f00f ( ), an abbreviation of f0 0f c7 c8, is
the hexadecimal encoding of an instruction that exhibits a
design flaw in the majority of Intel
Pentium, Pentium MMX, and
Pentium OverDrive processors.
Intel calls this issue the "invalid operand with locked CMPXCHG8B
instruction."
Description
The instruction represents:
lock cmpxchg8b eax
The
operand could be a
register other than eax. The
cmpxchg8b instruction is used
to compare the value in the
edx and
eax registers with an 8-
byte value at some memory location. In this example a
4-byte register is used as the destination operand, which is not
big enough to store the 8-byte result.
Under normal circumstances, this instruction would simply result in
an
exception; however, when used
with the
lock prefix (normally used to prevent two
processors from interfering with the same memory location), the
exception handler is never called, the processor stops servicing
interrupts and the CPU must be reset to
recover.
The instruction does not require any
special
privileges and, due to the proliferation of Intel
microprocessors, was considered a serious issue at the time.
Operating system vendors responded
by implementing
workarounds that detected
the condition and prevented the crash.
Although technically not an example of either, the f00f instruction
is often considered an
HCF
instruction or a
killer poke. Since the
f00f bug has become common knowledge, the term is sometimes used to
describe similar hardware design flaws such as the
Cyrix coma bug.
No permanent hardware damage would result from executing the f00f
instruction on a vulnerable system. Depending on the
file system,
operating system, and other circumstances,
it is possible for
data loss to occur if
the
disk buffers have not been flushed,
if drives were interrupted during a write operation, or if some
other non-
atomic operation was
interrupted.
No Intel processors since the introduction of the
Pentium Pro have been affected by the bug. The
myB2
stepping fixed this issue.
Information about this bug started spreading over the Internet on
or about
7 November 1997.
See also
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