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Update 计算机操作系统 - 内存管理.md

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重新绘制配图。
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notes/计算机操作系统 - 内存管理.md
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## 虚拟内存
虚拟内存的目的是为了让物理内存扩充成更大的逻辑内存，从而让程序获得更多的可用内存。
虚拟内存的目的是为了让物理内存扩充成更大的逻辑内存，从而让程序获得更多的可用内存。此外，每个进程都有独立的地址空间，一个进程无法直接访问另一个进程的内存，操作系统还可以按页设置读写和执行权限。
为了更好的管理内存，操作系统将内存抽象成地址空间。每个程序拥有自己的地址空间，这个地址空间被分割成多个块，每一块称为一页。这些页被映射到物理内存，但不需要映射到连续的物理内存，也不需要所有页都必须在物理内存中。当程序引用到不在物理内存中的页时，由硬件执行必要的映射，将缺失的部分装入物理内存并重新执行失败的指令。
为了更好的管理内存，操作系统将内存抽象成地址空间。每个程序拥有自己的地址空间，这个地址空间被分割成多个块，每一块称为一页。这些页被映射到物理内存，但不需要映射到连续的物理内存，也不需要所有页都必须在物理内存中。当程序引用到不在物理内存中的页时，硬件触发缺页中断，由操作系统将缺失的页装入物理内存，再重新执行失败的指令。
从上面的描述中可以看出，虚拟内存允许程序不用将地址空间中的每一页都映射到物理内存，也就是说一个程序不需要全部调入内存就可以运行，这使得有限的内存运行大程序成为可能。例如有一台计算机可以产生 16 位地址，那么一个程序的地址空间范围是 0\~64K。该计算机只有 32KB 的物理内存，虚拟内存技术允许该计算机运行一个 64K 大小的程序。
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/7b281b1e-0595-402b-ae35-8c91084c33c1.png"/> </div><br>
<div align="center"> <img src="pics/vm-mapping.png" width="640px"/> </div><br>
## 分页系统地址映射
@@ -32,9 +32,13 @@
一个虚拟地址分成两个部分，一部分存储页面号，一部分存储偏移量。
下图的页表存放着 16 个页，这 16 个页需要用 4 个比特位来进行索引定位。例如对于虚拟地址（0010 000000000100），前 4 位是存储页面号 2，读取表项内容为（110 1），页表项最后一位表示是否存在于内存中，1 表示存在。后 12 位存储偏移量。这个页对应的页框的地址为 （110 000000000100）。
下图的页表存放着 16 个页，这 16 个页需要用 4 个比特位来进行索引定位。例如对于虚拟地址 0x3ABC（0011 101010111100），前 4 位是页面号 3，读取表项内容为（101 1），页表项最后一位表示是否存在于内存中，1 表示存在。后 12 位是偏移量，直接拼接到页框号 101 后面，得到物理地址 0x5ABC（101 101010111100）。
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/cf4386a1-58c9-4eca-a17f-e12b1e9770eb.png" width="500"/> </div><br>
<div align="center"> <img src="pics/page-translate.png" width="720px"/> </div><br>
页表存放在内存中，每次访问内存之前都要先查一次页表。CPU 中的快表（Translation Lookaside Buffer, TLB）缓存最近用过的页表项，命中时不用再访问页表。
64 位系统的地址空间很大，如果用一张页表覆盖全部地址，页表本身就会非常大，因此使用多级页表，只为用到的地址分配下一级页表。例如 x86-64 通常使用 4 级页表，48 位虚拟地址分为 4 个 9 位的页表索引和 12 位的页内偏移。
## 页面置换算法
@@ -54,7 +58,7 @@
举例：一个系统为某进程分配了三个物理块，并有如下页面引用序列：
```html
```text
7，0，1，2，0，3，0，4，2，3，0，3，2，1，2，0，1，7，0，1
```
@@ -70,11 +74,11 @@
因为每次访问都需要更新链表，因此这种方式实现的 LRU 代价很高。
```html
```text
4，7，0，7，1，0，1，2，1，2，6
```
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/eb859228-c0f2-4bce-910d-d9f76929352b.png"/> </div><br>
<div align="center"> <img src="pics/lru-stack.png" width="720px"/> </div><br>
### 3. 最近未使用
@@ -105,7 +109,7 @@ FIFO 算法可能会把经常使用的页面置换出去，为了避免这一问
当页面被访问 (读或写) 时设置该页面的 R 位为 1。需要替换的时候，检查最老页面的 R 位。如果 R 位是 0，那么这个页面既老又没有被使用，可以立刻置换掉；如果是 1，就将 R 位清 0，并把该页面放到链表的尾端，修改它的装入时间使它就像刚装入的一样，然后继续从链表的头部开始搜索。
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/ecf8ad5d-5403-48b9-b6e7-f2e20ffe8fca.png"/> </div><br>
<div align="center"> <img src="pics/second-chance.png" width="720px"/> </div><br>
### 6. 时钟
@@ -113,19 +117,17 @@ FIFO 算法可能会把经常使用的页面置换出去，为了避免这一问
第二次机会算法需要在链表中移动页面，降低了效率。时钟算法使用环形链表将页面连接起来，再使用一个指针指向最老的页面。
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/5f5ef0b6-98ea-497c-a007-f6c55288eab1.png"/> </div><br>
<div align="center"> <img src="pics/clock.png" width="720px"/> </div><br>
## 分段
虚拟内存采用的是分页技术，也就是将地址空间划分成固定大小的页，每一页再与内存进行映射。
下图为一个编译器在编译过程中建立的多个表，有 4 个表是动态增长的，如果使用分页系统的一维地址空间，动态增长的特点会导致覆盖问题的出现。
例如一个程序有代码、全局数据、堆和栈几部分，其中堆和栈会动态增长。如果放在一维的地址空间中，就要预先给每一部分留出位置，某一部分增长到相邻部分的位置时就会发生覆盖。
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/22de0538-7c6e-4365-bd3b-8ce3c5900216.png"/> </div><br>
分段的做法是把每一部分作为一个段，一个段构成一个独立的地址空间。每个段的长度可以不同，并且可以动态增长。地址由段号和段内偏移组成，通过段表找到段的基址，再加上偏移得到物理地址。
分段的做法是把每个表分成段，一个段构成一个独立的地址空间。每个段的长度可以不同，并且可以动态增长。
<div align="center"> <img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/e0900bb2-220a-43b7-9aa9-1d5cd55ff56e.png"/> </div><br>
<div align="center"> <img src="pics/segments.png" width="740px"/> </div><br>
## 段页式
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<text x="200" y="202" class="c sub note">指针</text>
<text x="400" y="70" class="b">发生缺页时，从指针处开始检查：</text>
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<text x="400" y="228" class="sub">不用在链表中移动页面。</text>
<text x="30" y="326" class="sub">图中灰色的 C、D 在这次检查中被清零；淡紫色的 E 被换出。</text>
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<text x="30" y="284" class="sub">被访问的页面移到链表头（淡紫色）；访问 6 时 5 个页框已满，换出链表尾部最久没有访问的 4。11 次访问共缺页 6 次。</text>
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tools/diagram/src/page-translate.svg
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<text x="130" y="314" class="c sub">……共 16 项</text>
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<text x="30" y="352" class="sub">4 KB 的页需要 12 位偏移量，16 位地址剩下的 4 位就是页号，页表一共 16 项。</text>
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tools/diagram/src/second-chance.svg
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<text x="30" y="86" class="sub b" style="fill:#444">(a) 时间 20 缺页</text>
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<text x="468" y="85" class="c sub mono" style="fill:#444">D R=0</text>
<text x="468" y="58" class="c sub">装入 8</text>
<line x1="500" y1="81" x2="532" y2="81" class="arrow "/>
<rect x="534" y="64" width="60" height="34" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="564" y="85" class="c sub mono" style="fill:#444">E R=1</text>
<text x="564" y="58" class="c sub">装入 12</text>
<text x="150" y="120" class="sub" style="fill:#444">最老的 A 的 R = 1：清零，移到队尾，装入时间改为 20；再看 B，R = 0，换出 B</text>
<text x="30" y="188" class="sub b" style="fill:#444">(b) 处理之后</text>
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<rect x="246" y="166" width="60" height="34" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
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<rect x="342" y="166" width="60" height="34" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="372" y="187" class="c sub mono" style="fill:#444">E R=1</text>
<text x="372" y="160" class="c sub">装入 12</text>
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<rect x="438" y="166" width="60" height="34" rx="3" fill="#E6E3FA" stroke="#B8B0DE" stroke-width="1"/>
<text x="468" y="187" class="c sub mono" style="fill:#444">A R=0</text>
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<rect x="534" y="166" width="60" height="34" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="564" y="187" class="c sub mono" style="fill:#444">新页 R=1</text>
<text x="564" y="160" class="c sub">装入 20</text>
<text x="30" y="236" class="sub">队头是最早装入的页面。FIFO 会直接换出刚被访问过的 A，第二次机会算法换出的是又老又没被访问的 B。</text>
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tools/diagram/src/segments.svg
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<style>
text { font-family: "PingFang SC", "Microsoft YaHei", Helvetica, Arial, sans-serif; fill: #3F3F3F; font-size: 12px; }
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<text x="370" y="28" class="title">分段：每个段是独立的地址空间</text>
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<text x="75" y="240" class="c sub">段 0</text>
<text x="75" y="256" class="c sub">长 12K</text>
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<text x="167" y="240" class="c sub">段 1</text>
<text x="167" y="256" class="c sub">长 4K</text>
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<text x="259" y="240" class="c sub">段 2</text>
<text x="259" y="256" class="c sub">长 8K</text>
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<text x="259" y="108" class="c sub">可增长</text>
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<text x="351" y="240" class="c sub">段 3</text>
<text x="351" y="256" class="c sub">长 8K</text>
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<text x="535" y="92" class="c sub" style="fill:#444">基址</text>
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<text x="615" y="92" class="c sub" style="fill:#444">长度</text>
<rect x="440" y="100" width="50" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="465" y="116" class="c sub mono" style="fill:#444">0</text>
<rect x="490" y="100" width="90" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="535" y="116" class="c sub mono" style="fill:#444">0x4000</text>
<rect x="580" y="100" width="70" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="615" y="116" class="c sub mono" style="fill:#444">12K</text>
<rect x="440" y="124" width="50" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="465" y="140" class="c sub mono" style="fill:#444">1</text>
<rect x="490" y="124" width="90" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="535" y="140" class="c sub mono" style="fill:#444">0x9000</text>
<rect x="580" y="124" width="70" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="615" y="140" class="c sub mono" style="fill:#444">4K</text>
<rect x="440" y="148" width="50" height="24" rx="3" fill="#E6E3FA" stroke="#B8B0DE" stroke-width="1"/>
<text x="465" y="164" class="c sub mono" style="fill:#444">2</text>
<rect x="490" y="148" width="90" height="24" rx="3" fill="#E6E3FA" stroke="#B8B0DE" stroke-width="1"/>
<text x="535" y="164" class="c sub mono" style="fill:#444">0xC000</text>
<rect x="580" y="148" width="70" height="24" rx="3" fill="#E6E3FA" stroke="#B8B0DE" stroke-width="1"/>
<text x="615" y="164" class="c sub mono" style="fill:#444">8K</text>
<rect x="440" y="172" width="50" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="465" y="188" class="c sub mono" style="fill:#444">3</text>
<rect x="490" y="172" width="90" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="535" y="188" class="c sub mono" style="fill:#444">0x1C000</text>
<rect x="580" y="172" width="70" height="24" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="615" y="188" class="c sub mono" style="fill:#444">8K</text>
<text x="440" y="220" class="sub mono" style="fill:#444">地址 = (段号 2, 偏移 0x300)</text>
<text x="440" y="240" class="sub" style="fill:#444">0x300 &lt; 8K，没有越界</text>
<text x="440" y="260" class="sub mono" style="fill:#444">物理地址 = 0xC000 + 0x300 = 0xC300</text>
<text x="440" y="280" class="sub note">偏移 ≥ 段长时触发越界异常</text>
<text x="30" y="300" class="sub">各段长度不同、可以单独增长，并且可以按段设置权限（例如代码段只读）；但段的大小不固定，</text>
<text x="30" y="318" class="sub">内存中会留下大小不一的空闲碎片。段页式在每个段内再分页，兼顾两者。</text>
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tools/diagram/src/vm-mapping.svg
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<svg xmlns="http://www.w3.org/2000/svg" width="640" height="520" viewBox="0 0 640 520">
<style>
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.title { font-weight: bold; font-size: 15px; fill: #333; text-anchor: middle; }
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<defs>
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</marker>
<marker id="r" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="7" markerHeight="7" orient="auto-start-reverse">
<path d="M1,1.5 L9.5,5 L1,8.5 z" fill="#C0605A"/>
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<rect width="100%" height="100%" fill="#FFFFFF"/>
<text x="320" y="28" class="title">虚拟内存：64 KB 的地址空间，只有 32 KB 物理内存</text>
<text x="195" y="52" class="c sub b" style="fill:#444">虚拟页（4 KB 一页）</text>
<rect x="150" y="454" width="90" height="26" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="195" y="471" class="c sub mono" style="fill:#444">2</text>
<text x="142" y="471" class="sub mono e">0K～4K</text>
<text x="250" y="471" class="sub mono">0</text>
<rect x="150" y="428" width="90" height="26" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="195" y="445" class="c sub mono" style="fill:#444">7</text>
<text x="142" y="445" class="sub mono e">4K～8K</text>
<text x="250" y="445" class="sub mono">1</text>
<rect x="150" y="402" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="419" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="419" class="sub mono e">8K～12K</text>
<text x="250" y="419" class="sub mono">2</text>
<rect x="150" y="376" width="90" height="26" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="195" y="393" class="c sub mono" style="fill:#444">5</text>
<text x="142" y="393" class="sub mono e">12K～16K</text>
<text x="250" y="393" class="sub mono">3</text>
<rect x="150" y="350" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="367" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="367" class="sub mono e">16K～20K</text>
<text x="250" y="367" class="sub mono">4</text>
<rect x="150" y="324" width="90" height="26" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="195" y="341" class="c sub mono" style="fill:#444">0</text>
<text x="142" y="341" class="sub mono e">20K～24K</text>
<text x="250" y="341" class="sub mono">5</text>
<rect x="150" y="298" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="315" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="315" class="sub mono e">24K～28K</text>
<text x="250" y="315" class="sub mono">6</text>
<rect x="150" y="272" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="289" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="289" class="sub mono e">28K～32K</text>
<text x="250" y="289" class="sub mono">7</text>
<rect x="150" y="246" width="90" height="26" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="195" y="263" class="c sub mono" style="fill:#444">3</text>
<text x="142" y="263" class="sub mono e">32K～36K</text>
<text x="250" y="263" class="sub mono">8</text>
<rect x="150" y="220" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="237" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="237" class="sub mono e">36K～40K</text>
<text x="250" y="237" class="sub mono">9</text>
<rect x="150" y="194" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="211" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="211" class="sub mono e">40K～44K</text>
<text x="250" y="211" class="sub mono">10</text>
<rect x="150" y="168" width="90" height="26" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="195" y="185" class="c sub mono" style="fill:#444">6</text>
<text x="142" y="185" class="sub mono e">44K～48K</text>
<text x="250" y="185" class="sub mono">11</text>
<rect x="150" y="142" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="159" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="159" class="sub mono e">48K～52K</text>
<text x="250" y="159" class="sub mono">12</text>
<rect x="150" y="116" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="133" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="133" class="sub mono e">52K～56K</text>
<text x="250" y="133" class="sub mono">13</text>
<rect x="150" y="90" width="90" height="26" rx="3" fill="#C6E4FB" stroke="#7FB2D6" stroke-width="1"/>
<text x="195" y="107" class="c sub mono" style="fill:#444">4</text>
<text x="142" y="107" class="sub mono e">56K～60K</text>
<text x="250" y="107" class="sub mono">14</text>
<rect x="150" y="64" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="195" y="81" class="c sub mono" style="fill:#444">×</text>
<text x="142" y="81" class="sub mono e">60K～64K</text>
<text x="250" y="81" class="sub mono">15</text>
<text x="515" y="260" class="c sub b" style="fill:#444">物理页框</text>
<rect x="470" y="454" width="90" height="26" rx="3" fill="#C6ECE9" stroke="#7DBCB7" stroke-width="1"/>
<text x="515" y="471" class="c sub" style="fill:#444">页 5</text>
<text x="570" y="471" class="sub mono">0K～4K</text>
<text x="462" y="471" class="sub mono e">0</text>
<rect x="470" y="428" width="90" height="26" rx="3" fill="#FFFFFF" stroke="#BBBBBB" stroke-width="1"/>
<text x="515" y="445" class="c sub" style="fill:#444">空闲</text>
<text x="570" y="445" class="sub mono">4K～8K</text>
<text x="462" y="445" class="sub mono e">1</text>
<rect x="470" y="402" width="90" height="26" rx="3" fill="#C6ECE9" stroke="#7DBCB7" stroke-width="1"/>
<text x="515" y="419" class="c sub" style="fill:#444">页 0</text>
<text x="570" y="419" class="sub mono">8K～12K</text>
<text x="462" y="419" class="sub mono e">2</text>
<rect x="470" y="376" width="90" height="26" rx="3" fill="#C6ECE9" stroke="#7DBCB7" stroke-width="1"/>
<text x="515" y="393" class="c sub" style="fill:#444">页 8</text>
<text x="570" y="393" class="sub mono">12K～16K</text>
<text x="462" y="393" class="sub mono e">3</text>
<rect x="470" y="350" width="90" height="26" rx="3" fill="#C6ECE9" stroke="#7DBCB7" stroke-width="1"/>
<text x="515" y="367" class="c sub" style="fill:#444">页 14</text>
<text x="570" y="367" class="sub mono">16K～20K</text>
<text x="462" y="367" class="sub mono e">4</text>
<rect x="470" y="324" width="90" height="26" rx="3" fill="#C6ECE9" stroke="#7DBCB7" stroke-width="1"/>
<text x="515" y="341" class="c sub" style="fill:#444">页 3</text>
<text x="570" y="341" class="sub mono">20K～24K</text>
<text x="462" y="341" class="sub mono e">5</text>
<rect x="470" y="298" width="90" height="26" rx="3" fill="#C6ECE9" stroke="#7DBCB7" stroke-width="1"/>
<text x="515" y="315" class="c sub" style="fill:#444">页 11</text>
<text x="570" y="315" class="sub mono">24K～28K</text>
<text x="462" y="315" class="sub mono e">6</text>
<rect x="470" y="272" width="90" height="26" rx="3" fill="#C6ECE9" stroke="#7DBCB7" stroke-width="1"/>
<text x="515" y="289" class="c sub" style="fill:#444">页 1</text>
<text x="570" y="289" class="sub mono">28K～32K</text>
<text x="462" y="289" class="sub mono e">7</text>
<line x1="266" y1="467.0" x2="448" y2="415.0" stroke="#AAA" stroke-width="1" marker-end="url(#a)"/>
<line x1="266" y1="441.0" x2="448" y2="285.0" stroke="#AAA" stroke-width="1" marker-end="url(#a)"/>
<line x1="266" y1="389.0" x2="448" y2="337.0" stroke="#AAA" stroke-width="1" marker-end="url(#a)"/>
<line x1="266" y1="337.0" x2="448" y2="467.0" stroke="#AAA" stroke-width="1" marker-end="url(#a)"/>
<line x1="266" y1="259.0" x2="448" y2="389.0" stroke="#AAA" stroke-width="1" marker-end="url(#a)"/>
<line x1="266" y1="181.0" x2="448" y2="311.0" stroke="#AAA" stroke-width="1" marker-end="url(#a)"/>
<line x1="266" y1="103.0" x2="448" y2="363.0" stroke="#AAA" stroke-width="1" marker-end="url(#a)"/>
<text x="30" y="496" class="sub">左边格子中的数字是页框号，× 表示这一页不在内存中，访问它会触发缺页中断，由操作系统从磁盘调入。</text>
<text x="30" y="514" class="sub">只有 7 页在内存中，程序照样可以运行；各页映射到哪个页框没有顺序要求。</text>
</svg>
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