D.1 Task Priorities
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[This subclause specifies the priority model for real-time systems. In
addition, the methods for specifying priorities are defined.]
Static Semantics
Priority
The aspect Priority is an
expression,
which shall be of type Integer.
Aspect Description for Priority: Priority
of a task object or type, or priority of a protected object or type;
the priority is not in the interrupt range.
Interrupt_Priority
The aspect Interrupt_Priority is an
expression,
which shall be of type Integer.
Aspect Description for Interrupt_Priority:
Priority of a task object or type, or priority of a protected object
or type; the priority is in the interrupt range.
Legality Rules
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If the Priority aspect is specified for a subprogram, the
expression
shall be static, and its value shall be in the range of System.Priority.
Reason: This value is needed before it
gets elaborated, when the environment task starts executing.
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At most one of the Priority and Interrupt_Priority aspects may be specified
for a given entity.
Ramification: This includes specifying
via pragmas (see
J.15.11). Note that
13.1
prevents multiple specifications of a single representation aspect by
any means.
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Neither of the Priority or Interrupt_Priority aspects shall be specified
for a synchronized interface type.
Static Semantics
The following declarations
exist in package System:
subtype Any_Priority is Integer range implementation-defined;
subtype Priority is Any_Priority
range Any_Priority'First .. implementation-defined;
subtype Interrupt_Priority is Any_Priority
range Priority'Last+1 .. Any_Priority'Last;
Default_Priority : constant Priority := (Priority'First + Priority'Last)/2;
Implementation defined: The declarations
of Any_Priority and Priority.
The full range of priority values supported by an
implementation is specified by the subtype Any_Priority. The subrange
of priority values that are high enough to require the blocking of one
or more interrupts is specified by the subtype Interrupt_Priority. [The
subrange of priority values below System.Interrupt_Priority'First is
specified by the subtype System.Priority.]
Dynamic Semantics
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The Priority aspect has no effect if it is specified for a subprogram
other than the main subprogram; the Priority value is not associated
with any task.
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A
task priority is an integer value that indicates a degree of urgency
and is the basis for resolving competing demands of tasks for resources.
Unless otherwise specified, whenever tasks compete for processors or
other implementation-defined resources, the resources are allocated to
the task with the highest priority value. The
base priority of
a task is the priority with which it was created, or to which it was
later set by Dynamic_Priorities.Set_Priority (see
D.5).
At all times, a task also has an
active priority, which generally
is its base priority unless it inherits a priority from other sources.
Priority inheritance is the process by which the priority of a
task or other entity (for example, a protected object; see
D.3)
is used in the evaluation of another task's active priority.
Implementation defined: Implementation-defined
execution resources.
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The
expression
specified for the Priority or Interrupt_Priority aspect of a task type
is evaluated each time an object of the task type is created (see
9.1).
For the Priority aspect, the value of the
expression
is converted to the subtype Priority; for the Interrupt_Priority aspect,
this value is converted to the subtype Any_Priority. The priority value
is then associated with the task object.
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Likewise, the priority value is associated with the environment task
if the aspect is specified for the main subprogram.
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The initial value of a task's base priority is specified by default or
by means of a Priority or Interrupt_Priority aspect. [After a task is
created, its base priority can be changed only by a call to Dynamic_Priorities.Set_Priority
(see
D.5).] The initial base priority of a
task in the absence of an aspect is the base priority of the task that
creates it at the time of creation (see
9.1).
If the aspect Priority is not specified for the main subprogram, the
initial base priority of the environment task is System.Default_Priority.
[The task's active priority is used when the task competes for processors.
Similarly, the task's active priority is used to determine the task's
position in any queue when Priority_Queuing is specified (see
D.4).]
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At any time, the active priority of a task is the maximum of all the
priorities the task is inheriting at that instant. For a task that is
not held (see
D.11), its base priority is
a source of priority inheritance unless otherwise specified for a particular
task dispatching policy. Other sources of priority inheritance are specified
under the following conditions:
Discussion: Other parts of the annex,
e.g.
D.11, define other sources of priority
inheritance.
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During activation, a task being activated inherits the active priority
that its activator (see
9.2) had at the time
the activation was initiated.
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While starting a protected action on a protected object when the FIFO_Spinning
admission policy is in effect, a task inherits the ceiling priority of
the protected object (see
9.5,
D.3,
and
D.4.1).
Reason: {
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Priority inheritance is needed for FIFO_Spinning to ensure that lower
priority tasks that initiate spin waiting earlier than other higher priority
tasks continue to spin to ensure that they can be granted the resource
when it becomes available in order to support FIFO ordering. Note that
this rule only matters when tasks that can initiate a protected action
on an object P can be on a different processor than P. In particular,
this rule does not matter on a monoprocessor.
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While a task executes a protected action on a protected object, the task
inherits the ceiling priority of the protected object (see
9.5
and
D.3).
In all of these cases, the priority ceases to be
inherited as soon as the condition calling for the inheritance no longer
exists.
Implementation Requirements
The range of System.Interrupt_Priority shall include
at least one value.
The range of System.Priority shall include at least
30 values.
NOTE 1 The priority expression can
include references to discriminants of the enclosing type.
NOTE 2 It is a consequence of the
active priority rules that at the point when a task stops inheriting
a priority from another source, its active priority is re-evaluated.
This is in addition to other instances described in this Annex for such
re-evaluation.
NOTE 3 {
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An implementation can provide a nonstandard mode in which tasks inherit
priorities under conditions other than those specified above.
Extensions to Ada 83
The priority of a task is
per-object and not per-type.
Priorities need not be static anymore (except
for the main subprogram).
Wording Changes from Ada 83
The description of the Priority pragma has been
moved to this annex.
Wording Changes from Ada 95
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Corrigendum: Clarified that dynamic priority changes are not transitive
- that is, they don't apply to tasks that are being activated by or in
rendezvous with the task that had its priority changed.
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Generalized the definition of priority inheritance to take into account
the differences between the existing and new dispatching policies.
Extensions to Ada 2005
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Aspects Priority and Interrupt_Priority are new;
pragmas Priority
and Interrupt_Priority are now obsolescent.
Wording Changes from Ada 2012
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Corrigendum: Clarified when the Priority and Interrupt_Priority
aspect expressions are evaluated.
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Added an additional case of priority inheritance when the new admission
policy FIFO_Spinning is in effect.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
