General protection fault

A general protection fault (GPF) in the Intel x86 and AMD x86-64 instruction set architectures (ISAs), as well as other ISAs, is a fault (a type of interrupt) initiated by ISA-defined protection mechanisms in response to an access violation caused by some running code, either in the kernel or a user program. The mechanism is first described in Intel manuals and datasheets for the 80286 CPU, which was introduced in 1983; it is also described in section 9.8.13 in the Intel 80386 programmer's reference manual from 1986. A general protection fault is implemented as an interrupt (vector number 13 in decimal) in both the x86 and the AMD64 architectures. Some operating systems may also classify some exceptions not related to access violations, such as illegal opcode exceptions, as general protection faults, even though they have nothing to do with memory protection.

If the processor detects a protection violation, it stops executing the code and sends a GPF interrupt. In most cases the operating system removes the failing process from the execution queue, signals the user, and continues executing other processes. If, however, the operating system fails to catch the general protection fault, i.e. another protection violation occurs before the operating system returns from the previous GPF interrupt, the processor signals a double fault, stopping the operating system. If yet another failure (triple fault) occurs, the processor will be unable to recover. Since the 80286, the processor will enter a special halt state called Shutdown, which can only be exited through a hardware reset, if a triple fault occurs. The IBM PC AT, the first PC-compatible system to contain an 80286, has hardware that detects the Shutdown state and automatically resets the CPU when it occurs, and all descendants of the PC AT do the same, so in a PC, a triple fault causes an immediate system reset.

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