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<h2>
<a name="1102424"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">7.5 Inactive processes</font>
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<a name="1102425"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">An inactive process is a process that is waiting for some event to occur, such as a process executing a <strong>run</strong> instruction or a peripheral completing a DMA. An inactive process cannot continue even if the CPU is idle. Inactive processes are not polled, but should be rescheduled by the event for which they are waiting. </font>
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<a name="1102426"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">A process becomes inactive by saving its own state and then executing the <strong>stop</strong> instruction. <strong>stop</strong> automatically saves the <span class=Signal>Iptr</span> and <span class=Signal>Wptr</span> in the process control block. The code should save the <span class=Signal>Tdesc</span> in an appropriate place where the awaited event can find it. The <span class=Signal>Tdesc</span> points to the process control block. The <span class=Signal>Iptr</span> is loaded onto the evaluation stack using <strong>ldpi</strong>, the <span class=Signal>Wptr</span> using <strong>ldlp 0</strong> and the <span class=Signal>Tdesc</span> using <strong>ldtdesc</strong>. For example:</font>
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<dt> <a name="1102427"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif"><strong>ldtdesc; ld tdesc_save_address; stnl 0;<br>stop;</strong> </font>
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<a name="1102430"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Two mechanisms are provided for rescheduling inactive processes.</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1102432"> </a>Executing the <strong>run</strong> instruction. This allows other processes to reschedule an inactive process. For example, this is used when semaphore signalling to a waiting process and could be used by other communications, possibly implemented by an operating system. </font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1102433"> </a>Scheduling an exception. This allows external devices to reschedule an inactive process. For example, this is used by DMA peripherals to wake a process waiting for the DMA to complete. Exception scheduling is described in <a href="using12.htm#1103353"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Section⌨️ 快捷键说明
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