidental bug in its ROM, which should have caused a crash, was mitigated by the execution of an undocumented Motorola MC68030 instruction. This instruction inadvertently modified a register, allowing the system to boot successfully despite the error. The discovery was made while using the MAME emulator, which failed to replicate the hardware’s behavior, leading to a deeper exploration of the ROM code. Ultimately, this finding highlights how emulators can uncover hidden issues in vintage hardware, suggesting that the Classic II’s booting mechanism relied on a flaw that Apple developers likely overlooked during production.

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