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<H1>Section 9: Tasks and Synchronization</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3462"></A>The execution of an Ada program
consists of the execution of one or more <I>tasks</I>. <A NAME="I3463"></A><A NAME="I3464"></A>Each
task represents a separate thread of control that proceeds independently
and concurrently between the points where it <I>interacts</I> with other
tasks. The various forms of task interaction are described in this section,
and include: <A NAME="I3465"></A><A NAME="I3466"></A><A NAME="I3467"></A><A NAME="I3468"></A></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>To be honest: </B>The execution
of an Ada program consists of the execution of one or more partitions
(see <A HREF="AA-10-2.html">10.2</A>), each of which in turn consists
of the execution of an environment task and zero or more subtasks. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>the activation and termination of a task;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I3469"></A>a call on a protected subprogram of
a <I>protected object</I>, providing exclusive read-write access, or
concurrent read-only access to shared data;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a call on an entry, either of another task, allowing for
synchronous communication with that task, or of a protected object, allowing
for asynchronous communication with one or more other tasks using that
same protected object;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>a timed operation, including a simple delay statement,
a timed entry call or accept, or a timed asynchronous select statement
(see next item);</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>an asynchronous transfer of control as part of an asynchronous
select statement, where a task stops what it is doing and begins execution
at a different point in response to the completion of an entry call or
the expiration of a delay;</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>an abort statement, allowing one task to cause the termination
of another task. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> In addition, tasks can communicate indirectly
by reading and updating (unprotected) shared variables, presuming the
access is properly synchronized through some other kind of task interaction.</DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> <A NAME="I3470"></A>The properties of a task are
defined by a corresponding task declaration and <FONT FACE="Arial, Helvetica">task_body</FONT>,
which together define a program unit called a <I>task unit</I>. </DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> Over time, tasks proceed through various <I>states</I>.
<A NAME="I3471"></A><A NAME="I3472"></A><A NAME="I3473"></A><A NAME="I3474"></A><A NAME="I3475"></A><A NAME="I3476"></A><A NAME="I3477"></A><A NAME="I3478"></A>A
task is initially <I>inactive</I>; upon activation, and prior to its
<I>termination</I> it is either <I>blocked</I> (as part of some task
interaction) or <I>ready</I> to run. <A NAME="I3479"></A>While ready,
a task competes for the available <I>execution resources</I> that it
requires to run. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>10.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B><A NAME="I3480"></A><A NAME="I3481"></A>The
means for selecting which of the ready tasks to run, given the currently
available execution resources, is determined by the <I>task dispatching
policy</I> in effect, which is generally implementation defined, but
may be controlled by pragmas and operations defined in the Real-Time
Annex (see <A HREF="AA-D-2.html">D.2</A> and <A HREF="AA-D-5.html">D.5</A>).
</FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>1 Concurrent task execution
may be implemented on multicomputers, multiprocessors, or with interleaved
execution on a single physical processor. On the other hand, whenever
an implementation can determine that the required semantic effects can
be achieved when parts of the execution of a given task are performed
by different physical processors acting in parallel, it may choose to
perform them in this way.</FONT></DIV>
<H4 ALIGN=CENTER>Wording Changes from Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>The introduction has been rewritten.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>We use the term "concurrent"
rather than "parallel" when talking about logically independent
execution of threads of control. The term "parallel" is reserved
for referring to the situation where multiple physical processors run
simultaneously. </FONT></DIV>
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