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D.2.4 Non-Preemptive Dispatching

This subclause defines a non-preemptive task dispatching policy. 

Static Semantics

The policy_identifier Non_Preemptive_FIFO_Within_Priorities is a task dispatching policy.
  The following language-defined library package exists: 
package Ada.Dispatching.Non_Preemptive is
  pragma Preelaborate(Non_Preemptive);
  procedure Yield_To_Higher;
  procedure Yield_To_Same_Or_Higher renames Yield;
end Ada.Dispatching.Non_Preemptive;
  A call of Yield_To_Higher is a task dispatching point for this policy. If the task at the head of the highest priority ready queue has a higher active priority than the calling task, then the calling task is preempted.

Legality Rules

Non_Preemptive_FIFO_Within_Priorities shall not be specified as the policy_identifier of pragma Priority_Specific_Dispatching (see D.2.2).

Dynamic Semantics

When Non_Preemptive_FIFO_Within_Priorities is in effect, modifications to the ready queues occur only as follows:
When a blocked task becomes ready, it is added at the tail of the ready queue for its active priority.
When the active priority of a ready task that is not running changes, or the setting of its base priority takes effect, the task is removed from the ready queue for its old active priority and is added at the tail of the ready queue for its new active priority.
When the setting of the base priority of a running task takes effect, the task is added to the tail of the ready queue for its active priority.
When a task executes a delay_statement that does not result in blocking, it is added to the tail of the ready queue for its active priority. 
For this policy, blocking or termination of a task, a delay_statement, a call to Yield_To_Higher, and a call to Yield_To_Same_Or_Higher or Yield are the only task dispatching points (see D.2.1).

Implementation Requirements

 An implementation shall allow, for a single partition, both the task dispatching policy to be specified as Non_Preemptive_FIFO_Within_Priorities and also the locking policy (see D.3) to be specified as Ceiling_Locking. 

Implementation Permissions

 Since implementations are allowed to round all ceiling priorities in subrange System.Priority to System.Priority'Last (see D.3), an implementation may allow a task of a partition using the Non_Premptive_FIFO_Within_Priorities policy to execute within a protected object without raising its active priority provided the associated protected unit does not contain any subprograms with aspects Interrupt_Handler or Attach_Handler specified, nor does the unit have aspect Interrupt_Priority specified. When the locking policy (see D.3) is Ceiling_Locking, an implementation taking advantage of this permission shall ensure that a call to Yield_to_Higher that occurs within a protected action uses the ceiling priority of the protected object (rather than the active priority of the task) when determining whether to preempt the task. 

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