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<h2>
<a name="1204486"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">5.1 OS20 tasks</font>
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<a name="1204487"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">A task consists of a data structure, stack and a section of code. A task's data structure is known as its state; its exact content and structure are processor-dependent. In OS20 it is divided into two parts, and includes the following elements:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204492"> </a><strong>Dynamic state</strong></font></p>
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<dt> <a name="1207003"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This is defined in the data structure <font size=2 face=Courier><strong>tdesc_t</strong></font>, which is used directly by the CPU to execute the process. The fields of this structure vary depending on the processor type. The most important elements of this structure are the machine registers, in particular the instruction (<span class=Signal>Iptr</span>) and workspace (<span class=Signal>Wptr</span>) pointers. A task priority is also used to make scheduling decisions. While the task is running, the <span class=Signal>Iptr</span> and <span class=Signal>Wptr</span> are maintained by the CPU; when the task is not executing they are stored in <font size=2 face=Courier><strong>tdesc_t</strong></font>. On the ST20C1, the <span class=Signal>Tdesc</span> register points to the current task's <font size=2 face=Courier><strong>tdesc_t</strong></font>.</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204496"> </a><strong>Static state</strong></font></p>
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<dt> <a name="1207007"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This is defined in the data structure <font size=2 face=Courier><strong>task_t</strong></font>, which is used by OS20 to describe the task, and which does not usually change while the task is running. It includes the task's state (that is; being created, executing, terminated) and the stack range (used for stack checking).</font>
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<a name="1204498"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The dynamic state should be stored in internal memory to minimize context switch time. The state is divided into two in this way so that only the minimum amount of internal memory needs to be used to store <font size=2 face=Courier><strong>tdesc_t</strong></font>.</font>
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<a name="1204499"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">A task is identified by its <font size=2 face=Courier><strong>task_t</strong></font> structure and this should always be used when referring to the task. A pointer to the <font size=2 face=Courier><strong>task_t</strong></font> structure is called the task's ID; see <a href="task13.htm#1204834"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Section⌨️ 快捷键说明
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