The Busy Bit
Each TSS descriptor in the GDT has a Busy bit (see Figure 11-7 on page 205). When control is first passed to the task, its Busy bit is set to one by the processor.
If the task is terminated with a far jump to another task (performing an unconditional jump to another task implies that the current task has completed and is therefore not busy anymore), an IRET to return to a task that called it, or an IRET to return to an interrupted task, the task's Busy bit is cleared to zero.
If the task is exited via a far CALL to another task, or an interrupt/exception causes a task switch ...
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