flag，是為了避免Instruction tracing影響中斷的響應。隨后的IRET指令會從棧中的EFLAGS中恢復TF 
flag的值。</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">在Interrupt Gate和Trap 
Gate之間的唯一區別是，當通過一個Interrupt 
Gate來處理一個Interrupt/Exception時，CPU會在將EFLAGS壓棧之后，清除EFLAGS寄存器的IF flag。清除IF 
flag，是為了避免在當前ISR執行時受到其它Interrupts的影響。隨后的IRET指令會從棧中的EFLAGS中恢復IF flag的值。而通過Trap 
Gate來處理Interrupt/Exception則不會影響IF 
flag的設置。</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"><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">需要注意的是，在恢復EFLAGS的時候，只有CPL=0的時候，IOPL域才能夠被恢復；只有CPL在數值上小于或等于IOPL時，IF位才能夠被恢復。</SPAN></FONT></SPAN></SPAN></FONT></SPAN></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 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></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"><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">下面是用C++實現的IRET指令：</SPAN></FONT></SPAN></SPAN></FONT></SPAN></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 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></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"><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">inline 
void&nbsp;iret(void)</SPAN></FONT></SPAN></SPAN></FONT></SPAN></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 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></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 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">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; __asm__ 
__volatile__ ("iret": : 
:"memory");</SPAN></FONT></SPAN></SPAN></FONT></SPAN></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 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></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 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></SPAN></FONT></SPAN></SPAN></FONT></SPAN>&nbsp;</P><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 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">
<P class=MsoNormal 
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3" 
align=left><FONT size=4><STRONG>Task Gate</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" 
align=left><FONT face=宋體><FONT face="Times New Roman TUR"><FONT 
face=宋體></FONT></FONT></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=left><FONT face=宋體><FONT face="Times New Roman TUR"><FONT 
face=宋體>如果</FONT></FONT><FONT face="Times New Roman TUR">調用Task 
Gate</FONT>，會引起<FONT face="Times New Roman TUR">Context</FONT>切換。如果<FONT 
face="Times New Roman TUR">Interrupts/Exceptions</FONT>使用這種方式的話，會有如下幾個優點：</FONT></P><FONT 
face=宋體>
<UL>
  <LI>被中斷的程序的<FONT face="Times New Roman TUR">Context</FONT>會被完全被保存； 
  <LI>允許<FONT face="Times New Roman TUR">ISR</FONT>使用新的<FONT 
  face="Times New Roman TUR">CPL</FONT>為<FONT 
  face="Times New Roman TUR">0</FONT>的<FONT 
  face="Times New Roman TUR">stack</FONT>；這樣如果<FONT 
  face="Times New Roman TUR">Interrupt/Exception</FONT>是由于內核<FONT 
  face="Times New Roman TUR">stack</FONT>越界引起的話，可以通過<FONT 
  face="Times New Roman TUR">TSS</FONT>指定的新的<FONT 
  face="Times New Roman TUR">CPL=0</FONT>的<FONT 
  face="Times New Roman TUR">stack</FONT>來解決這個故障，而不是讓系統崩潰； 
  <LI>
  <P><FONT 
  face="Times New Roman TUR">ISR</FONT>可以被放在一個獨立的地址空間中，這種情況下，你需要通過設定一個獨立的<FONT 
  face="Times New Roman TUR">LDT</FONT>。</P></LI></UL>
<P>但也必須注意它的缺點：由于在任務切換時，當時的機器狀態需要全部被保存，所以它的性能要比<FONT 
face="Times New Roman TUR">Interrupt Gate</FONT>和<FONT 
face="Times New Roman TUR">Trap Gate</FONT>低。從而導致中斷延遲越來越多。</P>
<DIV>將<FONT face="Times New Roman TUR">ISR</FONT>作為一個<FONT 
face="Times New Roman TUR">Task</FONT>，然后通過<FONT face="Times New Roman TUR">Task 
Gate</FONT>實現從被中斷任務向<FONT face="Times New Roman TUR">ISR 
Task</FONT>的切換，與普通的任務切換沒有什么兩樣。我們注意到，在<FONT face="Times New Roman TUR">Task Gate 
Descriptor</FONT>中，一個主要的元素就是</FONT><FONT face="Times New Roman TUR">TSS Segment 
Selector</FONT><FONT face=宋體>，這個<FONT 
face="Times New Roman TUR">Selector</FONT>指向一個存放在<FONT 
face="Times New Roman TUR">GDT</FONT>中的<FONT face="Times New Roman TUR">TSS 
Descriptor</FONT>。它所指向的<FONT face="Times New Roman TUR">TSS</FONT>是供<FONT 
face="Times New Roman TUR">ISR Task</FONT>使用的。這個<FONT 
face="Times New Roman TUR">TSS</FONT>中的<FONT face="Times New Roman TUR">Previous 
Task Link</FONT>域指向那個被中斷任務的<FONT face="Times New Roman TUR">TSS</FONT>。如果<FONT 
face="Times New Roman TUR">Exception</FONT>返回一個<FONT 
face="Times New Roman TUR">error code</FONT>的話，那么<FONT 
face="Times New Roman TUR">error code</FONT>會被保存在<FONT 
face="Times New Roman TUR">ISR Task</FONT>的<FONT 
face="Times New Roman TUR">stack</FONT>中。</FONT></DIV>
<P align=center><IMG src="interrupt-handling_4.files/task_gate.gif" 
tppabs="http://pagoda-ooos.51.net/os_book/interrupt/handling/task_gate.gif"></P>
<HR width="100%" SIZE=2>

<P><SPAN lang=EN-US><FONT face="Times New Roman"><SPAN 
style="mso-tab-count: 1"><FONT face="Times New Roman TUR" 
size=5><STRONG>2.4.3&nbsp;Task Restart</STRONG><SPAN lang=EN-US><FONT 
face="Times New Roman" size=5><SPAN style="mso-tab-count: 1">&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=宋體><FONT size=+0><FONT size=3><FONT 
face="Times New Roman TUR">為了能夠保證一個被中斷的Task能夠在ISR運行結束后繼續運行，除了Aborts之外的其他Exceptions，會保證在引起異常的精確指令位置來觸發這個Exception。而所有的Interrupts則保證在一個指令結束后才會被觸發。也就是說Interrupts/Exceptions都不會破壞指令的原子性。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></P>
<P class=MsoNormal