Intel IA-32 - Manual (Page 218)

5-42 Vol. 3A
INTERRUPT AND EXCEPTION HANDLING
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 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. See Section 6.3, “Task Switching,” 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 commit-
to-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 infor-
mation 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, general-
protection 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 exception-handler task
will then cause the processor to check the registers as it loads them from the TSS.