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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="I3301"></A>The execution of an Ada program
consists of the execution of one or more <I>tasks</I>. <A NAME="I3302"></A><A NAME="I3303"></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="I3304"></A><A NAME="I3305"></A><A NAME="I3306"></A><A NAME="I3307"></A></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="I3308"></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="I3309"></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="I3310"></A><A NAME="I3311"></A><A NAME="I3312"></A><A NAME="I3313"></A><A NAME="I3314"></A><A NAME="I3315"></A><A NAME="I3316"></A><A NAME="I3317"></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="I3318"></A>While ready,
a task competes for the available <I>execution resources</I> that it
requires to run. </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>
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