Intel IA-32 - Manual (Page 260)

6-18 Vol. 3A
TASK MANAGEMENT
6.4.1 Use of Busy Flag To Prevent Recursive Task Switching
A TSS allows only one context to be saved for a task; therefore, once a task is called
(dispatched), a recursive (or re-entrant) call to the task would cause the current state of the task
to be lost. The busy flag in the TSS segment descriptor is provided to prevent re-entrant task
switching and a subsequent loss of task state information. The processor manages the busy flag
as follows:
1. When dispatching a task, the processor sets the busy flag of the new task.
2. If during a task switch, the current task is placed in a nested chain (the task switch is being
generated by a CALL instruction, an interrupt, or an exception), the busy flag for the
current task remains set.
3. When switching to the new task (initiated by a CALL instruction, interrupt, or exception),
the processor generates a general-protection exception (#GP) if the busy flag of the new
task is already set. If the task switch is initiated with an IRET instruction, the exception is
not raised because the processor expects the busy flag to be set.
4. When a task is terminated by a jump to a new task (initiated with a JMP instruction in the
task code) or by an IRET instruction in the task code, the processor clears the busy flag,
returning the task to the “not busy” state.
The processor prevents recursive task switching by preventing a task from switching to itself or
to any task in a nested chain of tasks. The chain of nested suspended tasks may grow to any
length, due to multiple calls, interrupts, or exceptions. The busy flag prevents a task from being
invoked if it is in this chain.
The busy flag may be used in multiprocessor configurations, because the processor follows a
LOCK protocol (on the bus or in the cache) when it sets or clears the busy flag. This lock keeps
two processors from invoking the same task at the same time. See Section 7.1.2.1, “Automatic
Locking,” for more information about setting the busy flag in a multiprocessor applications.
6.4.2 Modifying Task Linkages
In a uniprocessor system, in situations where it is necessary to remove a task from a chain of
linked tasks, use the following procedure to remove the task:
1. Disable interrupts.
2. Change the previous task link field in the TSS of the pre-empting task (the task that
suspended the task to be removed). It is assumed that the pre-empting task is the next task
(newer task) in the chain from the task to be removed. Change the previous task link field
to point to the TSS of the next oldest task in the chain or to an even older task in the chain.
3. Clear the busy (B) flag in the TSS segment descriptor for the task being removed from the
chain. If more than one task is being removed from the chain, the busy flag for each task
being remove must be cleared.
4. Enable interrupts.