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<a name="1204850"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">1.5 Priority</font>
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<a name="1204851"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Task scheduling is determined by priority. Normally the highest priority task is the one set to run, with all lower priority tasks paused until the highest priority task deschedules.</font>
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<a name="1204852"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">In some cases, when there are two or more tasks of the same priority waiting to run, they are each run for a short period, dividing the use of the CPU between the tasks. This is called timeslicing.</font>
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<a name="1204853"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">A task's priority is set when the task is created, although it may be changed later. OS20 provides the user with sixteen levels of priority.</font>
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<a name="1204857"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Some members of the ST20 family of micro-cores implement an additional level of priority via hardware processes.</font>
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<a name="1204861"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">OS20 supports the following system of priority for tasks running on an ST20C2 processor.</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204862"> </a>Tasks are normally run as low priority processes, and within this low priority rating may be given a further priority level specified by the user. Low priority tasks of equal priority are timesliced to share the processor time. Low priority tasks only run when there are no high priority processes waiting to run.</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204863"> </a>Tasks may be created to run as high priority processes, in which case they are never timesliced and run until they terminate or have to wait for a time or communication before they deschedule themselves. High priority tasks should be kept as short as possible to prevent them from monopolizing system resources. High priority tasks can interrupt low priority tasks that are running.</font></p>
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<a name="1204867"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">On an ST20C1 there is no hardware priority support. OS20 allows the user to define individual task priorities, and tasks of equal priority are timesliced. High priority processes are not supported on the ST20C1.</font>
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<a name="1204871"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">To implement multi-priority scheduling, OS20 uses a scheduling kernel which needs to be installed and started before any tasks are created. This is described in <a href="ker.htm#1204411"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Chapter⌨️ 快捷键说明
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