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D.2.1 The Task Dispatching Model

The task dispatching model specifies preemptive scheduling, based on conceptual priority-ordered ready queues.
Dynamic Semantics
A task runs (that is, it becomes a running task) only when it is ready (see 9.2) and the execution resources required by that task are available. Processors are allocated to tasks based on each task's active priority.
It is implementation defined whether, on a multiprocessor, a task that is waiting for access to a protected object keeps its processor busy.
Task dispatching is the process by which one ready task is selected for execution on a processor. This selection is done at certain points during the execution of a task called task dispatching points. A task reaches a task dispatching point whenever it becomes blocked, and whenever it becomes ready. In addition, the completion of an accept_statement (see 9.5.2), and task termination are task dispatching points for the executing task. Other task dispatching points are defined throughout this Annex.
Task dispatching policies are specified in terms of conceptual ready queues, task states, and task preemption. A ready queue is an ordered list of ready tasks. The first position in a queue is called the head of the queue, and the last position is called the tail of the queue. A task is ready if it is in a ready queue, or if it is running. Each processor has one ready queue for each priority value. At any instant, each ready queue of a processor contains exactly the set of tasks of that priority that are ready for execution on that processor, but are not running on any processor; that is, those tasks that are ready, are not running on any processor, and can be executed using that processor and other available resources. A task can be on the ready queues of more than one processor.
Each processor also has one running task, which is the task currently being executed by that processor. Whenever a task running on a processor reaches a task dispatching point, one task is selected to run on that processor. The task selected is the one at the head of the highest priority nonempty ready queue; this task is then removed from all ready queues to which it belongs.
A preemptible resource is a resource that while allocated to one task can be allocated (temporarily) to another instead. Processors are preemptible resources. Access to a protected object (see 9.5.1) is a nonpreemptible resource. When a higher-priority task is dispatched to the processor, and the previously running task is placed on the appropriate ready queue, the latter task is said to be preempted.
A new running task is also selected whenever there is a nonempty ready queue with a higher priority than the priority of the running task, or when the task dispatching policy requires a running task to go back to a ready queue. These are also task dispatching points.
Implementation Permissions
An implementation is allowed to define additional resources as execution resources, and to define the corresponding allocation policies for them. Such resources may have an implementation defined effect on task dispatching (see D.2.2).
An implementation may place implementation-defined restrictions on tasks whose active priority is in the Interrupt_Priority range.


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