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6-44 Vol. 3
INTERRUPT AND EXCEPTION HANDLING
This exception can generated either in the context of the original task or in the
context of the new task (see
Section 7.3, “Task Switching”). Until the processor has
completely verified the presence of the new TSS, the exception is generated in the
context of the original task. Once the existence of the new TSS is verified, the task
switch is considered complete. Any invalid-TSS conditions detected after this point
are handled in the context of the new task. (A task switch is considered complete
when the task register is loaded with the segment selector for the new TSS and, if the
switch is due to a procedure call or interrupt, the previous task link field of the new
TSS references the old TSS.)
The invalid-TSS handler must be a task called using a task gate. Handling this excep-
tion inside the faulting TSS context is not recommended because the processor state
may not be consistent.
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 7.3, “Task Switching,” for more informa-
tion on the task switch process and the possible recovery actions that can be taken.
TSS segment selector index The TSS segment upper descriptor is not the correct type.
TSS segment selector index The TSS segment descriptor contains a non-canonical base.
TSS segment selector index There is a limit violation in attempting to load SS selector or ESP
from a TSS on a call or exception which changes privilege levels in
legacy mode.
TSS segment selector index There is a limit violation or canonical fault in attempting to load RSP
or IST from a TSS on a call or exception which changes privilege
levels in IA-32e mode.
Table 6-6. Invalid TSS Conditions (Contd.)
Error Code Index Invalid Condition