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D.2.4 Non-Preemptive Dispatching

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{AI95-00298-01} [This clause defines a non-preemptive task dispatching policy.] 

Static Semantics

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{AI95-00298-01} {AI95-00355-01} The policy_identifier Non_Preemptive_FIFO_Within_Priorities is a task dispatching policy.

Legality Rules

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{AI95-00355-01} Non_Preemptive_FIFO_Within_Priorities shall not be specified as the policy_identifier of pragma Priority_Specific_Dispatching (see D.2.2).
3.a/2
Reason: The non-preemptive nature of this policy could cause the policies of higher priority tasks to malfunction, missing deadlines and having unlimited priority inversion. That would render the use of such policies impotent and misleading. As such, this policy only makes sense for a complete system. 

Dynamic Semantics

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{AI95-00298-01} When Non_Preemptive_FIFO_Within_Priorities is in effect, modifications to the ready queues occur only as follows:
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{AI95-00298-01} When a blocked task becomes ready, it is added at the tail of the ready queue for its active priority.
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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.
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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.
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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. 
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Ramification: If the delay does result in blocking, the task moves to the “delay queue”, not to the ready queue. 
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For this policy, a non-blocking delay_statement is the only non-blocking event that is a task dispatching point (see D.2.1).{task dispatching point [partial]} {dispatching point [partial]}

Implementation Requirements

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{AI95-00333-01} 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. 
10.a/2
Reason: This is the preferred combination of the Non_Preemptive_FIFO_Within_Priorities policy with a locking policy, and we want that combination to be portable. 

Implementation Permissions

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{AI95-00298-01} 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 to execute within a protected object without raising its active priority provided the associated protected unit does not contain pragma Interrupt_Priority, Interrupt_Handler, or Attach_Handler.

Extensions to Ada 95

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{AI95-00298-01} {AI95-00355-01} {extensions to Ada 95} Policy Non_Preemptive_FIFO_Within_Priorities is new. 

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