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<a name="1204438"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">3.1 Implementation</font>
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<a name="1204439"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The kernel maintains two vitally important pieces of information:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204440"> </a>the currently executing task, and thus what priority is currently being executed,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204441"> </a>a list of all the tasks which are currently ready to run.</font></p>
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<dt> <a name="1205271"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This is stored as a number of queues, one for each priority, with the tasks stored in the order in which they will be executed.</font>
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<a name="1205195"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The kernel is invoked whenever a scheduling decision has to be made. This happens on three possible occasions.</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1205196"> </a>When a task is about to be scheduled, the scheduler is called to determine if the new task is of higher priority than the currently executing task. If it is, then the state of the current task is saved, and the new one installed in its place, so that the new task starts to run. This is termed "preemption", because the new task has preempted the old one.</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204444"> </a>When a task deschedules, for example it is waiting on a message queue which does not have any messages available, then the scheduler is invoked to decide which task to run next. The kernel examines the list of processes which are ready to run, and picks the one with the highest priority.</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204445"> </a>Periodically the scheduler is called to timeslice the currently executing task. If there are other tasks which are of the same priority as the current task, then the state of the current task is saved onto the back of the current priority queue, and the task at the front of the queue installed in its place. In this way all processes of the same priority get a chance to run.</font></p>
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<a name="1204446"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">In this way the kernel ensures that it is always the highest priority task(s) which run.</font>
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<a name="1204447"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The scheduler code is installed as a scheduler trap handler, which causes the ST20 hardware to invoke the scheduling software whenever a scheduling operation is required.</font>
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<font size=1 face="Verdana, Arial, Helvetica, sans-serif"> © 2001, 2002, 2003 STMicroelectronics. All Rights Reserved.<br>
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