D.1 Task Priorities

This subclause specifies the priority model for real-time systems. In addition, the methods for specifying priorities are defined.

Paragraphs 2 through 6 were moved to Annex J, “Obsolescent Features”. 

 For a task type (including the anonymous type of a single_task_declaration), protected type (including the anonymous type of a single_protected_declaration), or subprogram, the following language-defined representation aspects may be specified:

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.

 At most one of the Priority and Interrupt_Priority aspects may be specified for a given entity.

 Neither of the Priority or Interrupt_Priority aspects shall be specified for a synchronized interface type.

The following declarations exist in package System: 

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.

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.

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. 

The effect of specifying a Priority or Interrupt_Priority aspect for a protected type or single_protected_declaration is discussed in D.3.

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.

Likewise, the priority value is associated with the environment task if the aspect is specified for the main subprogram.

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).

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: 

In all of these cases, the priority ceases to be inherited as soon as the condition calling for the inheritance no longer exists.

The range of System.Interrupt_Priority shall include at least one value.

The range of System.Priority shall include at least 30 values.