interrupt-priority_3.htm

编写自己的操作系统

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face="Times New Roman TUR">当一个Interrupt/Exception正在被处理时，如果又发生了其它Interrupts/Exceptions，CPU会去查看它们中间是否存在着比当前正在处理的Interrupt/Exception 
优先级更高的，如果存在，则保留当前中断处理的Context，然后从这些Interrupts/Exceptions中挑选一个优先级最高的，并开始处理它。然后，剩下的那些未被处理的Exceptions将被丢弃，未被处理的Interrupts则继续处于Pending状态，等待被处理。那些被丢弃的Exceptions，都是由软件引起的（由硬件引起的Exceptions具有最高的Priority，永远不可能被丢弃），所以，等所有的Interrupts/Exceptions被处理结束时候，执行点会重新回到那些引起这些Exceptions的点，那时候，Exceptions会被重新生成并被处理。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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style="mso-tab-count: 1">下图表现了CPU的指令循环（Instruction 
Cycle）：</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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style="mso-tab-count: 1"><IMG src="interrupt-priority_3.files/flowchrt.gif" 
tppabs="http://pagoda-ooos.51.net/os_book/interrupt/priority/flowchrt.gif"></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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<P><STRONG></STRONG></P><SPAN lang=EN-US><FONT face="Times New Roman" 
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face="Times New Roman TUR" size=5><STRONG>2.3.3 Control - Disable &amp; Enable 
Interrupts</STRONG></FONT></P>
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<P class=MsoNormal 
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size=+0><FONT size=3><FONT 
face="Times New Roman TUR">依赖于状态寄存器EFLAGS的IF和RF位的设置，CPU可以禁止一些Interrupts的发生。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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lang=EN-US><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
face="Times New Roman TUR" size=4><SPAN style="mso-tab-count: 1"><STRONG>2.3.3.1 
Masking Maskable Hardware 
Interrupts</STRONG></SPAN></FONT></SPAN></SPAN></SPAN></SPAN></FONT></SPAN></P></FONT></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT><FONT 
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size=3><FONT 
face="Times New Roman TUR">EFLAGS寄存器的</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></SPAN></FONT></SPAN></SPAN></SPAN>IF位的设置可以决定是否屏蔽那些来自于INTR 
pin的中断。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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<P class=MsoNormal 
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style="mso-tab-count: 1">如果IF位被清除，CPU会禁止外部中断传递给INTR 
pin；</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT><FONT 
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style="mso-tab-count: 1">如果IF位被设置，CPU则允许外部中断传递给INTR pin。INTR 
pin外接PIC-8259A。</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT><FONT 
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style="mso-tab-count: 1">IF位的设置不影响NMI，也不影响处理器产生的Exceptions。当CPU被Reset之后，IF位被清除。</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
<P class=MsoNormal 
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size=5><SPAN style="mso-tab-count: 1"><SPAN lang=EN-US><FONT 
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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 
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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">通过STI(Set 
Interrupt-Enable Flag)和CLI(Clear Interrupt 
Flag)指令可以设置和清除IF位。但在执行这些指令时，必须保证当前CPL小于或等于IOPL；如果CPL &gt; IOPL，则会产生一个General 
Protection Exception。</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
<P class=MsoNormal 
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<P class=MsoNormal 
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style="mso-tab-count: 1">我们可以使用C的Macro实现这两个操作：</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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<P class=MsoNormal 
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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">#define sti() 
__asm__ __volatile__ ("sti": : 
:"memory")</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
<P class=MsoNormal 
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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">#define cli() 
__asm__ __volatile__ ("cli": : 
:"memory")</SPAN></FONT></SPAN></SPAN></FONT></SPAN></FONT></P>
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