Editing General protection fault (section)

==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]].

===Segment limits exceeded===
Segment limits can be exceeded:
* with [[code 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]] (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{{clarify|note=What does this mean?|date=May 2017}}

===Miscellaneous===
Other causes of general protection faults are:
* attempting to access an interrupt/exception handler from [[virtual 8086 mode]] when the handler's code segment descriptor [[privilege level]] (DPL) is greater than zero
* attempting to write a one into the reserved bits of [[control register|CR4]]
* attempting to execute [[privilege (computing)|privileged]] instructions when the current privilege level (CPL) is not zero
* attempting to execute a single instruction with a length greater than 15 bytes (possibly by prepending the instruction with superfluous prefixes)<ref>{{cite web|url=https://xem.github.io/minix86/manual/intel-x86-and-64-manual-vol3/o_fe12b1e2a880e0ce-223.html|title=Page 223|access-date=2023-09-24}}</ref>
* writing to a reserved bit in an [[model-specific register|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 [[Streaming SIMD Extensions|SSE]]: Memory operand is not 16-byte aligned.