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D.2.1 The Task Dispatching Model

The task dispatching model specifies task scheduling, based on conceptual priority-ordered ready queues. 

Static Semantics

  The following language-defined library package exists: 
package Ada.Dispatching is
  pragma Pure(Dispatching);
  Dispatching_Policy_Error : exception;
end Ada.Dispatching;
  Dispatching serves as the parent of other language-defined library units concerned with task dispatching.

Dynamic Semantics

A task can become a running task only if it is ready (see 9) 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 when it terminates. Other task dispatching points are defined throughout this Annex for specific policies. 
Task dispatching policies are specified in terms of conceptual ready queues and task states. 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 it goes back to one or more ready queues; a task (possibly the same task) is then 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. 
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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. 
An implementation may place implementation-defined restrictions on tasks whose active priority is in the Interrupt_Priority range. 
   For optimization purposes, an implementation may alter the points at which task dispatching occurs, in an implementation-defined manner. However, a delay_statement always corresponds to at least one task dispatching point.
7  Section 9 specifies under which circumstances a task becomes ready. The ready state is affected by the rules for task activation and termination, delay statements, and entry calls. When a task is not ready, it is said to be blocked.
8  An example of a possible implementation-defined execution resource is a page of physical memory, which needs to be loaded with a particular page of virtual memory before a task can continue execution.
9  The ready queues are purely conceptual; there is no requirement that such lists physically exist in an implementation.
10  While a task is running, it is not on any ready queue. Any time the task that is running on a processor is added to a ready queue, a new running task is selected for that processor.
11  In a multiprocessor system, a task can be on the ready queues of more than one processor. At the extreme, if several processors share the same set of ready tasks, the contents of their ready queues is identical, and so they can be viewed as sharing one ready queue, and can be implemented that way. Thus, the dispatching model covers multiprocessors where dispatching is implemented using a single ready queue, as well as those with separate dispatching domains.
12  The priority of a task is determined by rules specified in this subclause, and under D.1, “Task Priorities”, D.3, “Priority Ceiling Locking”, and D.5, “Dynamic Priorities”.
13  The setting of a task's base priority as a result of a call to Set_Priority does not always take effect immediately when Set_Priority is called. The effect of setting the task's base priority is deferred while the affected task performs a protected action.

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