📄 chapter6.htm
字号:
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">以形成一个独一无二的页首地址。</SPAN></P>
<P class=MsoNormal
style="MARGIN-LEFT: 63pt; TEXT-INDENT: -21pt; mso-list: l13 level1 lfo4; tab-stops: list 63.0pt"><SPAN
style="FONT-FAMILY: Wingdings">n<SPAN
style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">我们在</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中查找是否有一个本页的转译项在里面。如果有，那么我们将得到对应的物理地址的高位，最终得到可用地址。</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">是一个做特殊用途的存储器件，可以运用各种有效的方法来匹配地址。它可以参考一个全局的标志位来在查找某些入口时忽略</SPAN>ASID<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">位，因此这些</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">入口可以用来映射所有进程中的某一段共享的虚地址空间。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 63pt"><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">类似的，</SPAN>VPN<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">也可以在存储的时候使用某些掩码位，使</SPAN>VPN<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中某些位在匹配过程中被排除在外，这使得</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">入口能够映射更大范围的虚地址。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 63pt"><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在某些</SPAN>MIPS
MMU<SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中这两种特殊机制均被采纳。</SPAN></P>
<P class=MsoNormal
style="MARGIN-LEFT: 63pt; TEXT-INDENT: -21pt; mso-list: l18 level1 lfo6; tab-stops: list 63.0pt"><SPAN
style="FONT-FAMILY: Wingdings">n<SPAN
style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">通常在</SPAN>PFN<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中还存储了一些额外的位信息（</SPAN>flags<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）以用于控制哪些访问可以被允许——最明显的，允许读操作而不允许写操作。我们会在</SPAN>6.2<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">节中讨论</SPAN>MIPS<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">体系结构的标志位。</SPAN></P>
<P class=MsoNormal
style="MARGIN-LEFT: 63pt; TEXT-INDENT: -21pt; mso-list: l18 level1 lfo6; tab-stops: list 63.0pt"><SPAN
style="FONT-FAMILY: Wingdings">n<SPAN
style="FONT: 7pt 'Times New Roman'">
</SPAN></SPAN><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">如果在</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">中入口匹配失败，那么系统必须定位或者分配一个适当的入口（使用常驻内存页表的相应信息）并将其装入</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，然后再次进行一次转译过程</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 42pt"> <O:P> </O:P></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt; TEXT-INDENT: 21pt"><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">在</SPAN>VAX<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">小型机中，这个过程是被微代码（</SPAN>microcode<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）所控制的，对程序员而言整个过程完全是自动进行的。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt; TEXT-INDENT: 21pt"> <O:P>
</O:P></P>
<P class=MsoNormal><SPAN
style="FONT-SIZE: 14pt; mso-bidi-font-size: 12.0pt">6.1.5 MIPS</SPAN><SPAN
lang=ZH-CN
style="FONT-SIZE: 14pt; FONT-FAMILY: SimSun; mso-bidi-font-size: 12.0pt; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">如此设计的起源</SPAN><SPAN
style="FONT-SIZE: 14pt; mso-bidi-font-size: 12.0pt"><O:P> </O:P></SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">为了在尽可能少使用硬件的前提下提供一套与</SPAN>VAX<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">相同的功能，</SPAN>MIPS<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的设计者们需要找些好办法。由微代码控制的</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">重装入（</SPAN><B
style="mso-bidi-font-weight: normal">refill</B><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）是不能接受的，因此他们勇敢的迈出了一步：把这个工作交给软件来完成。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
style="mso-tab-count: 1"> </SPAN><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">这意味着除了有一个寄存器用来存放当前的</SPAN>ASID<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">，</SPAN>MMU<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">器件仅仅是个</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">而已，也就是一个简单的高速、定长的转译表。系统软件可以（通常也就是如此）把</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">作为一个快存来面向常驻内存的页表，然而</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">硬件本身并不能把自己当作快存来使用，而只能这样：当某一个地址无法进行转译时，</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">会触发一个特殊的异常（</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">重装入异常）来引发调用软件程序。不过，</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的细节设计和相应的控制寄存器上都作了十分周密的考虑，以帮助软件更加富有效率的运行。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"> <O:P> </O:P></P>
<P class=MsoNormal><B><SPAN
style="FONT-SIZE: 15pt; mso-bidi-font-size: 12.0pt">6.2 MIPS TLB</SPAN><SPAN
lang=ZH-CN
style="FONT-SIZE: 15pt; FONT-FAMILY: SimSun; mso-bidi-font-size: 12.0pt; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">的特点</SPAN></B></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt">MIPS TLB<SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">通常都是在芯片上实现的：即使在快存命中的情形下，内存转译也这一步仍然必须进行，因此在机器上这是一个十分重要的“关键路径”（</SPAN>critical
path<SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）。这意味着它必须很小，尤其在早期那个年代，因此，把它的规模控制的很小就十分明智。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
style="mso-tab-count: 1"> </SPAN><SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">基本上这是块全相连的存储单元。每个入口都是一块拥有键值（</SPAN>key<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">）域和数据域的关联存储器；当你提供某个键值后，由硬件来进行匹配并给出匹配成功的入口内的数据。通常全相连存储器效率很高但在硬件上过于浪费，而</SPAN>MIPS<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">系列的</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">含有</SPAN>32<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">到</SPAN>64<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">个入口不等；这种规模的存储量在芯片设计中是相对容易处理的。</SPAN></P>
<P class=MsoNormal style="MARGIN-LEFT: 21pt"><SPAN
style="mso-tab-count: 1"> </SPAN>R4000<SPAN lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">风格的</SPAN>CPU<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">至今都在使用这样一种</SPAN>TLB<SPAN
lang=ZH-CN
style="FONT-FAMILY: SimSun; mso-ascii-font-family: 'Times New Roman'; mso-hansi-font-family: 'Times New Roman'">：每个入口内容被扩大为</SPAN>2<SPAN
lang=ZH-CN
⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -