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Unformatted text preview: hat the exception was caused by an event external to the currently running program (for example, if an external interrupt handler using a task gate attempted a task switch to an invalid TSS). Saved Instruction Pointer If the exception condition was detected before the task switch was carried out, the saved contents of CS and EIP registers point to the instruction that invoked the task switch. If the exception condition was detected after the task switch was carried out, the saved contents of CS and EIP registers point to the first instruction of the new task. Program State Change The ability of the invalid-TSS handler to recover from the fault depends on the error condition than causes the fault. Refer to Section 6.3., “Task Switching” in Chapter 6, Task Management for more information on the task switch process and the possible recovery actions that can be taken. If an invalid TSS exception occurs during a task switch, it can occur before or after the committo-new-task point. If it occurs before the commit point, no program state change occurs. If it occurs after the commit point (when the segment descriptor information for the new segment selectors have been loaded in the segment registers), the processor will load all the state information from the new TSS before it generates the exception. During a task switch, the processor first loads all the segment registers with segment selectors from the TSS, then checks their contents for validity. If an invalid TSS exception is discovered, the remaining segment registers are loaded but not checked for validity and therefore may not be usable for referencing memory. The invalid TSS handler should not rely on being able to use the segment selectors found in the CS, SS, DS, ES, FS, and GS registers without causing another exception. The exception handler should load all segment registers before trying to resume the new task; otherwise, generalprotection exceptions (#GP) may result later under conditions that make diagnosis more difficult. The Intel recommended way of dealing situation is to use a task for the invalid TSS exception handler. The task switch back to the interrupted task from the invalid-TSS exceptionhandler task will then cause the processor to check the registers as it loads them from the TSS. 5-36 INTERRUPT AND EXCEPTION HANDLING Interrupt 11—Segment Not Present (#NP)
Exception Class Description Indicates that the present flag of a segment or gate descriptor is clear. The processor can generate this exception during any of the following operations: Fault. • • • • While attempting to load CS, DS, ES, FS, or GS registers. [Detection of a not-present segment while loading the SS register causes a stack fault exception (#SS) to be generated.] This situation can occur while performing a task switch. While attempting to load the LDTR using an LLDT instruction. Detection of a not-present LDT while loading the LDTR during a task switch operation causes an invalid-TSS exception (#TS) to be generated. When executing the LTR instruction and the TSS is marked not present. While attempting to use a gate descriptor or TSS that is marked segment-not-present, but is otherwise valid. An operating system typically uses the segment-not-present exception to implement virtual memory at the segment level. If the exception handler loads the segment and returns, the interrupted program or task resumes execution. A not-present indication in a gate descriptor, however, does not indicate that a segment is not present (because gates do not correspond to segments). The operating system may use the present flag for gate descriptors to trigger exceptions of special significance to the operating system. Exception Error Code An error code containing the segment selector index for the segment descriptor that caused the violation is pushed onto the stack of the exception handler. If the EXT flag is set, it indicates that the exception resulted from an external event (NMI or INTR) that caused an interrupt, which subsequently referenced a not-present segment. The IDT flag is set if the error code refers to an IDT entry (e.g., an INT instruction referencing a not-present gate). Saved Instruction Pointer The saved contents of CS and EIP registers normally point to the instruction that generated the exception. If the exception occurred while loading segment descriptors for the segment selectors in a new TSS, the CS and EIP registers point to the first instruction in the new task. If the exception occurred while accessing a gate descriptor, the CS and EIP registers point to the instruction that invoked the access (for example a CALL instruction that references a call gate). 5-37 INTERRUPT AND EXCEPTION HANDLING Program State Change If the segment-not-present exception occurs as the result of loading a register (CS, DS, SS, ES, FS, GS, or LDTR), a program-state change does accompany the exception, because the register is not loaded. Recovery from this exception is possible by simply loading the missing segment into memory and setting the present flag in the segment descriptor. If the segment-not-present exception occurs while accessing a gate descriptor, a program-state change does not accompany the exception....
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