Triple fault
Encyclopedia
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 encountered. This typically causes the motherboard
hardware to initiate a CPU reset which in turn causes the whole computer
to reboot.
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.
, 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.
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, 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 and more cleanly, permitting multitasking operating systems to switch back and forth at high speed.
Some operating system kernels, such as Linux
, still use triple faults as a last effort in their rebooting process if an ACPI
reboot fails. This is done 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.
Exception handling
Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....
generated by the CPU
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
when an exception occurs while the CPU is trying to invoke the double fault
Double fault
On the x86 architecture, a double fault exception occurs if the processor encounters a problem while trying to service a pending interrupt or exception. An example situation when a double fault would occur is when an interrupt is triggered but the segment in which the interrupt handler resides is...
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 encountered. This typically causes the motherboard
Motherboard
In personal computers, a motherboard is the central printed circuit board in many modern computers and holds many of the crucial components of the system, providing connectors for other peripherals. The motherboard is sometimes alternatively known as the mainboard, system board, or, on Apple...
hardware to initiate a CPU reset which in turn causes the whole computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
to reboot.
Possible causes of triple faults
Triple faults indicate a problem with the operating systemOperating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer 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
Interrupt descriptor table
The Interrupt Descriptor Table is a data structure used by the x86 architecture to implement an interrupt vector table. The IDT is used by the processor to determine the correct response to interrupts and exceptions....
. When the next interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
happens, the processor cannot call either the needed interrupt handler or the double fault handler because the descriptors in the IDT
Interrupt descriptor table
The Interrupt Descriptor Table is a data structure used by the x86 architecture to implement an interrupt vector table. The IDT is used by the processor to determine the correct response to interrupts and exceptions....
are corrupted.
Virtual machines
In QEMUQEMU
QEMU is a processor emulator that relies on dynamic binary translation to achieve a reasonable speed while being easy to port on new host CPU architectures....
, 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.
Other uses
The Intel 80286Intel 80286
The Intel 80286 , introduced on 1 February 1982, was a 16-bit x86 microprocessor with 134,000 transistors. Like its contemporary simpler cousin, the 80186, it could correctly execute most software written for the earlier Intel 8086 and 8088...
processor was the first to introduce the now-ubiquitous protected mode
Protected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
. However, the 286 could not revert to the basic 8086-compatible "real mode
Real mode
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
" without resetting the processor. 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 and more cleanly, permitting multitasking operating systems to switch back and forth at high speed.
Some operating system kernels, such as Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, still use triple faults as a last effort in their rebooting process if an ACPI
Advanced Configuration and Power Interface
In computing, the Advanced Configuration and Power Interface specification provides an open standard for device configuration and power management by the operating system....
reboot fails. This is done 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.