Editing Triple fault

{{for|the geological term|triple junction}}
{{One source|date=May 2012}}

On the [[x86]] [[computer architecture]], a '''triple fault''' is a special kind of [[Exception handling|exception]] generated by the [[Central processing unit|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 [[Intel 80286|80286]] will cause a shutdown cycle to occur when a triple fault is encountered. This typically causes the [[motherboard]] hardware to initiate a CPU reset, which, in turn, causes the whole computer to reboot.<ref name="Collins_2000_Triple"/><ref name="Collins_2000_Reset"/>

==Possible causes of triple faults==
Triple faults indicate a problem with the [[operating system]] [[Kernel (operating system)|kernel]] or [[device driver]]s. 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]] (IDT). If the IDT is corrupted, when the next [[interrupt]] happens, the processor will be unable to call either the needed interrupt handler or the double fault handler because the descriptors in the IDT are corrupted.{{Citation needed|date=July 2010}}

==Virtual machines==
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In [[QEMU]], a triple fault produces a dump of the virtual machine in the console, with the instruction pointer set to the instruction that triggered the first exception.

In [[VirtualBox]], a triple fault causes a [[Guru Meditation]] error to be displayed to the user. A virtual machine in this state has most features disabled and cannot be restarted. If the VirtualBox Debugger is open, a message is printed indicating a Triple fault has occurred, followed by a register dump and [[Disassembler|disassembly]] of the last instruction executed, similar to the output of the <code>rg</code> debugger command.

When using [[Intel VT-x]], a triple fault causes a VM exit, with exit reason 2. The exit reason is saved to the VMCS and may be handled by the VMM software.

In [[VMware]], an error message will be displayed and the virtual machine will need to be reset.

==Other uses==
The [[Intel 80286]] processor was the first x86 processor to introduce the now-ubiquitous [[protected mode]].  However, the 286 could not revert to the basic 8086-compatible "[[real mode]]" without resetting the processor, which can only be done using hardware external to the CPU.  On the [[IBM Personal Computer AT|IBM AT]] and compatibles, the documented method of doing this was to use a special function on the [[Intel 8042]] keyboard controller, which would assert the RESET pin of the processor.  However, intentionally triple-faulting the CPU was found to cause the transition to occur much faster (0.8 milliseconds instead of 15+ milliseconds) and more cleanly, permitting multitasking operating systems to switch back and forth at high speed.<ref name="Osterman_2005_286"/>

Some operating system kernels, such as [[Linux kernel|Linux]], still use triple faults as a last effort in their rebooting process if an [[Advanced Configuration and Power Interface|ACPI]] reboot fails. This is done by setting the IDT register to 0 and then issuing an interrupt.<ref name="Collins_2000_Triple"/> Since the table now has length 0, all attempts to access it fail and the processor generates a triple fault.

==References==
{{Reflist|refs=
<ref name="Collins_2000_Triple">{{cite web |author-first=Robert |author-last=Collins |title=Triple Faulting the CPU |date=2000 |work=Productivity Enhancements and Programming Tricks |url=http://www.rcollins.org/Productivity/TripleFault.html |access-date=2015-11-22 |url-status=live |archive-url=https://web.archive.org/web/20170909120639/http://www.rcollins.org/Productivity/TripleFault.html |archive-date=2017-09-09}}</ref>
<ref name="Collins_2000_Reset">{{cite web |title=ELEGANT RESET |author-first=Robert |author-last=Collins |date=2000 |url=http://www.rcollins.org/ftp/source/3fault/reset.asm |access-date=2017-09-09 |url-status=live |archive-url=https://web.archive.org/web/20170909113216/http://www.rcollins.org/ftp/source/3fault/reset.asm |archive-date=2017-09-09}}</ref>
<ref name="Osterman_2005_286">{{cite web |title=Faster Syscall Trap redux |author-first=Larry |author-last=Osterman |date=2005-02-08 |work=Larry Osterman's WebLog |publisher=[[MSDN Blogs]] |url=https://blogs.msdn.microsoft.com/larryosterman/2005/02/08/faster-syscall-trap-redux/ |access-date=2010-07-23 |url-status=live |archive-url=https://web.archive.org/web/20170909113828/https://blogs.msdn.microsoft.com/larryosterman/2005/02/08/faster-syscall-trap-redux/ |archive-date=2017-09-09}}</ref>
}}

[[Category:Computer errors]]