操作系统引导探究.htm

操作系统引导探究

style="TEXT-INDENT: 42.75pt; mso-char-indent-count: 4.07"><SPAN lang=EN   
style="COLOR: blue">DL</SPAN><SPAN   
style="COLOR: blue; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">寄存器：存驱动器号，表示欲读哪一个驱动器</SPAN><SPAN   
lang=EN style="COLOR: blue"><o:p></o:p></SPAN></P>  
<P class=MsoNormal   
style="TEXT-INDENT: 42.75pt; mso-char-indent-count: 4.07"><SPAN lang=EN   
style="COLOR: blue">CH</SPAN><SPAN   
style="COLOR: blue; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">寄存器：存磁头号，表示欲读哪一个磁头</SPAN><SPAN   
lang=EN style="COLOR: blue"><o:p></o:p></SPAN></P>  
<P class=MsoNormal   
style="TEXT-INDENT: 42.75pt; mso-char-indent-count: 4.07"><SPAN lang=EN   
style="COLOR: blue">CL</SPAN><SPAN   
style="COLOR: blue; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">寄存器：存扇区号，表示欲读的启始扇区</SPAN><SPAN   
lang=EN style="COLOR: blue"><o:p></o:p></SPAN></P>  
<P class=MsoNormal   
style="TEXT-INDENT: 42.75pt; mso-char-indent-count: 4.07"><SPAN lang=EN   
style="COLOR: blue">AL</SPAN><SPAN   
style="COLOR: blue; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">寄存器：存计数值，表示欲读入的扇区数量</SPAN><SPAN   
lang=EN style="COLOR: blue"><o:p></o:p></SPAN></P>  
<P class=MsoNormal style="TEXT-INDENT: 21pt; mso-char-indent-count: 2.0"><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在设置了这几个寄存器后，我们就可以使用</SPAN>   
<SPAN class=SpellE><SPAN lang=EN style="COLOR: blue">int</SPAN></SPAN><SPAN   
lang=EN style="COLOR: blue"> 13</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">这条指令调用</SPAN><SPAN   
lang=EN>BIOS 13</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">号中断读取指定的磁盘扇区，它将磁盘扇区读入</SPAN><SPAN   
lang=EN style="COLOR: blue">ES:BX</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">处，因此，在调用它之前，我们实际上还需要设置</SPAN><SPAN   
lang=EN>ES</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">与</SPAN><SPAN   
lang=EN>BX</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">寄存器，以指出数据在内存中存放的位置。</SPAN></P>  
<P class=MsoNormal><SPAN lang=EN><o:p>&nbsp;</o:p></SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><B   
style="mso-bidi-font-weight: normal"><SPAN   
style="COLOR: #ff6600; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">四、保护模式下段模式内存地址的访问</SPAN><SPAN   
lang=EN style="COLOR: #ff6600"><o:p></o:p></SPAN></B></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'">写程序离不开对内存的访问，然而在保护模式下内存的访问与在实模式下内存的访问完全不同，这里我们将详细描述一下保护模式下内存的访问方法。当然，这里并不打算完整的介绍保护模式下所有的内存访问方法与机制，只介绍从实模式转到保护模式下所需要进行的转换，完整的内存访问请你最好参见《</SPAN><SPAN   
lang=EN>Intel</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">用户手册》，当然，随着</SPAN><SPAN   
class=SpellE><SPAN lang=EN>pyos</SPAN></SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的实验进行，我也会在后面的实验报告与心得体会中渐渐描述，现在不描述主要原因是我还为通过</SPAN><SPAN   
class=SpellE><SPAN lang=EN>pyos</SPAN></SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">进行实验，对与没经过实验验证的东西，我并不打算也不敢妄下结论，因为在前言中已然说过，这篇文章只是我的一些心得体会，如果我没有实验过，就没有心得，也没有体会，也就无法描述了。</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'">言归正转，我们还是先来看看实模式下内存访问方法吧。</SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><B   
style="mso-bidi-font-weight: normal"><SPAN lang=EN style="COLOR: #ff6600">4.1   
</SPAN></B><B style="mso-bidi-font-weight: normal"><SPAN   
style="COLOR: #ff6600; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">实模式下的内存访问</SPAN><SPAN   
lang=EN style="COLOR: #ff6600"><o:p></o:p></SPAN></B></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'">计算机在加电时，处于“实模式”，在计算机中有一个</SPAN><SPAN   
lang=EN style="COLOR: blue">CR0</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">寄存器，又称为</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'">号控制寄存器，在这个寄存器中，最低位也即第</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'">位，被称为</SPAN><SPAN   
lang=EN style="COLOR: blue">PM</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">（</SPAN><SPAN   
lang=EN>Protected <SPAN class=SpellE>Modle</SPAN></SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：保护模式）位，当它被清零的时候表示</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'">在“实模式”下工作，当它被置位的时候，表示</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'">在“保护模式”下工作。在计算机加电的时候，它是被清零的，所在这个时候的计算机，处于“实模式”。</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'">“实模式”下的内存访问通过段寄存器与偏移量构成，比如前面描述中常常出现的</SPAN><SPAN   
lang=EN>0x:0000:0x0001</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">就是一个实模式下的内存地址。分号前面的值表时段寄存器中的值，分号后面的值表是偏移量，实际物理地址的形成如下图所示：</SPAN></P>  
<P class=MsoNormal><SPAN lang=EN></SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><IMG   
alt="" src="操作系统引导探究_files/20044621278430.gif" onload=""><BR>(图四）</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'">然而在保护模式下，内存地址却不是如上图所示的方法形成的。那么它又时怎样形成的呢？</SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><B   
style="mso-bidi-font-weight: normal"><SPAN lang=EN style="COLOR: red">4.2   
</SPAN></B><B style="mso-bidi-font-weight: normal"><SPAN   
style="COLOR: red; FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">保护模式下的内存地址形成</SPAN></B></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'">保护模式下内存地址就复杂多了，</SPAN>   
<SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">我们首先要分清三个概念：<SPAN   
style="COLOR: blue">逻辑地址</SPAN>、<SPAN style="COLOR: blue">线性地址</SPAN>与<SPAN   
style="COLOR: blue">物理地址</SPAN>。物理地址很好理解，逻辑地址也好理解，就是程序所使用的地址。那么什么是线性地址呢？</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'">其实如果不使用分页机制的话，线性地址就是物理地址，它与物理地址是一一对应的，线性地址</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'">，也就是物理地址</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'">。但我们知道，</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</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'">拥有</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'">根地址线，也就是可以访问：</SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><SPAN   
lang=EN><SUB></SUB><SPAN style="mso-spacerun: yes"><IMG alt=""   
src="操作系统引导探究_files/200446212726779.gif" onload="">&nbsp;</SPAN><SPAN   
class=GramE>=<SPAN style="mso-spacerun: yes">&nbsp; </SPAN>4GB</SPAN></SPAN></P>  
<P class=MsoNormal><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的内存空间，这实再是一个太大的空间了！现在很少有机器的物理内存能有这么大。那怎么在有限的物理空间中使用</SPAN><SPAN   
lang=EN>4GB</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的空间呢？人们把物理内存分成许多页，当某些页被使用的时候，某些页就没有被使用，而没有被使用的页就可以用来载入</SPAN><SPAN   
lang=EN>4GB</SPAN><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">上的空间，这就称为从线性地址到物理地址的映射，这是一个多对一的映射，也就是说多个线性空间中的页对应一个物理空间中的页，希望下面一幅图能有助于你理解这样的分页机制。</SPAN></P>  
<P class=MsoNormal><SPAN lang=EN></SPAN></P>  
<P class=MsoNormal style="TEXT-ALIGN: center" align=center><SPAN   
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'"><IMG   
alt="" src="操作系统引导探究_files/200446212741748.gif" onload=""><BR>（图五）</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'">上面是一种最简单的映射方式，术语称作“直接相连”映射，它大约只能用来说明问题，而在一个实际的操作系统中通常是“全相联相连”映射，也就是说线性地址中的页可以是映射到物理地址中的任何一个页中，只要那块物理地址空间现在是空闲的。不过，通过上图也能说明问题，当线性地址中的页</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'">需要被访问时，</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'">通过地址映射机制将其转换到物理地址，发现其对应物理地址中的页</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'">。于是</SPAN><SPAN