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
2/3
3/3
4/3
5/3
Name Resolution Rules
6/3
{
AI05-0229-1}
The expected
type for the expression
in a Priority or Interrupt_Priority pragma is Integer. Static Semantics
6.1/3
6.2/3
Priority
The aspect Priority is an expression,
which shall be of type Integer.
6.a/3
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.
6.3/3
Interrupt_Priority
The aspect Interrupt_Priority is an expression,
which shall be of type Integer.
6.b/3
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
7/3
8/3
8.a
Reason: This value is needed before it
gets elaborated, when the environment task starts executing.
8.1/3
{
AI05-0229-1}
At most one of the Priority and Interrupt_Priority
aspects may be specified for a given entity.8.b/3
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.
8.2/3
{
AI05-0229-1}
Neither of the Priority or Interrupt_Priority aspects
shall be specified for a synchronized interface type.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.]
13/3
This paragraph was
deleted.{
AI05-0229-1}
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
14/3
{
AI05-0229-1}
The A Priority
aspect pragma
has no effect if it
is specified for it
occurs in the declarative_part
of the subprogram_body
of a subprogram other than the main subprogram
;
the Priority value is not associated with any task.
15
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.
16/3
17/3
{
AI05-0229-1}
The
expression
specified for the in
a Priority or Interrupt_Priority
aspect
of a task pragma that appears in a task_definition
is evaluated for each task object (see
9.1).
For
the a Priority
aspect pragma,
the value of the
expression
is converted to the subtype Priority; for
the an
Interrupt_Priority
aspect pragma,
this value is converted to the subtype Any_Priority. The priority value
is then associated with the task object whose
task
declaration specifies the aspect task_definition
contains the pragma.
18/3
{
AI05-0229-1}
Likewise, the priority value is associated with the environment task
if the
aspect is specified for pragma
appears in the declarative_part
of the main subprogram.
19/3
{
AI05-0229-1}
The initial value of a task's base priority is specified by default or
by means of a Priority or Interrupt_Priority
aspect 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
an
aspect a pragma. is the base priority
of the task that creates it at the time of creation (see
9.1).
If
the aspect a pragma
Priority
is not specified for 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.
29/3
6 {
AI05-0248-1}
An implementation may provide a
nonstandard non-standard
mode in which tasks inherit priorities under conditions other than those
specified above.
29.a/3
Extensions to Ada 83
29.b
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. Extensions to Ada 2005
29.g/3
{
AI05-0229-1}
Aspects Priority and Interrupt_Priority
are new; pragmas
Priority and Interrupt_Priority are now obsolescent.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
