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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>2.4 
Handling &amp; Error Code</STRONG></SPAN></FONT></SPAN></P><SPAN 
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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>2.4.1 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">当一个Interrupt/Exception发生时，CPU将以这个Interrupt/Exception的Vector 
Number为索引，到IDT中查找相应的Gate Descriptor。IDT中的Gate Descriptor有3种：Interrupt Gate 
Descriptor，Trap Gate Descriptor，Task Gate Descriptor。根据被设置的Gate 
Descriptor的不同，对ISR的调用方法和处理过程也不同。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></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 TUR" size=3><SPAN 
style="mso-tab-count: 1"></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 TUR" size=3><SPAN 
style="mso-tab-count: 1">当某些Exceptions发生时，会返回Error 
Code，以通知相应的ISR是那个Segment引起了这个Exception。</SPAN></FONT></SPAN></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 TUR" size=3><SPAN 
style="mso-tab-count: 1"><STRONG><FONT face="Times New Roman" size=5>&nbsp;</P>
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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"><STRONG>2.4.2 ISR Handling</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">一个ISR(Interrupt Service 
Routine)是一块专门用来处理某个Interrupt/Exception的程序。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></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 TUR" size=3><SPAN 
style="mso-tab-count: 1"></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 TUR" size=3><SPAN 
style="mso-tab-count: 1">如果一个Interrupt/Exception引用的是一个Interrupt Gate或Trap 
Gate，则当这个Interrupt/Exception发生时，不发生任务切换，其ISR仍然引用当前任务的Context。Interrupt/Trap Gate 
Descriptor中的Segment 
Selector引用的是一个放在GDT或当前LDT中的可执行代码段。Offset域引用的是其ISR的入口地址。</SPAN></FONT></SPAN></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><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"></SPAN></FONT></SPAN></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><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><IMG src="interrupt-handling_4.files/isr.gif" 
tppabs="http://pagoda-ooos.51.net/os_book/interrupt/handling/isr.gif"></SPAN></FONT></SPAN></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 TUR" size=3><SPAN 
style="mso-tab-count: 1">当CPU调用一个ISR时，会将EFLAGS，CS，EIP寄存器压栈，其中CS，EIP寄存器的内容事实上是ISR的返回地址。如果一个Exception发生时，存在一个Error 
Code，那么这个Error Code会在EIP之后被压栈。</SPAN></FONT></SPAN></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 TUR" size=3><SPAN 
style="mso-tab-count: 1"></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 TUR" size=3><SPAN 
style="mso-tab-count: 1">如果ISR和被打断的任务具有相同的特权等级，则CPU不改变Stack，让ISR继续使用当前的Stack。</SPAN></FONT></SPAN></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 TUR" size=3><SPAN 
style="mso-tab-count: 1"></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 TUR" size=3><SPAN 
style="mso-tab-count: 1">如果ISR的特权等级从数字上小于被打断任务的特权等级，也就是说如果ISR的权限大于被打断任务的权限，则会发生Stack 
Switch。为了能够在ISR结束之后，能够恢复使用原来的Stack，CPU会将SS和ESP寄存器的内容也压栈，并且这些内容都被压在新的Stack中，而不是Stack 
Switch之前的那个Stack。</SPAN></FONT></SPAN></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><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><IMG src="interrupt-handling_4.files/intr_stack.gif" 
tppabs="http://pagoda-ooos.51.net/os_book/interrupt/handling/intr_stack.gif"></SPAN></FONT></SPAN></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><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"></SPAN></FONT></SPAN></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3" 
align=left><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><SPAN lang=EN-US><FONT face="Times New Roman TUR" 
size=3><SPAN 
style="mso-tab-count: 1">问题是，CPU在Interrupt/Exception发生的时刻，发现特权等级不一致的时候，如何知道该使用那个栈？答案是通过当前任务的TSS。在TSS中存放着针对0，1，2三个特权等级的SS和ESP内容，当特权等级切换到某个特权等级的时候，CPU会从当前任务的TSS中找到相应特权等级的SS和ESP用来进行Stack 
Switch。</SPAN></FONT></SPAN></SPAN></FONT></SPAN></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3" 
align=left><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><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>&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=left><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><SPAN lang=EN-US><FONT face="Times New Roman TUR" 
size=3><SPAN 
style="mso-tab-count: 1">当ISR执行结束之后，需要继续执行之前被终止的任务，为了能够正确的做到这一点，ISR的返回必须使用IRET指令。</SPAN></FONT></SPAN></SPAN></FONT></SPAN><SPAN 
lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><SPAN lang=EN-US><FONT face="Times New Roman TUR" 
size=3><SPAN 
style="mso-tab-count: 1">IRET指令会自动从Stack中恢复EFLAGS寄存器的值，并根据Stack中的CS，EIP内容挑转到被中断的位置。</SPAN></FONT></SPAN></SPAN></FONT></SPAN><SPAN 
lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><SPAN lang=EN-US><FONT face="Times New Roman TUR" 
size=3><SPAN style="mso-tab-count: 1">如果之前在Interrupt/Exception发生的时候，进行了Stack 
Switch，此时IRET会根据栈中的SS，ESP的内容将Stack切换回去。</SPAN></FONT></SPAN></SPAN></FONT></SPAN></P>
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3" 
align=left><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><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>&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=left><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN 
style="mso-tab-count: 1"><SPAN lang=EN-US><FONT face="Times New Roman TUR" 
size=3><SPAN style="mso-tab-count: 1">当通过一个Interrupt Gate或Trap 
Gate来处理一个Interrupt/Exception时，CPU会在将EFLAGS压栈之后，清除EFLAGS寄存器的TF flag。清除TF
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