📄 booting-protecte_mode_7.htm
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lang=EN-US># 这是存放IDTR所需的两部分内容的位置</SPAN></P>
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lang=EN-US>idt_48:<BR> .word 0
# idt limit = 0<BR> .word 0, 0 # idt base =
0L</SPAN></P>
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lang=EN-US># 对于IDTR的处理，只需要这一条指令即可</SPAN></P>
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lang=EN-US> lidt
idt_48 # load idt with 0,0</SPAN></P>
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lang=EN-US>#</SPAN></P><SPAN lang=EN-US>
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lang=EN-US># 通过设置8259A PIC，屏蔽所有可屏蔽中断</SPAN></P>
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lang=EN-US>#</SPAN></P>
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lang=EN-US> movb $0xFF,
%al # mask all interrupts for
now<BR> outb %al,
$0xA1<BR> call
delay<BR> <BR> movb $0xFB, %al # mask all
irq's but irq2 which<BR> outb %al,
$0x21 # is cascaded</SPAN></P>
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lang=EN-US># 保证所有的协处理都被正确的Reset</SPAN></P></SPAN>
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lang=EN-US> xorw
%ax, %ax<BR> outb %al,
$0xf0<BR> call delay</SPAN></P>
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lang=EN-US> outb
%al, $0xf1<BR> call
delay</SPAN></P>
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lang=EN-US># Delay is needed after doing I/O</SPAN></P>
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lang=EN-US>delay:<BR> outb %al,$0x80<BR> ret<BR></P></SPAN>
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lang=EN-US><FONT face="Times New Roman" size=5><STRONG>1.7.5 Let's
Go!</STRONG></FONT></SPAN></P><SPAN lang=EN-US><FONT face="Times New Roman"
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lang=EN-US>好了，一切准备就绪，点火:)</SPAN></P>
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lang=EN-US>进入Protected Mode，还是进入Real
Mode，完全靠CR0寄存器的PE标志位来控制：如果PE=1，则CPU切换到PM，否则，则进入RM。</SPAN></P>
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align=center><SPAN lang=EN-US><IMG src="booting-protecte_mode_7.files/cr0.gif"
tppabs="http://pagoda-ooos.51.net/os_book/booting/protected-mode/cr0.GIF"></SPAN></P>
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align=left><SPAN lang=EN-US>设置CR0-PE位的方法有两种：</SPAN></P>
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lang=EN-US>第一种是80286所使用的LMSW指令，后来的80386及更高型号的CPU为了保持向后兼容，都保留了这个指令。这个指令只能影响最低的4
bit，即PE，MP，EM和TS，对其它的没有影响。</SPAN></P>
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align=left><SPAN lang=EN-US>#通过LMSW指令进入Protected Mode</SPAN></P>
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align=left><SPAN lang=EN-US>#</SPAN></P>
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align=left><SPAN lang=EN-US> movw $1,
%ax # protected mode (PE) bit</SPAN></P>
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align=left><SPAN lang=EN-US> lmsw
%ax # This is it!</SPAN></P>
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lang=EN-US>第二种是Intel所建议的在80386以后的CPU上使用的进入PM的方式，即通过MOV指令。MOV指令可以设置CR0寄存器的所有域的值。</SPAN></P>
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align=left><SPAN lang=EN-US>#</SPAN></P>
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align=left><SPAN lang=EN-US>#通过MOV指令进入Protected Mode</SPAN></P>
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align=left><SPAN lang=EN-US>#</SPAN></P>
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align=left><SPAN lang=EN-US> movl %cr0,
%eax</SPAN></P>
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align=left><SPAN lang=EN-US> xorb $1,
%al #
set PE = 1</SPAN></P>
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align=left><SPAN lang=EN-US> movl %eax,
%cr0 # go!!</SPAN></P>
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