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D.2.3 Preemptive Dispatching

1/3
{AI95-00321-01} {AI05-0299-1} [This subclause defines a preemptive task dispatching policy.] 

Static Semantics

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

Dynamic Semantics

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{AI95-00321-01} When FIFO_Within_Priorities is in effect, modifications to the ready queues occur only as follows:
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{AI95-00321-01} 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, except in the case where the active priority is lowered due to the loss of inherited priority, in which case the task is added at the head 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. 
7.a/2
Ramification: If the delay does result in blocking, the task moves to the “delay queue”, not to the ready queue. 
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{AI95-00321-01} Each of the events specified above is a task dispatching point (see D.2.1).
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{AI95-00321-01} A task dispatching point occurs for the currently running task of a processor whenever there is a nonempty ready queue for that processor with a higher priority than the priority of the running task. The currently running task is said to be preempted and it is added at the head of the ready queue for its active priority.

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 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 FIFO_Within_Priorities policy with a locking policy, and we want that combination to be portable. 

Documentation Requirements

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{AI95-00321-01} Priority inversion is the duration for which a task remains at the head of the highest priority nonempty ready queue while the processor executes a lower priority task. The implementation shall document:
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The maximum priority inversion a user task can experience due to activity of the implementation (on behalf of lower priority tasks), and 
12.a/2
Documentation Requirement: The maximum priority inversion a user task can experience from the implementation.
13/2
whether execution of a task can be preempted by the implementation processing of delay expirations for lower priority tasks, and if so, for how long. 
13.a/2
Documentation Requirement: The amount of time that a task can be preempted for processing on behalf of lower-priority tasks.
NOTES
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14  {AI95-00321-01} If the active priority of a running task is lowered due to loss of inherited priority (as it is on completion of a protected operation) and there is a ready task of the same active priority that is not running, the running task continues to run (provided that there is no higher priority task).
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15  {AI95-00321-01} Setting the base priority of a ready task causes the task to move to the tail of the queue for its active priority, regardless of whether the active priority of the task actually changes.

Wording Changes from Ada 95

15.a/2
{AI95-00321-01} This subclause is new; it mainly consists of text that was found in D.2.1 and D.2.2 in Ada 95. This was separated out so the definition of additional policies was easier.
15.b/2
{AI95-00333-01} We require that implementations allow this policy and Ceiling_Locking together.
15.c/2
{AI95-00355-01} We explicitly defined FIFO_Within_Priorities to be a task dispatching policy. 

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