In the 1990s, a strange bug in Intel’s Pentium processors caused real panic. A few bytes of machine code, F0 0F C7 C8, could completely freeze the CPU. No error message. No reboot. No recovery. The system just died.
This flaw became known as the F00F bug, named after its hexadecimal pattern. It wasn’t a normal software crash. The CPU itself stopped responding, leaving even the operating system helpless.
What it revealed:
- Hardware bugs can be just as dangerous as software ones.
- Intel and OS developers had to rethink how they handle “impossible” instructions.
- Even one line of code can bring everything down.
Even today, we rely on billions of assumptions inside our chips. When one of them fails, the results can be catastrophic. The F00F bug shows that undefined behavior isn’t just a programming issue, it’s a hardware story too.
You can read the full story, including technical details and how engineers found it and fixed it, here:
The Forbidden Intel Opcode That Melts CPUs
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