📄 booting-protecte_mode_7.htm
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# base address = 0<BR> .word 0x9200 #
data read/write<BR> .word 0x00CF # granularity
= 4096,
386<BR> #
(+5th nibble of limit)</SPAN></P>
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lang=EN-US><FONT face="Times New Roman" size=5><STRONG>1.7.3 Load
GDT</STRONG></FONT></SPAN></P><SPAN lang=EN-US><FONT face="Times New Roman"
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lang=EN-US>设置好GDT之后，我们需要通过LGDT指令将设定的gdt的入口地址和gdt表的大小装入GDTR寄存器。</SPAN></P>
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lang=EN-US>GDTR寄存器包括两部分：32-bit的线性基地址，以及16-bit的GDT大小（以字节为单位）。需要注意的是，对于32-bit线性基地址，必须是32-bit绝对物理地址，而不是相对于某个段的偏移量。而我们在Booting阶段，在进入Protected
Mode之前，我们CS和DS设置很可能不是0，所以我们必须计算出gdt的绝对物理地址。</SPAN></P>
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lang=EN-US>为了执行LGDT指令，你需要把这两部分内容放在内存的某个位置，然后将这个位置的内存地址作为操作数传递给LGDT指令。然后LGDT指令会自动将保存在这个位置的这两部分值装入GDTR寄存器。</SPAN></P>
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lang=EN-US>#</SPAN></P>
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lang=EN-US># 这是存放GDTR所需的两部分内容的位置</SPAN></P>
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lang=EN-US>#</SPAN></P>
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lang=EN-US>gdt_48:<BR> .word 0x8000 # gdt
limit=2048,<BR>
# 256 GDT entries</SPAN></P>
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lang=EN-US> .word 0,
0 # gdt base (filled in
later)</SPAN></P>
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lang=EN-US>#</SPAN></P>
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lang=EN-US># 下面这段代码用来计算GDT的32-bit线性地址，并将其装入GDTR寄存器。</SPAN></P>
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lang=EN-US>#</SPAN></P>
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xorl %eax, %eax # Compute
gdt_base<BR> movw %ds, %ax #
(Convert %ds:gdt to a linear
ptr)<BR> shll $4,
%eax<BR> addl $gdt,
%eax<BR> movl %eax,
(gdt_48+2)<BR> lgdt
gdt_48 # load gdt with whatever is appropriate</SPAN></P>
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lang=EN-US><FONT face="Times New Roman"
size=5><STRONG>1.7.4 其它准备</STRONG></FONT></SPAN></P><SPAN lang=EN-US><FONT
face="Times New Roman" size=3>
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lang=EN-US>在进入Protected Mode之前，除了需要设置和装入GDT之外，还需要做如下一些事情：</SPAN></P>
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lang=EN-US>1、屏蔽所有可屏蔽中断；</SPAN></P>
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lang=EN-US>2、装入IDTR；</SPAN></P>
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lang=EN-US>3、所有协处理器被正确的Reset。</SPAN></P>
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lang=EN-US>由于在Real Mode和Protected Mode下的中断处理机制有一些不同，所以在进入Protected
Mode之前，务必禁止所有可屏蔽中断，这可以通过下面两种方法之一：</SPAN></P>
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lang=EN-US></SPAN> </P>
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lang=EN-US>1、使用CLI指令；</SPAN></P>
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lang=EN-US>2、对8259A可编程中断控制器编程以屏蔽所有中断。</SPAN></P>
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lang=EN-US></SPAN> </P>
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lang=EN-US>即使当我们进入Protected Mode之后，也不能马上将中断打开，这时因为我们必须在OS Kernel中对相关的Protected
Mode中断处理所需的数据结构正确的初始化之后，才能打开中断，否则会产生处理器异常。</SPAN></P>
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lang=EN-US></SPAN> </P>
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lang=EN-US>在Real Mode下，中断处理使用IVT(Interrupt Vector Table)，在Protected
Mode下，中断处理使用IDT（Interrupt Descriptor Table），所以，我们必须在进入Protected
Mode之前设置IDTR。</SPAN></P>
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lang=EN-US></SPAN> </P>
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lang=EN-US>IDTR的格式和GDTR相同，IDTR的装入方式和GDTR也相同。由于IDT中相关的中断处理程序需要让OS
Kernel来设定，所以在Booting阶段，我们只需要将IDTR中IDT的基地址和Size都设为0就可以了，随后，等进入Protected
Mode之后，由OS Kernel来真正设置它。</SPAN></P>
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lang=EN-US></SPAN> </P>
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lang=EN-US>关于中断机制和中断处理，请参考 <A
href="interrupt_and_exception.htm"
tppabs="http://pagoda-ooos.51.net/os_book/interrupt_and_exception.htm">Interrupt
& Exception</A> ，这里不再赘述。</SPAN></P>
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lang=EN-US></SPAN> </P><SPAN lang=EN-US>
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