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A general protection fault (GPF) in the x86
instruction set architecture In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...
s (ISAs) is a fault (a type of
interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
) initiated by ISA-defined protection mechanisms in response to an access violation caused by some running code, either in the
kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...
or a user program. The mechanism is first described in Intel manuals and datasheets for the
Intel 80286 The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non- multiplexed address and data buses and also the ...
CPU, which was introduced in 1983; it is also described in section 9.8.13 in the
Intel 80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistorsvector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
number 13 (0Dh)). Some
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
s may also classify some exceptions not related to access violations, such as illegal
opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine language instruction that specifies the operat ...
exceptions, as general protection faults, even though they have nothing to do with memory protection. If a CPU 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 CPU signals a
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 ...
, stopping the operating system. If yet another failure ( triple fault) occurs, the CPU is unable to recover; since 80286, the CPU enters a special halt state called "Shutdown", which can only be exited through a
hardware reset A hardware reset or hard reset of a computer system is a hardware operation that re-initializes the core hardware components of the system, thus ending all current software operations in the system. This is typically, but not always, followed by b ...
. The
IBM PC AT The IBM Personal Computer/AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 80 ...
, the first
PC-compatible IBM PC compatible computers are similar to the original IBM PC, XT, and AT, all from computer giant IBM, that are able to use the same software and expansion cards. Such computers were referred to as PC clones, IBM clones or IBM PC clones. ...
system to contain an 80286, has hardware that detects the Shutdown state and automatically resets the CPU when it occurs. All descendants of the PC AT do the same, so in a PC, a triple fault causes an immediate system reset.


Specific behavior

In
Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...
, the general protection fault presents with varied language, depending on product version: In
Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
and
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...
, the errors are reported separately (e.g.
segmentation fault In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restrict ...
for memory errors).


Memory errors

In memory errors, the faulting program accesses
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
that it should not access. Examples include: * Attempting to write to a read-only portion of memory * Attempting to execute
byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
s in memory which are not designated as instructions * Attempting to read as data bytes in memory which are designated as instructions * Other miscellaneous conflicts between the designation of a part of memory and its use However, many modern operating systems implement their memory access-control schemes via paging instead of segmentation, so it is often the case that invalid memory references in operating systems such as Windows are reported via
page fault In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added t ...
s instead of general protection faults. Operating systems typically provide an abstraction layer (such as exception handling or signals) that hides whatever internal processor mechanism was used to raise a memory access error from a program, for the purposes of providing a standard interface for handling many different types of processor-generated error conditions. In terms of the x86 architecture, general protection faults are specific to segmentation-based protection when it comes to memory accesses. However, general protection faults are still used to report other protection violations (aside from memory access violations) when paging is used, such as the use of instructions not accessible from the
current privilege level In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behavior (by providing computer security). Comput ...
(CPL). While it is theoretically possible for an operating system to utilize both paging and segmentation, for the most part, common operating systems typically rely on paging for the bulk of their memory access control needs.


Privilege errors

There are some things on a computer which are reserved for the exclusive use of the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
. If a program which is not part of the operating system attempts to use one of these features, it may cause a general protection fault. Additionally, there are storage locations which are reserved both for the operating system and the processor itself. As a consequence of their reservation, they are read-only and an attempt to write
data In the pursuit of knowledge, data (; ) is a collection of discrete values that convey information, describing quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpret ...
to them by an unprivileged program produces an error.


Technical causes for faults

General protection faults are raised by the processor when a protected instruction is encountered which exceeds the permission level of the currently executing task, either because a user-mode program is attempting a protected instruction, or because the operating system has issued a request which would put the processor into an undefined state. General protection faults are caught and handled by modern operating systems. Generally, if the fault originated in a user-mode program, the user-mode program is terminated. If, however, the fault originated in a core system driver or the operating system itself, the operating system usually saves diagnostic information either to a file or to the screen and stops operating. It either restarts the computer or displays an error screen, such as a Blue Screen of Death or
kernel panic A kernel panic (sometimes abbreviated as KP) is a safety measure taken by an operating system's kernel upon detecting an internal fatal error in which either it is unable to safely recover or continuing to run the system would have a higher ...
.


Segment limits exceeded

Segment limits can be exceeded: * with
code segment In computing, a code segment, also known as a text segment or simply as text, is a portion of an object file or the corresponding section of the program's virtual address space that contains executable instructions. Segment The term "segment" ...
(CS), data segment (DS), or ES, FS, or GS (extra segment) registers; or * accessing descriptor tables such as the Global Descriptor Table (GDT), the Interrupt descriptor table (IDT) and the Local Descriptor Table (LDT).


Segment permissions violated

Segment permissions can be violated by: * jumping to non-executable segments * writing to code segments, or read only segments * reading execute-only segments


Segments illegally loaded

This can occur when: * a stack segment (SS) is loaded with a segment selector for a read only, executable, null segment, or segment with descriptor privilege not matching the current privilege in CS * a code segment (CS) loaded with a segment selector for a data, system, or null segment * SS, DS, ES, FS, or GS are segments loaded with a segment selector for a system segment * SS, DS, ES, FS, or GS are segments loaded with a segment selector for an execute-only code segment * accessing memory using DS, ES, FS, or GS registers, when they contain a null selector


Switching

Faults can occur in the
task state segment The task state segment (TSS) is a structure on x86-based computers which holds information about a task. It is used by the operating system kernel for task management. Specifically, the following information is stored in the TSS: * Processor reg ...
(TSS) structure when: * switching to a busy task during a call or jump instruction * switching to an available task during an interrupt return (IRET) instruction * using a segment selector on a switch pointing to a TSS descriptor in the LDT


Miscellaneous

Other causes of general protection faults are: * attempting to access an interrupt/exception handler from
virtual 8086 mode In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running a ...
when the handler's code segment descriptor
privilege level In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behavior (by providing computer security). Computer ...
(DPL) is greater than zero * attempting to write a one into the reserved bits of CR4 * attempting to execute privileged instructions when the current privilege level (CPL) is not zero * writing to a reserved bit in an MSR instruction * accessing a gate containing a null segment selector * executing a software interrupt when the CPL is greater than the DPL set for the interrupt gate * the segment selector in a call, interrupt or trap gate does not point to a code segment * violating privilege rules * enabling paging whilst disabling protection * referencing the interrupt descriptor table following an interrupt or exception that is not an interrupt, trap, or a task gate * Legacy SSE: Memory operand is not 16-byte aligned.


References


Further reading

*''Intel Architecture Software Developer's Manual''–Volume 3: System Programming {{Operating System Operating system technology Computer errors de:Allgemeine Schutzverletzung