操作系统引导探究.htm

操作系统引导探究

lang=EN>CPU</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">将物理地址页</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中的内容放到硬盘上的一个地方（虚拟内存），然后将线性地址中的页</SPAN><SPAN   
lang=EN>5</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">载入物理内存页</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中。这里就当可以比较明显的区别什么是线性地址，什么是物理地址了。</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">然而，当不使用分页机制的时候，线性地址就会被</SPAN><SPAN   
lang=EN>CPU</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">当做物理地址来使用，线性地址会被直接放在</SPAN><SPAN   
lang=EN>CPU</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的地址信号线上，而在编写应用程序的时候，我们通常使用却是另一种地址——逻辑地址，从逻辑地址到线性地址也存在着与上述机制类似的一种映射机制，不过这个机制常常称为<SPAN   
style="COLOR: blue">“段模式”</SPAN>，它是由操作系统与</SPAN><SPAN lang=EN>CPU</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">硬件共同完成的。操作系统的任务就是分配映射表，而</SPAN><SPAN   
lang=EN>CPU</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">硬件的任务就是按着映射表进行映射。而这样的映射表在操作系统编写中又称之为“描述符表”，有两种重要的描述符表，一种是<SPAN   
style="COLOR: blue">“全局描述符表（</SPAN></SPAN><SPAN lang=EN   
style="COLOR: blue">GDT</SPAN><SPAN   
style="COLOR: blue; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）”</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，另一种是<SPAN   
style="COLOR: blue">“局部描述符表（</SPAN></SPAN><SPAN lang=EN   
style="COLOR: blue">LDT</SPAN><SPAN   
style="COLOR: blue; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）”</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，这两种表的用途不同，但它们的用法却是近似的，下面我们就来描述一下全局描述符表。</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">说到表，学过数据结构的人都知道，其实它就是一种数据结构，全局描述符表也是一种数据结构，当这种结构放在一块连续的内存这间中就称之为表了。表由表项组成，全局描述符表由它的表符全局描述符组成，其实单纯的术语就叫<SPAN   
style="COLOR: blue">“描述符”</SPAN>，只因为它放在全局描述符表中就成了全局描述符了。这个描述符由</SPAN><SPAN   
lang=EN>8</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">个字节组成，下面我们就来看看它的结构：</SPAN></P>  
<P class=MsoNormal><SPAN lang=EN></SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><SPAN lang=EN><IMG   
alt="" src="操作系统引导探究_files/20044621285758.gif" onload=""><BR>(</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">图六</SPAN><SPAN   
lang=EN>)</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21pt; mso-char-indent-count: 2.0"><SPAN   
lang=EN>TYPE</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：</SPAN>   
<SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">表明此段的类型，</SPAN><SPAN   
lang=EN>4</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">位中的最高位被置</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的时候表是它是数据段，相应的于下的三位，从左到右依次为</SPAN><SPAN   
lang=EN>E</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">、</SPAN><SPAN   
lang=EN>W</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">、</SPAN><SPAN   
lang=EN>A</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，即数据段的</SPAN><SPAN   
lang=EN>TYPE</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为：</SPAN><SPAN   
lang=EN>0EWA</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">。其中</SPAN><SPAN   
lang=EN>E</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">表示向下增长位，置</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">时表示向下增长，</SPAN><SPAN   
lang=EN>W</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">表示可写位，置</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">表洋可定，</SPAN><SPAN   
lang=EN>A</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">表示被访问位（如果</SPAN><SPAN   
lang=EN>CPU</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">访问了它，此位将会被置</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）。</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN lang=EN>S</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：为</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">时表示其为代码或数据段，为</SPAN><SPAN   
lang=EN>0</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">时表示为系统段</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN lang=EN>DPL</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：表示物权级，从</SPAN><SPAN   
lang=EN>00~11</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，共</SPAN><SPAN   
lang=EN>0</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，</SPAN><SPAN   
lang=EN>2</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，</SPAN><SPAN   
lang=EN>3</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">四个特权级</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN lang=EN>P</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：为</SPAN><SPAN   
lang=EN>0</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">是表示此描术符无效，不能被使用</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN lang=EN>AVL</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：留给系统程序随便用的</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN lang=EN>D/B</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：为</SPAN><SPAN   
lang=EN>0</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的时候表示它是一个</SPAN><SPAN   
lang=EN>16</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">位的段，为</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">时表示它是一个</SPAN><SPAN   
lang=EN>32</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">位的段</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN lang=EN>G</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：为</SPAN><SPAN   
lang=EN>0</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">时，表示段限的单位是</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">字节，为</SPAN><SPAN   
lang=EN>1</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">时表示段限的单位是</SPAN><SPAN   
lang=EN>4KB</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，并且段偏移量的最低</SPAN><SPAN   
lang=EN>12</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">位将不被检测是否在段限之中。（这一点现在可能不好理解，但我下面马上会解释）。</SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21.75pt"><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">这里面有两个部份，比较有意思，一个就是<SPAN   
style="COLOR: blue">“基址”</SPAN>，一个就是<SPAN   
style="COLOR: blue">“段限”</SPAN>。基址应当比较好理解，它给出的是一个段在物理内存中的地址，对于“段限”，顾名思义，就是段大小的限制。不过它有点特别，对于一个段的最大可访问的地址</SPAN><SPAN   
lang=EN>CPU</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">是通过下面的公式计算得到的：</SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><SPAN lang=EN