📄 booting-a20_4.htm
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<H3
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 0cm; mso-char-indent-count: 0; mso-char-indent-size: 0cm"><FONT
size=5><FONT face="Times New Roman"><SPAN lang=EN-US
style="FONT-SIZE: 14pt; mso-bidi-font-size: 12.0pt"><FONT size=5>1.4.2 A20
Gate in Protected Mode</FONT></SPAN></FONT></FONT></H3>
<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'"></SPAN> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><FONT
face="Times New Roman TUR"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">从</SPAN><SPAN
lang=EN-US>80286</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">开始，系统出现了一种新的机制，被称为保护模式。到了</SPAN><SPAN
lang=EN-US>80386</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，保护模式得到了进一步的完善和发展，并且对于</SPAN><SPAN
lang=EN-US>80386</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">以后的芯片，保护模式的变化就非常小了。</SPAN></FONT></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"></FONT></SPAN> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><FONT
face="Times New Roman TUR"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">我们在上一节已经谈到，如果要访问更多的内存，则必须进入保护模式，那么，在保护模式下，</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">对于内存访问有什么影响呢？</SPAN></FONT></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"></FONT></SPAN> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><FONT
face="Times New Roman TUR"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为了搞清楚这一点，我们先来看一看</SPAN><SPAN
lang=EN-US>A20</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的工作原理。</SPAN><SPAN
lang=EN-US>A20</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，从它的名字就可以看出来，其实它就是对于</SPAN><SPAN
lang=EN-US>20-bit</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">（从</SPAN><SPAN
lang=EN-US>0</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">开始数）的特殊处理</SPAN><SPAN
lang=EN-US>(</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">也就是对第</SPAN><SPAN
lang=EN-US>21</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">根地址线的处理</SPAN><SPAN
lang=EN-US>)</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。如果</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">被禁止，对于</SPAN><SPAN
lang=EN-US>80286</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">来说，其地址为</SPAN><SPAN
lang=EN-US>24bit</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，其地址表示为</SPAN><SPAN
lang=EN-US>EFFFFF</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">；对于</SPAN><SPAN
lang=EN-US>80386</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">极其随后的</SPAN><SPAN
lang=EN-US>32-bit</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">芯片来说，其地址表示为</SPAN><SPAN
lang=EN-US>FFEFFFFF</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。这种表示的意思是如果</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">被禁止，则其第</SPAN><SPAN
lang=EN-US>20-bit</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在</SPAN><SPAN
lang=EN-US>CPU</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">做地址访问的时候是无效的，永远只能被作为</SPAN><SPAN
lang=EN-US>0</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">；如果</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">被打开，则其第</SPAN><SPAN
lang=EN-US>20-bit</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">是有效的，其值既可以是</SPAN><SPAN
lang=EN-US>0</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，又可以是</SPAN><SPAN
lang=EN-US>1</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。</SPAN></FONT></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'"></SPAN> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt" align=center><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt" align=center><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><IMG
height=139 alt="" hspace=0 src="booting-a20_4.files/a20_1.jpg" width=423
border=2
tppabs="http://pagoda-ooos.51.net/os_book/booting/a20/a20_1.jpg"></SPAN></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><FONT
face="Times New Roman TUR"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"></SPAN></FONT> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><FONT
face="Times New Roman TUR"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">所以，在保护模式下，如果</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">被禁止，则可以访问的内存只能是奇数</SPAN><SPAN
lang=EN-US>1M</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">段，即</SPAN><SPAN
lang=EN-US>1M,3M,5M…</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，也就是</SPAN><SPAN
lang=EN-US>00000-FFFFF, 200000-2FFFFF,300000-3FFFFF…</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。如果</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">被打开，则可以访问的内存则是连续的。</SPAN></FONT></P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt" align=center><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"></SPAN></P><SPAN
lang=EN-US style="FONT-SIZE: 14pt; mso-bidi-font-size: 12.0pt"><FONT
face="Times New Roman">
<H3 align=center><IMG height=251 alt="" hspace=0
src="booting-a20_4.files/a20_2.jpg" width=246 border=2
tppabs="http://pagoda-ooos.51.net/os_book/booting/a20/a20_2.jpg"></H3>
<H3
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<HR width="100%" SIZE=2>
</H3>
<P
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 0cm; mso-char-indent-count: 0; mso-char-indent-size: 0cm"> </P></FONT></SPAN>
<H3
style="MARGIN: 0cm 0cm 0pt; TEXT-INDENT: 0cm; mso-char-indent-count: 0; mso-char-indent-size: 0cm"><FONT
face="Times New Roman"><SPAN lang=EN-US
style="FONT-SIZE: 14pt; mso-bidi-font-size: 12.0pt"><FONT size=5>1.4.3 How to
Enable </FONT></SPAN><SPAN lang=EN-US
style="FONT-SIZE: 14pt; mso-bidi-font-size: 12.0pt"><FONT size=5>A20
Gate?<o:p></o:p></FONT></SPAN></FONT></H3>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><SPAN lang=EN-US><FONT
face="Times New Roman"></FONT></SPAN> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><FONT
face="Times New Roman TUR"><SPAN lang=EN-US>多数</SPAN><SPAN
lang=EN-US>PC</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">都使用键盘控制器（</SPAN><SPAN
lang=EN-US>8042</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">芯片）来处理</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。</SPAN></FONT></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"></FONT></SPAN> </P>
<P class=MsoNormal style="MARGIN: 0cm 0cm 0pt"><FONT
face="Times New Roman TUR"><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">从理论上讲，打开</SPAN><SPAN
lang=EN-US>A20 Gate</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的方法是通过设置</SPAN><SPAN
lang=EN-US>8042</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">芯片输出端口（</SPAN><SPAN
lang=EN-US>64h</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）的</SPAN><SPAN
lang=EN-US>2nd-bit</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，但事实上，当你向</SPAN><SPAN
lang=EN-US>8042</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">芯片输出端口进行写操作的时候，在键盘缓冲区中，或许还有别的数据尚未处理，因此你必须首先处理这些数据。</SPAN></FONT></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"></FONT></SPAN> </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">流程如下：</FONT></SPAN></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l2 level1 lfo9"><FONT
face="Times New Roman TUR"><SPAN lang=EN-US>1.<SPAN
style="FONT: 7pt 'Times New Roman'"><FONT
size=3> </FONT></SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">禁止中断；</SPAN></FONT></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l2 level1 lfo9"><FONT
face="Times New Roman TUR"><SPAN lang=EN-US>2.<SPAN
style="FONT: 7pt 'Times New Roman'"><FONT
size=3> </FONT></SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">等待，直到<FONT
face="Times New Roman TUR">8042 Input buffer</FONT>为空为止；</SPAN></FONT></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l2 level1 lfo9"><FONT
face="Times New Roman TUR"><SPAN lang=EN-US>3.<SPAN
style="FONT: 7pt 'Times New Roman'"><FONT
size=3> </FONT></SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">发送禁止键盘操作命令到<FONT
face="Times New Roman TUR">8042 Input buffer</FONT>；</SPAN></FONT></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l2 level1 lfo9"><FONT
face="Times New Roman TUR"><SPAN lang=EN-US>4.<SPAN
style="FONT: 7pt 'Times New Roman'"><FONT
size=3> </FONT></SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">等待，直到<FONT
face="Times New Roman TUR">8042 Input buffer</FONT>为空为止；</SPAN></FONT></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l2 level1 lfo9"><FONT
face="Times New Roman TUR"><SPAN lang=EN-US>5.<SPAN
style="FONT: 7pt 'Times New Roman'"><FONT
size=3> </FONT></SPAN></SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">发送读取<FONT
face="Times New Roman TUR">8042 Output Port</FONT>命令；</SPAN></FONT></P>
<P class=MsoNormal
style="MARGIN: 0cm 0cm 0pt 21pt; TEXT-INDENT: -21pt; tab-stops: list 21.0pt; mso-list: l2 level1 lfo9"><FONT
face="Times New Roman TUR"><SPAN lang=EN-US>6.<SPAN
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