?? interrupt-handling_4.htm
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<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><SPAN lang=EN-US><FONT
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Handling & Error Code</STRONG></SPAN></FONT></SPAN></P><SPAN
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face="Times New Roman TUR" size=5><STRONG>2.4.1 Overview</STRONG></FONT></P>
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style="mso-tab-count: 1"><FONT face=宋體><FONT size=+0><FONT size=3><FONT
face="Times New Roman TUR">當一個Interrupt/Exception發生時，CPU將以這個Interrupt/Exception的Vector
Number為索引，到IDT中查找相應的Gate Descriptor。IDT中的Gate Descriptor有3種：Interrupt Gate
Descriptor，Trap Gate Descriptor，Task Gate Descriptor。根據被設置的Gate
Descriptor的不同，對ISR的調用方法和處理過程也不同。</FONT></FONT></FONT></FONT></SPAN></FONT></SPAN></P>
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lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN
style="mso-tab-count: 1">當某些Exceptions發生時，會返回Error
Code，以通知相應的ISR是那個Segment引起了這個Exception。</SPAN></FONT></SPAN></P>
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face="Times New Roman TUR"><STRONG>2.4.2 ISR Handling</STRONG></FONT></P>
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style="mso-tab-count: 1"><FONT face=宋體><FONT size=+0><FONT size=3><FONT
face="Times New Roman TUR">一個ISR(Interrupt Service
Routine)是一塊專門用來處理某個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> </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">如果一個Interrupt/Exception引用的是一個Interrupt Gate或Trap
Gate，則當這個Interrupt/Exception發生時，不發生任務切換，其ISR仍然引用當前任務的Context。Interrupt/Trap Gate
Descriptor中的Segment
Selector引用的是一個放在GDT或當前LDT中的可執行代碼段。Offset域引用的是其ISR的入口地址。</SPAN></FONT></SPAN></P>
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align=center><SPAN lang=EN-US><FONT face="Times New Roman TUR" size=3><SPAN
style="mso-tab-count: 1"></SPAN></FONT></SPAN></P>
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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">當CPU調用一個ISR時，會將EFLAGS，CS，EIP寄存器壓棧，其中CS，EIP寄存器的內容事實上是ISR的返回地址。如果一個Exception發生時，存在一個Error
Code，那么這個Error Code會在EIP之后被壓棧。</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> </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和被打斷的任務具有相同的特權等級，則CPU不改變Stack，讓ISR繼續使用當前的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> </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的特權等級從數字上小于被打斷任務的特權等級，也就是說如果ISR的權限大于被打斷任務的權限，則會發生Stack
Switch。為了能夠在ISR結束之后，能夠恢復使用原來的Stack，CPU會將SS和ESP寄存器的內容也壓棧，并且這些內容都被壓在新的Stack中，而不是Stack
Switch之前的那個Stack。</SPAN></FONT></SPAN></P>
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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">問題是，CPU在Interrupt/Exception發生的時刻，發現特權等級不一致的時候，如何知道該使用那個棧？答案是通過當前任務的TSS。在TSS中存放著針對0，1，2三個特權等級的SS和ESP內容，當特權等級切換到某個特權等級的時候，CPU會從當前任務的TSS中找到相應特權等級的SS和ESP用來進行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> </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">當ISR執行結束之后，需要繼續執行之前被終止的任務，為了能夠正確的做到這一點，ISR的返回必須使用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指令會自動從Stack中恢復EFLAGS寄存器的值，并根據Stack中的CS，EIP內容挑轉到被中斷的位置。</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發生的時候，進行了Stack
Switch，此時IRET會根據棧中的SS，ESP的內容將Stack切換回去。</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> </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">當通過一個Interrupt Gate或Trap
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TF flag。清除TF
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