Pentium F00F bug

Intel Pentium processor

The Pentium F00F bug is a design flaw in Intel's P5 microarchitecture, and present in CPUs based on it, including the original Pentium, Pentium MMX, and Pentium OverDrive. Discovered in 1997, it can result in the processor ceasing to function until the computer is physically rebooted. This bug has been circumvented through later operating system updates. Intel eventually fixed the flaw in their B2 stepping level of the chips, released c. 1999.

The name is shorthand for F0 0F C7 C8, the hexadecimal encoding of one offending instruction. More formally, the bug is called the invalid operand with locked CMPXCHG8B instruction bug.

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