Intel 80286, 80287 manual Invalid Task State Segment Interrupt, Not Present Interrupt

INTERRUPTS AND EXCEPTIONS

The interrupt is generated by the processor extension data channel within the 80286 during the limit test performed on each transfer of data between memory and the processor extension. This interrupt can be handled in the same task but is not restartable.

As with all external interrupts, Interrupt 9 is an asynchronous demand caused by the processor exten- sion referencing something outside a segment boundary. Since Interrupt 9 can occur any time after the processor extension is started, the 80286 does not save any information that identifies what partic- ular operation had been initiated in the processor extension. The processor extension maintains special registers that identify the last instruction it executed and the address of the desired operand.

After this interrupt occurs, no WAIT or escape instruction, except FNINIT, can be executed until the interrupt condition is cleared or the processor extension is reset. The interrupt signals that the processor extension is requesting an invalid data transfer. The processor extension will always be busy when waiting on data. Deadlock results if the CPU executes an instruction that causes it to wait for the processor extension before resetting the processor extension. Deadlock means the CPU is waiting for the processor extension to become idle while the processor extension waits for the CPU to service its data request.

The FNINIT instruction is guaranteed to reset the processor extension without causing deadlock. After the interrupt is cleared, this restriction is lifted. It is then possible to read the instruction and operand address via FSTENV or FSAVE, causing the segment overrun in the processor extension's special registers.

The task interrupted by interrupt 9 is not necessarily the task that executed the ESC instruction that caused the interrupt. The operating system should keep track of which task last used the NPX '(see section 11.4). If the interrupted task did not execute the ESC instruction, it can be restarted. The task that executed the ESC instruction cannot.

9.6.4 Invalid Task State Segment (Interrupt 10)

Interrupt 10 is invoked if during a task switch the new TSS pointed to by the task gate is invalid. The EXT bit indicates whether the exception was caused by an event outside the control of the program.

A TSS is considered invalid in the cases shown in table 9-5.

Once the existence of the new TSS is verified, the task switch is considered complete, with the backlink set to the old task if necessary. All errors are handled in the context of the new task.

Exception 10 must be handled through a task gate to insure a proper TSS to process it. The handler must reset the busy bit in the newTSS.

9.6.5 Not Present (Interrupt 11)

Exception II occurs when an attempt is made to load a not-present segment or to use. a control descrip- tor that is marked not-present. (If, however, the missing segment is an LDT that is needed in a task switch, exception 10 occurs.) This exception is fully restartable.

Any segment load instruction can cause this exception. Interrupt 11 is always processed in the context of the task in which it occurs.

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