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<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><SPAN lang=EN-US><FONT 
face="Times New Roman" size=6><SPAN style="mso-tab-count: 1"><STRONG>2.3 
Priority &amp; Control</STRONG></SPAN></FONT></SPAN></P><SPAN lang=EN-US><FONT 
face="Times New Roman" size=5><SPAN style="mso-tab-count: 1">
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<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face="Times New Roman TUR" size=5><STRONG>2.3.1 Overview</STRONG></FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><SPAN 
lang=EN-US><FONT face="Times New Roman" size=5><SPAN 
style="mso-tab-count: 1"><FONT face=宋体><FONT size=3><FONT 
face="Times New Roman TUR"></FONT></FONT></FONT></SPAN></FONT></SPAN>&nbsp;</P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><SPAN 
lang=EN-US><FONT face="Times New Roman" size=5><SPAN 
style="mso-tab-count: 1"><FONT face=宋体><FONT size=+0><FONT size=3><FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>有很多种，当一个<FONT 
face="Times New Roman TUR">Interrupt/Exception</FONT>发生时，系统会自动调用它的<FONT 
face="Times New Roman TUR">ISR</FONT>，在<FONT 
face="Times New Roman TUR">ISR</FONT>被执行期间，可能会发生新的<FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>，这样，<FONT 
face="Times New Roman TUR">ISR</FONT>的执行就会被新的<FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>打断，而新的<FONT 
face="Times New Roman TUR">ISR</FONT>会被执行，当新的<FONT 
face="Times New Roman TUR">ISR</FONT>被执行结束之后，再继续执行旧的<FONT 
face="Times New Roman TUR">ISR</FONT>。这就是<FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>的嵌套问题。</FONT></FONT></FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3></FONT>&nbsp;</P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3" 
align=center><IMG src="interrupt-priority_3.files/int_iter.gif" 
tppabs="http://pagoda-ooos.51.net/os_book/interrupt/priority/int_iter.gif"></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3>这种嵌套可以有很多层，甚至可以无穷嵌套下去——如果中断发生的频率超过<FONT 
face="Times New Roman TUR">ISR</FONT>执行速度的话——但这种情况一般不会发生，因为<FONT 
face="Times New Roman TUR">ISR</FONT>一般都写的非常短小。但如果发生多层嵌套，有时候会存在问题。比如，当<FONT 
face="Times New Roman TUR">OS</FONT>正在运行的时候，<FONT 
face="Times New Roman TUR">CPU检测到一个内部总线错误</FONT>，引发了一个<FONT 
face="Times New Roman TUR">Machine-Check Exception</FONT>，这个时候，<FONT 
face="Times New Roman TUR">CPU</FONT>应该马上来处理这个<FONT 
face="Times New Roman TUR">Exception</FONT>。但当<FONT 
face="Times New Roman TUR">CPU</FONT>正在执行这个<FONT 
face="Times New Roman TUR">Exception</FONT>的<FONT 
face="Times New Roman TUR">ISR</FONT>的时候，又接到多个键盘<FONT 
face="Times New Roman TUR">Interrupt</FONT>。这个时候硬件（内部总线）已经发生了错误，<FONT 
face="Times New Roman TUR">CPU</FONT>应该优先解决这个错误，如果这时候<FONT 
face="Times New Roman TUR">CPU</FONT>转而去处理键盘<FONT 
face="Times New Roman TUR">Interrupt</FONT>的话，可能会引发灾难性的后果。</FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3></FONT>&nbsp;</P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3>针对这种情况，一个方法还起来好像能够解决这个问题，那就是，当一个<FONT 
face="Times New Roman TUR">Interrupt/Exception</FONT>的<FONT 
face="Times New Roman TUR">ISR</FONT>正在被执行的时候，不响应任何其它<FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>，也就是说<FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>应该被一个一个按照<FONT 
face="Times New Roman TUR">FIFO（First In, First Out）</FONT>的原则处理。</FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3></FONT>&nbsp;</P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3>但不幸的是，这样仍然存在问题。我们将上一个例子反过来：当<FONT 
face="Times New Roman TUR">CPU</FONT>正在处理一个<FONT 
face="Times New Roman TUR">Keyboard Interrupt</FONT>的时候，<FONT 
face="Times New Roman TUR">CPU</FONT>突然检测到一个内部总线错误，引发一个<FONT 
face="Times New Roman TUR">Machine-Check Exception</FONT>，这个时候<FONT 
face="Times New Roman TUR">CPU</FONT>应该马上去处理这个<FONT 
face="Times New Roman TUR">Exception</FONT>，而不是继续处理<FONT 
face="Times New Roman TUR">Keyboard Interrupt</FONT>。</FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3></FONT>&nbsp;</P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face=宋体 size=3>从上面两个例子可以看出，那个<FONT face="Times New Roman TUR">Machine-Check 
Exception</FONT>总是应该比<FONT face="Times New Roman TUR">Keyboard 
Interrupt</FONT>优先处理。所以我们需要为不同的<FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>定义不同的<FONT 
face="Times New Roman TUR">Priority</FONT>。</FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><STRONG></P>
<P>
<HR width="100%" SIZE=2>
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<P></P>
<P><STRONG></STRONG></P><SPAN lang=EN-US><FONT face="Times New Roman" 
size=5><SPAN style="mso-tab-count: 1">
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face="Times New Roman TUR" size=5><STRONG>2.3.2 Priority</STRONG></FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face="Times New Roman TUR"><SPAN lang=EN-US><FONT face="Times New Roman" 
size=5><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
face="Times New Roman" size=5><SPAN style="mso-tab-count: 1"><FONT face=宋体><FONT 
size=+0><FONT size=3><STRONG><FONT 
face="Times New Roman TUR"></FONT></STRONG></FONT></FONT></FONT></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT>&nbsp;</P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face="Times New Roman TUR"><SPAN lang=EN-US><FONT face="Times New Roman" 
size=5><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
face="Times New Roman" size=5><SPAN style="mso-tab-count: 1"><FONT face=宋体><FONT 
size=+0><FONT size=3><FONT 
face="Times New Roman TUR">当多个Interrupts/Exceptions同时或在很短的时间内相继发生的时候，Intel 
80x86芯片可以以一种可以预知的顺序来先后处理它们。能够预知的原因就是每个Interrupt/Exception都有自己的Priority。对于这批Interrrupts/Exceptions，CPU优先执行高优先级的，对于相同优先级的，CPU以FIFO的顺序执行它们。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face="Times New Roman TUR"><SPAN lang=EN-US><FONT face="Times New Roman" 
size=5><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT>&nbsp;</P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face="Times New Roman TUR"><SPAN lang=EN-US><FONT face="Times New Roman" 
size=5><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1">对于所有可能的Interrupts/Exceptions，Intel为它们定义了各自的Priority。这些Priority分类按照Interrupts/Exceptions的来源进行。下表总结了这些分类：</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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face="Times New Roman" size=5><SPAN style="mso-tab-count: 1"><SPAN 
lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><IMG src="interrupt-priority_3.files/ie_prior.gif" 
tppabs="http://pagoda-ooos.51.net/os_book/interrupt/priority/ie_prior.gif"></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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align=center><FONT face="Times New Roman TUR"><SPAN lang=EN-US><FONT 
face="Times New Roman" size=5><SPAN style="mso-tab-count: 1"><SPAN 
lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P><FONT 
face="Times New Roman TUR"><SPAN lang=EN-US><FONT face="Times New Roman" 
size=5><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
face="Times New Roman" size=5><SPAN style="mso-tab-count: 1"><FONT face=宋体 
size=3>
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style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3"><FONT 
face="Times New Roman TUR"><SPAN lang=EN-US><FONT face="Times New Roman" 
size=5><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
face="Times New Roman" size=5><SPAN style="mso-tab-count: 1"><FONT face=宋体><FONT 
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