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D.2.5 Round Robin Dispatching

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This clause defines the task dispatching policy Round_Robin_Within_Priorities and the package Round_Robin. 

Static Semantics

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The policy_identifier Round_Robin_Within_Priorities is a task dispatching policy.
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The following language-defined library package exists: 
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with System;
with Ada.Real_Time;
package Ada.Dispatching.Round_Robin is
  Default_Quantum : constant Ada.Real_Time.Time_Span :=
             implementation-defined;
  procedure Set_Quantum (Pri     : in System.Priority;
                         Quantum : in Ada.Real_Time.Time_Span);
  procedure Set_Quantum (Low, High : in System.Priority;
                         Quantum   : in Ada.Real_Time.Time_Span);
  function Actual_Quantum (Pri : System.Priority) return Ada.Real_Time.Time_Span;
  function Is_Round_Robin (Pri : System.Priority) return Boolean;
end Ada.Dispatching.Round_Robin;
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When task dispatching policy Round_Robin_Within_Priorities is the single policy in effect for a partition, each task with priority in the range of System.Interrupt_Priority is dispatched according to policy FIFO_Within_Priorities.

Dynamic Semantics

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The procedures Set_Quantum set the required Quantum value for a single priority level Pri or a range of priority levels Low .. High. If no quantum is set for a Round Robin priority level, Default_Quantum is used.
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The function Actual_Quantum returns the actual quantum used by the implementation for the priority level Pri.
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The function Is_Round_Robin returns True if priority Pri is covered by task dispatching policy Round_Robin_Within_Priorities; otherwise it returns False.
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A call of Actual_Quantum or Set_Quantum raises exception Dispatching.Dispatching_Policy_Error if a predefined policy other than Round_Robin_Within_Priorities applies to the specified priority or any of the priorities in the specified range.
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 For Round_Robin_Within_Priorities, the dispatching rules for FIFO_Within_Priorities apply with the following additional rules:
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Implementation Requirements

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

Documentation Requirements

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 An implementation shall document the quantum values supported. 
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 An implementation shall document the accuracy with which it detects the exhaustion of the budget of a task. 
NOTES
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16  Due to implementation constraints, the quantum value returned by Actual_Quantum might not be identical to that set with Set_Quantum.
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17  A task that executes continuously with an inherited priority will not be subject to round robin dispatching.






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