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<H1> D.1 Task Priorities</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [This clause specifies the priority model for
real-time systems. In addition, the methods for specifying priorities
are defined.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented" Style="margin-bottom: 0.2em">The form of
a <FONT FACE="Arial, Helvetica">pragma</FONT> Priority is as follows:
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6764"></A>Priority(<A NAME="I6765"></A><FONT FACE="Arial, Helvetica">expression</FONT>);</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="SyntaxIndented" Style="margin-bottom: 0.2em">The form of
a <FONT FACE="Arial, Helvetica">pragma</FONT> Interrupt_Priority is as
follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6766"></A>Interrupt_Priority[(<A NAME="I6767"></A><FONT FACE="Arial, Helvetica">expression</FONT>)];
</DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6768"></A><A NAME="I6769"></A>The expected
type for the <FONT FACE="Arial, Helvetica">expression</FONT> in a Priority
or Interrupt_Priority pragma is Integer. </DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> A Priority pragma is allowed only immediately
within a <FONT FACE="Arial, Helvetica">task_definition</FONT>, a <FONT FACE="Arial, Helvetica">protected_definition</FONT>,
or the <FONT FACE="Arial, Helvetica">declarative_part</FONT> of a <FONT FACE="Arial, Helvetica">subprogram_body</FONT>.
An Interrupt_Priority pragma is allowed only immediately within a <FONT FACE="Arial, Helvetica">task_definition</FONT>
or a <FONT FACE="Arial, Helvetica">protected_definition</FONT>. At most
one such pragma shall appear within a given construct.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> For a Priority pragma that appears in the <FONT FACE="Arial, Helvetica">declarative_part</FONT>
of a <FONT FACE="Arial, Helvetica">subprogram_body</FONT>, the <FONT FACE="Arial, Helvetica">expression</FONT>
shall be static, and its value shall be in the range of System.Priority.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>This value is needed
before it gets elaborated, when the environment task starts executing.
</FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The following declarations
exist in package System: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Examples"><TT><B>subtype</B> Any_Priority <B>is</B> Integer <B>range</B> <I>implementation-defined</I>;<BR>
<B>subtype</B> Priority <B>is</B> Any_Priority<BR>
<B>range</B> Any_Priority'First .. <I>implementation-defined</I>;<BR>
<B>subtype</B> Interrupt_Priority <B>is</B> Any_Priority<BR>
<B>range</B> Priority'Last+1 .. Any_Priority'Last;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Examples"><TT>Default_Priority : <B>constant</B> Priority := (Priority'First + Priority'Last)/2;</TT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>The
declarations of Any_Priority and Priority.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> 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.]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> The priority specified by a Priority or Interrupt_Priority
pragma is the value of the <FONT FACE="Arial, Helvetica">expression</FONT>
in the pragma, if any. If there is no <FONT FACE="Arial, Helvetica">expression</FONT>
in an Interrupt_Priority pragma, the priority value is Interrupt_Priority'Last.</DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal"> A Priority pragma has no effect if it occurs
in the <FONT FACE="Arial, Helvetica">declarative_part</FONT> of the <FONT FACE="Arial, Helvetica">subprogram_body</FONT>
of a subprogram other than the main subprogram.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6770"></A><A NAME="I6771"></A><A NAME="I6772"></A><A NAME="I6773"></A><A NAME="I6774"></A>A
<I>task priority</I> 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 <I>base priority</I> of
a task is the priority with which it was created, or to which it was
later set by Dynamic_Priorities.Set_Priority (see <A HREF="AA-D-5.html">D.5</A>).
At all times, a task also has an <I>active priority</I>, which generally
reflects its base priority as well as any priority it inherits from other
sources. <I>Priority inheritance</I> is the process by which the priority
of a task or other entity (e.g. a protected object; see <A HREF="AA-D-3.html">D.3</A>)
is used in the evaluation of another task's active priority. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Implementation-defined
execution resources.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal"> The effect of specifying such a pragma in a <FONT FACE="Arial, Helvetica">protected_definition</FONT>
is discussed in <A HREF="AA-D-3.html">D.3</A>.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6775"></A>The <FONT FACE="Arial, Helvetica">expression</FONT>
in a Priority or Interrupt_Priority pragma that appears in a <FONT FACE="Arial, Helvetica">task_definition</FONT>
is evaluated for each task object (see <A HREF="AA-9-1.html">9.1</A>).
For a Priority pragma, the value of the <FONT FACE="Arial, Helvetica">expression</FONT>
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 <FONT FACE="Arial, Helvetica">task_definition</FONT>
contains the pragma. <A NAME="I6776"></A><A NAME="I6777"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Normal"> Likewise, the priority value is associated with
the environment task if the pragma appears in the <FONT FACE="Arial, Helvetica">declarative_part</FONT>
of the main subprogram.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Normal"> 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 <A HREF="AA-D-5.html">D.5</A>).]
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
<A HREF="AA-9-1.html">9.1</A>). 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 <A HREF="AA-D-4.html">D.4</A>).]</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> 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 <A HREF="AA-D-11.html">D.11</A>),
its base priority is always a source of priority inheritance. Other sources
of priority inheritance are specified under the following conditions:
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>Other parts
of the annex, e.g. <A HREF="AA-D-11.html">D.11</A>, define other sources
of priority inheritance. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21/1</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>{<I><A HREF="defect1.html#8652/0072">8652/0072</A></I>}
During activation, a task being activated inherits the active priority
<U>that</U><S>of the</S> its activator (see <A HREF="AA-9-2.html">9.2</A>)<U>
had at the time the activation was initiated</U>.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>22/1</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>{<I><A HREF="defect1.html#8652/0072">8652/0072</A></I>}
During rendezvous, the task accepting the entry call inherits the <S>active
</S>priority of the <U>entry call</U><S>caller</S> (see <A HREF="AA-9-5-3.html">9.5.3</A><U>
and <A HREF="AA-D-4.html">D.4</A></U>).</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>23</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>During a protected action on a protected object, a task
inherits the ceiling priority of the protected object (see <A HREF="AA-9-5.html">9.5</A>
and <A HREF="AA-D-3.html">D.3</A>).</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>24</FONT></DIV>
<DIV Class="Normal"> In all of these cases, the priority ceases to
be inherited as soon as the condition calling for the inheritance no
longer exists.</DIV>
<H4 ALIGN=CENTER>Implementation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>25</FONT></DIV>
<DIV Class="Normal"> The range of System.Interrupt_Priority shall
include at least one value.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>26</FONT></DIV>
<DIV Class="Normal"> The range of System.Priority shall include at
least 30 values.</DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>27</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>4 The priority expression
can include references to discriminants of the enclosing type.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>28</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>29</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>6 An implementation may provide
a non-standard mode in which tasks inherit priorities under conditions
other than those specified above. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>29.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The use of
a Priority or Interrupt_Priority pragma does not require the package
System to be named in a <FONT FACE="Arial, Helvetica">with_clause</FONT>
for the enclosing <FONT FACE="Arial, Helvetica">compilation_unit</FONT>.
</FONT></DIV>
<H4 ALIGN=CENTER>Extensions to Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>29.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I6778"></A>The priority
of a task is per-object and not per-type.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>29.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Priorities need not be static
anymore (except for the main subprogram).</FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>29.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The description of the Priority
pragma has been moved to this annex.</FONT></DIV>
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