D.1 Task Priorities
1
[This clause specifies the priority model for real-time
systems. In addition, the methods for specifying priorities are defined.]
Syntax
2
The form of
a pragma Priority is as follows:
3
pragma Priority(
expression);
4
The form of
a pragma Interrupt_Priority is as follows:
5
pragma Interrupt_Priority[(
expression)];
Name Resolution Rules
6
{expected type (Priority
pragma argument) [partial]} {expected
type (Interrupt_Priority pragma argument) [partial]} The
expected type for the
expression in a Priority
or Interrupt_Priority pragma is Integer.
Legality Rules
7
A Priority pragma is allowed only immediately within
a task_definition, a protected_definition,
or the declarative_part of a subprogram_body.
An Interrupt_Priority pragma is allowed only immediately within a task_definition
or a protected_definition. At most one such
pragma shall appear within a given construct.
8
For a Priority pragma that appears in the declarative_part
of a subprogram_body, the expression
shall be static, and its value shall be in the range of System.Priority.
8.a
Reason: This value is needed before it
gets elaborated, when the environment task starts executing.
Static Semantics
9
The following declarations
exist in package System:
10
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;
11
Default_Priority : constant Priority := (Priority'First + Priority'Last)/2;
11.a
Implementation defined: The declarations
of Any_Priority and Priority.
12
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.]
13
The priority specified by a Priority or Interrupt_Priority
pragma is the value of the expression in the
pragma, if any. If there is no expression
in an Interrupt_Priority pragma, the priority value is Interrupt_Priority'Last.
Dynamic Semantics
14
A Priority pragma has no effect if it occurs in the
declarative_part of the subprogram_body
of a subprogram other than the main subprogram.
15
{task priority}
{priority}
{priority inheritance}
{base priority}
{active priority}
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
reflects its base priority as well as any priority it inherits from other
sources.
Priority inheritance is the process by which the priority
of a task or other entity (e.g. a protected object; see
D.3)
is used in the evaluation of another task's active priority.
15.a
Implementation defined: Implementation-defined
execution resources.
16
The effect of specifying such a pragma in a
protected_definition
is discussed in
D.3.
17
{creation (of a task
object)} The
expression
in a Priority or Interrupt_Priority pragma that appears in a
task_definition
is evaluated for each task object (see
9.1).
For a Priority pragma, the value of the
expression
is converted to the subtype Priority; for an Interrupt_Priority pragma,
this value is converted to the subtype Any_Priority. The priority value
is then associated with the task object whose
task_definition
contains the pragma.
{implicit subtype
conversion (pragma Priority) [partial]} {implicit
subtype conversion (pragma Interrupt_Priority) [partial]}
18
Likewise, the priority value is associated with the
environment task if the pragma appears in the declarative_part
of the main subprogram.
19
The initial value of a task's base priority is specified
by default or by means of a Priority or Interrupt_Priority pragma. [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 a pragma is the
base priority of the task that creates it at the time of creation (see
9.1). If a pragma Priority does not apply to
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).]
20/2
{
AI95-00357-01}
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
always 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:
20.a
Discussion: Other parts of the annex,
e.g.
D.11, define other sources of priority
inheritance.
21/1
- {8652/0072}
{AI95-00092-01}
During activation, a task being activated inherits the active priority
that of the
its activator (see 9.2) had at the time the activation was initiated.
22/1
- {8652/0072}
{AI95-00092-01}
During rendezvous, the task accepting the entry call inherits the active
priority of the entry call caller
(see 9.5.3 and D.4).
23
- During a protected action on a protected
object, a task inherits the ceiling priority of the protected object
(see 9.5 and D.3).
24
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
25
The range of System.Interrupt_Priority shall include
at least one value.
26
The range of System.Priority shall include at least
30 values.
27
4 The priority expression can include references
to discriminants of the enclosing type.
28
5 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.
29
6 An implementation may provide a non-standard
mode in which tasks inherit priorities under conditions other than those
specified above.
29.a
Ramification: The use of a Priority or
Interrupt_Priority pragma does not require the package System to be named
in a with_clause for the enclosing compilation_unit.
Extensions to Ada 83
29.b
{
extensions to Ada 83}
The
priority of a task is per-object and not per-type.
29.c
Priorities need not be static anymore (except
for the main subprogram).
Wording Changes from Ada 83
29.d
The description of the Priority pragma has been
moved to this annex.
Wording Changes from Ada 95
29.e/2
{
8652/0072}
{
AI95-00092-01}
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.29.f/2
{
AI95-00357-01}
Generalized the definition of priority inheritance
to take into account the differences between the existing and new dispatching
policies.
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