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3.1 简介
在没有虚存机制的计算机中，程序指令所访问的内存地址就是物理内存地址。而在有虚存机制并且该机制打开的情况下，程序指令所访问的内存地址都是虚拟内存地址。某个虚拟内存地址可以通过设置被映射到任何物理内存地址，如图 3-1 所示。
图3-1 虚拟内存可被映射到任意物理地址
程序运行时，虚存地址到物理地址的转换工作是由专门的硬件完成的，它叫做内存管理单元（Memory Management Unit，MMU）。转换过程如图3-2所示。
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图3-2 虚拟地址转换成物理地址的过程
从图3-2中可以看出，当CPU访问某虚存地址时，不管是读指令或读写数据，该地址首先会被发送给内存管理单元进行转换。在获得转换后的物理地址后，CPU再对该物理地址进行读写操作。
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3.2 虚拟内存的优点
3.2.1 安全性
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3.2.2 提高空间利用率
利用虚存机制还可以充分利用物理内存空间Normal 0 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;}
3.2.3 多进程的支持
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3.3 PDP11/40的虚拟内存机制
PDP 11/40的内存管理单元把物理内存等分成很多大小为64字节的连续块（block），并从0开始顺序编号，如图3-7所示：
尽管PDP 11/40的操作模式都是16位模式，但它的CPU和总线都支持18位地址模式，所以其最大地址空间实际可达256K，4096个物理块。
PDP 11/40的内存管理单元为用户模式和内核模式分别提供了8组、每组两个内存映射寄存器：
8个16位页地址寄存器PAR（Page Address Register）：PAR0～7
8个16位页描述寄存器 PDR（Page Descriptor Register）：PDR0～7
这样，实际上系统中一共有8×2×2=32个内存映射寄存器。每对PAR/PDR可称为一个
32 位的 APR （活动页寄存器 —— Active Page Register ）， APR 在物理上并不存在，它是为了理解和叙述上的方便而引入的。活动页寄存器示意图如图 3-8 所示。
内存管理单元对虚拟内存按页管理，它把虚拟内存分成很多活动页，每一个活动页最大不超过8K，由一个32位的活动页寄存器来描述它映射到物理内存的情况。因此64K的虚拟内存需要8个活动页寄存器才能完成映射。虚拟内存与活动页寄存器对应的情况如图3-9 所示。
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图3-9 虚拟内存与活动页寄存器对应图
3.3.1 页地址寄存器（PAR）
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页地址寄存器是32位活动页寄存器的第一个字（word），其位11～0共12位是页地址字段PAF（Page Address Field），用于指定物理内存的块号，并以该块地址作为本虚拟页的起始地址。例如：PAF = 100，则本虚拟页从100号物理块开始，起始物理地址是100 × 64 = 6400=0x1900。
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页描述寄存器是32位活动页寄存器的第二个字，它用来描述本页的一些属性，比如访问权限和大小等。
（ 1 ）位 2 ～ 1 ：访问控制字段（ Access Control Field ），用于设定页的访问权限。
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| 值 |
模 式 |
备 注 |
| 00 |
不存在 |
任何形式的访问都会导致异常退出 |
| 01 |
只读 |
只允许读操作，写操作会导致异常退出 |
| 10 |
未使用 |
任何形式的访问都会导致异常退出 |
| 11 |
读写 |
读写操作都允许 |
如果程序指令对页实行非该字段允许的操作，则会导致产生自陷而退出（abort）。
（2）位3：方向延续字段（Expansion Direction），该字段指定页内存空间的延续方向。
ED=0：空间向上延续（upwards）。页地址寄存器中的PAF字段指定的物理块是该页的最低地址块，并且地址空间向高地址延续。
ED=1：空间向下延续（downwards）。页地址寄存器中的PAF字段指定的物理块并不是该页的最低地址块，而是它的一个基线，地址空间从PAF+128处向低地址延续。
（3）6：修改标志位（Written Info）。表示该页是否被修改过。这和现代虚拟内存页中的脏页标志（dirty flag）非常相似。主要用于指导在进程换出操作时，该页是否需要被换出。如果该位被置（W=1），表明该页在程序运行期间被修改过，则需要换出（写入磁盘）以防数据丢失，否则该页不需要换出。但UNIX系统在进行换出操作时，并没有判断该位，即不管该页是否被修改都换出。
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