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face=System>&nbsp;&nbsp; cli</FONT></P>
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face=System>&nbsp;&nbsp; ret</FONT></P>
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face=System></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=System>wait_input_empty:</FONT></P>
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face=System>rp1: inb&nbsp; &nbsp;&nbsp;$0x64</FONT></P>
<P class=MsoNormal 
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face=System>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; testb&nbsp; %al, 
0x02</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=System>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; jnz&nbsp;&nbsp; &nbsp; 
rp1</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=System>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ret</FONT></P>
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face=System></FONT>&nbsp;</P>
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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=System>wait_output_full:</FONT></P>
<P class=MsoNormal 
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face=System>rp2:&nbsp; inb&nbsp;&nbsp; $0x64</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=System>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; testb %al, 
0x01</FONT></P>
<P class=MsoNormal 
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face=System>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; jz&nbsp;&nbsp; 
&nbsp;&nbsp; rp2</FONT></P>
<P class=MsoNormal 
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align=left><FONT face=System>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
ret</FONT></P>
<P class=MsoNormal 
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size=2></FONT>&nbsp;</P>
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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">以上描述的是一种和IBM PC完全兼容的，通过键盘控制器控制A20 
Gate的方法。但是，正象我们在前面所提到的，A20 Gate与键盘操作完全没有关系，IBM之所以将A20 
Gate的功能控制放在键盘控制器上，完全是一种为了弥补Intel 80286与Intel 
8086/8088不完全兼容的缺陷，而采取的Hacker行为，所以在许多新型PC上存在着一种通过芯片来直接控制A20 Gate的BIOS功能，我们在Real 
Mode下只需要调用BIOS中断就可以实现A20 Gate的控制功能。</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"></FONT>&nbsp;</P>
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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">这个BIOS中断为 INT 15h, AX=2401h。被称为Fast A20。</FONT></P>
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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"></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=System>movw&nbsp;$0x2401, %ax</FONT></P>
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face=System>int&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; $0x15</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"></FONT>&nbsp;</P>
<HR width="100%" SIZE=2>

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face="Times New Roman Greek" size=4><STRONG><FONT size=5>1.4.4 How to Detect if 
A20 Gate&nbsp;has been&nbsp;Enabled?</FONT></STRONG></FONT></P>
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style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l5 level1 lfo3">&nbsp;</P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face="Times New Roman TUR">我们在之前已经提到，如果A20 Gate被打开了，则在实模式下，程序员可以直接访问<SPAN 
lang=EN-US>100000H~10FFEFH</SPAN><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">之间的内存，如果A20 
Gate被禁止，则在实模式下，若程序员访问<SPAN lang=EN-US>100000H~10FFEFH</SPAN><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">之间的内存，则会被硬件自动转换为0H~0FFEFH之间的内存，所以我们可以利用这个差异来检测A20 
Gate是否被打开。</SPAN></SPAN></FONT></SPAN></P><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">
<P class=MsoNormal 
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face="" size=2></FONT></SPAN>&nbsp;</P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System># This routine tests whether or not A20 is enabled.&nbsp; If so, 
it</FONT></SPAN></P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System># exits with zf = 0.</FONT></SPAN></P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System>#</FONT></SPAN></P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System># The memory address used, 0x200, is the int $0x80 vector, 
which</FONT></SPAN></P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System># should be safe.</FONT></SPAN></P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System></FONT></SPAN>&nbsp;</P>
<P class=MsoNormal 
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System>A20_TEST_ADDR = 4*0x80</FONT></SPAN></P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System>A20_TEST_LOOPS = 3</FONT></SPAN></P>
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align=left><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System></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 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System>a20_test:<BR>&nbsp;pushw&nbsp; %cx<BR>&nbsp;pushw&nbsp; 
%ax<BR>&nbsp;xorw&nbsp;&nbsp;&nbsp;&nbsp; %cx, %cx<BR>&nbsp;movw&nbsp;&nbsp; 
%cx, 
%fs&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
# Low memory<BR>&nbsp;decw&nbsp;&nbsp;&nbsp; %cx<BR>&nbsp;movw&nbsp;&nbsp; %cx, 
%gs&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
# High memory area<BR>&nbsp;movw&nbsp;&nbsp; $A20_TEST_LOOPS, 
%cx<BR>&nbsp;movw&nbsp;&nbsp; %fs:(A20_TEST_ADDR), %ax<BR>&nbsp;pushw&nbsp; 
%ax</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 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System></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 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System>a20_test_wait:<BR>&nbsp;incw&nbsp;&nbsp;&nbsp;&nbsp; 
%ax<BR>&nbsp;movw&nbsp;&nbsp; %ax, 
%fs:(A20_TEST_ADDR)<BR>&nbsp;call&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
delay&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 
# Serialize and make delay constant<BR>&nbsp;cmpw&nbsp;&nbsp; 
%gs:(A20_TEST_ADDR+0x10), %ax<BR>&nbsp;loope&nbsp;&nbsp; 
a20_test_wait</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 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"></SPAN><SPAN 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; popw&nbsp;&nbsp;&nbsp; 
%fs:(A20_TEST_ADDR)<BR>&nbsp;popw&nbsp;&nbsp;&nbsp; 
%ax<BR>&nbsp;popw&nbsp;&nbsp;&nbsp; %cx<BR>&nbsp;ret</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 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System></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 
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><FONT 
face=System>delay:<BR>&nbsp;outb&nbsp;&nbsp;&nbsp;&nbsp; 
%al,$0x80<BR>&nbsp;ret&nbsp;</FONT></SPAN></P></SPAN></SPAN></SPAN></FONT></SPAN></BODY></HTML>
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