Triple fault

A triple fault is a special kind of exception generated by the CPU when an exception occurs while the CPU is trying to invoke the double fault exception handler, which itself handles exceptions occurring while trying to invoke a regular exception handler.

x86 processors beginning with the 80286 will cause a SHUTDOWN cycle to occur when a triple fault is hit. This typically causes the motherboard hardware to initiate a CPU reset which in turn causes the whole computer to reboot.

Possible causes of triple faults

Triple faults indicate a problem with the operating system kernel or device drivers. In modern operating systems, a triple fault is typically caused by a buffer overflow or underflow in a device driver which writes over the interrupt descriptor table. When the next interrupt happens, the processor cannot call either the needed interrupt handler or the double fault handler because the descriptors in the IDT are corrupted.

Triple faults may also be caused by incorrect kernel settings, such as using the wrong hardware abstraction layer in Windows.

Virtual machines

In Microsoft Virtual PC, if the virtual machine experiences a triple fault a warning box pops up with the message "An unrecoverable processor error has occurred".

In QEMU, a triple fault produces a dump of the virtual machine in the console, with EIP (or whatever PC may be called for the selected architecture) set to the instruction that triggered the first exception.

Other uses

* Some operating system kernels, such as Linux, use triple faults as a last effort in their rebooting process. It does this by setting the IDTR register to 0 and then issuing an interrupt. Since the table now has length 0, all attempts to access it fail and the processor generates a triple fault. Note that this is not the preferred method of restarting the computer; using the ACPI is tried first.
* The Intel 80286 processor was the first to introduce the now-ubiquitous protected mode. However, the 286 could not revert to the basic 8086-compatible "real mode" without resetting the processor. The documented method of doing this was to use a special function on the Intel 8042 keyboard controller. However, intentionally triple-faulting the CPU was found to cause the transition to occur much faster and more cleanly, permitting multitasking operating system to switch back and forth at high speed. [http://blogs.msdn.com/larryosterman/archive/2005/02/08/369243.aspx]

ee also

* Double fault