AT&T 格式Linux 汇编语法

刺骨的言语ヽ痛彻心扉 2022-08-18 11:55 264阅读 0赞

AT&T汇编是UNIX下惯用的汇编格式   
      l   ,w,b是AT&T汇编中用来表示操作属性的限定符   
        
      l是长字（4字节)，   
      w是双字   
      b是一个字节   
        
      加在指令的后边   
      相当于intel中的   
      dword   ptr   
      word   ptr   
      byte   ptr

movl 12(%ebp), %eax，等同于Intel格式中的mov EAX, [EBP + 12]，AT&T中，源操作数在左，目的操作数在右。“l”是Longword，相当于Intel格式中的dword ptr操作限定符；
    表示将地址SS:[EBP +12]指向的双字数据传送至EAX寄存器。
    
    addl 8(%ebp), %eax，等同于Intel格式中的add EAX, [EBP + 8]，表示将SS:[EBP + 8]指向的一个双字数据同寄存器EAX中的原值相加，所得的结果保存在EAX寄存器。

摘自：http://blog.sina.com.cn/s/blog\_51e9c0ab010099ow.html

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      <a style="text-decoration:underline; color:rgb(185,76,0)" rel="nofollow"><span style="color:#000000; word-wrap:normal; word-break:normal">一、</span></a>AT&amp;T 格式Linux 汇编语法格式 
     </div> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">在 AT&amp;T 汇编格式中，寄存器名要加上 '%' 作为前缀；而在 Intel 汇编格式中，寄存器名不需要加前缀。例如：</li> 
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           <strong>AT&amp;T&nbsp;格式</strong> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>Intel&nbsp;格式</strong> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            pushl %eax 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            push eax 
          </div> </td> 
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      </table> 
     </div> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">在 AT&amp;T 汇编格式中，用 '$' 前缀表示一个立即操作数；而在 Intel 汇编格式中，立即数的表示不用带任何前缀。例如：</li> 
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         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
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           <strong>AT&amp;T&nbsp;格式</strong> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>Intel&nbsp;格式</strong> 
          </div> </td> 
        </tr> 
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         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            pushl $1 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            push 1 
          </div> </td> 
        </tr> 
       </tbody> 
      </table> 
     </div> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">AT&amp;T 和 Intel 格式中的源操作数和目标操作数的位置正好相反。在 Intel 汇编格式中，目标操作数在源操作数的左边；而在 AT&amp;T 汇编格式中，目标操作数在源操作数的右边。例如：</li> 
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           <strong>AT&amp;T&nbsp;格式</strong> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>Intel&nbsp;格式</strong> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            addl $1, %eax 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            add eax, 1 
          </div> </td> 
        </tr> 
       </tbody> 
      </table> 
     </div> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">在 AT&amp;T 汇编格式中，操作数的字长由操作符的最后一个字母决定，后缀'b'、'w'、'l'分别表示操作数为字节（byte，8 比特）、字（word，16 比特）和长字（long，32比特）；而在 Intel 汇编格式中，操作数的字长是用 "byte ptr" 和 "word ptr" 等前缀来表示的。例如：</li> 
     </ol> 
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          <div> 
           <strong>AT&amp;T&nbsp;格式</strong> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>Intel&nbsp;格式</strong> 
          </div> </td> 
        </tr> 
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         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            movb val, %al 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            mov al, byte ptr val 
          </div> </td> 
        </tr> 
       </tbody> 
      </table> 
     </div> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:0px 0px 0px 30px; padding:0px; border:0px; list-style:decimal"> 在 AT&amp;T 汇编格式中，绝对转移和调用指令（jump/call）的操作数前要加上'*'作为前缀，而在 Intel 格式中则不需要。</li> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">远程转移指令和远程子调用指令的操作码，在 AT&amp;T 汇编格式中为 "ljump" 和 "lcall"，而在 Intel 汇编格式中则为 "jmp far" 和 "call far"，即：</li> 
     </ol> 
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           <strong>AT&amp;T&nbsp;格式</strong> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>Intel&nbsp;格式</strong> 
          </div> </td> 
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         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            ljump $section, $offset 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            jmp far section:offset 
          </div> </td> 
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            lcall $section, $offset 
          </div> </td> 
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            call far section:offset 
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     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">与之相应的远程返回指令则为：</li> 
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           <strong>AT&amp;T&nbsp;格式</strong> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>Intel&nbsp;格式</strong> 
          </div> </td> 
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         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            lret $stack_adjust 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            ret far stack_adjust 
          </div> </td> 
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     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">在 AT&amp;T 汇编格式中，内存操作数的寻址方式是</li> 
     </ol> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
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        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           section:disp(base, index, scale) 
         </div> </td> 
       </tr> 
      </tbody> 
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     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">而在 Intel 汇编格式中，内存操作数的寻址方式为：</li> 
     </ol> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
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        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           section:[base + index*scale + disp] 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">由于 Linux 工作在保护模式下，用的是 32 位线性地址，所以在计算地址时不用考虑段基址和偏移量，而是采用如下的地址计算方法：</li> 
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     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
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        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
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           disp + base + index * scale 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <ol style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:7.5pt 0cm; padding:0px; border:0px; list-style:decimal">下面是一些内存操作数的例子：</li> 
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           <strong>AT&amp;T&nbsp;格式</strong> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>Intel&nbsp;格式</strong> 
          </div> </td> 
        </tr> 
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         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            movl -4(%ebp), %eax 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            mov eax, [ebp - 4] 
          </div> </td> 
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        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            movl array(, %eax, 4), %eax 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            mov eax, [eax*4 + array] 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            movw array(%ebx, %eax, 4), %cx 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            mov cx, [ebx + 4*eax + array] 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            movb $4, %fs:(%eax) 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            mov fs:eax, 4 
          </div> </td> 
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       </tbody> 
      </table> 
     </div> 
     <div> 
      <a style="text-decoration:underline; color:rgb(185,76,0)" rel="nofollow"><span style="color:#000000; word-wrap:normal; word-break:normal">二、</span></a>Hello World! 
     </div> 
     <div>
       既然所有程序设计语言的第一个例子都是在屏幕上打印一个字符串 "Hello World!"，那我们也以这种方式来开始介绍 Linux 下的汇编语言程序设计。 
     </div> 
     <div>
       在 Linux 操作系统中，你有很多办法可以实现在屏幕上显示一个字符串，但最简洁的方式是使用 Linux 内核提供的系统调用。使用这种方法最大的好处是可以直接和操作系统的内核进行通讯，不需要链接诸如 libc 这样的函数库，也不需要使用 ELF 解释器，因而代码尺寸小且执行速度快。 
     </div> 
     <div>
       Linux 是一个运行在保护模式下的 32 位操作系统，采用 flat memory 模式，目前最常用到的是 ELF 格式的二进制代码。一个 ELF 格式的可执行程序通常划分为如下几个部分：.text、.data 和 .bss，其中 .text 是只读的代码区，.data 是可读可写的数据区，而 .bss 则是可读可写且没有初始化的数据区。代码区和数据区在 ELF 中统称为 section，根据实际需要你可以使用其它标准的 section，也可以添加自定义 section，但一个 ELF 可执行程序至少应该有一个 .text 部分。下面给出我们的第一个汇编程序，用的是 AT&amp;T 汇编语言格式： 
     </div> 
     <div>
       例1. AT&amp;T 格式 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
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         <div>
           #hello.s 
         </div> 
         <div>
           .data&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; # 数据段声明 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; msg : .string "Hello, world!\\n" # 要输出的字符串 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; len = . - msg&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;# 字串长度 
         </div> 
         <div>
           .text&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; # 代码段声明 
         </div> 
         <div>
           .global _start&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;# 指定入口函数 
         </div> 
         <div>
           _start:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; # 在屏幕上显示一个字符串 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; movl $len, %edx&nbsp; &nbsp; # 参数三：字符串长度 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; movl $msg, %ecx&nbsp; &nbsp; # 参数二：要显示的字符串 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; movl $1, %ebx&nbsp; &nbsp; &nbsp; &nbsp; # 参数一：文件描述符(stdout) 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; movl $4, %eax&nbsp; &nbsp; &nbsp; &nbsp; # 系统调用号(sys_write) 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int&nbsp; &nbsp; $0x80&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;# 调用内核功能 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;# 退出程序 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; movl $0,%ebx&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;# 参数一：退出代码 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; movl $1,%eax&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;# 系统调用号(sys_exit) 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int&nbsp; &nbsp; $0x80&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;# 调用内核功能 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       初次接触到 AT&amp;T 格式的汇编代码时，很多程序员都认为太晦涩难懂了，没有关系，在 Linux 平台上你同样可以使用 Intel 格式来编写汇编程序： 
     </div> 
     <div>
       例2. Intel 格式 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           ; hello.asm 
         </div> 
         <div>
           section .data&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ; 数据段声明 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; msg db "Hello, world!", 0xA&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 要输出的字符串 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; len equ $ - msg&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 字串长度 
         </div> 
         <div>
           section .text&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ; 代码段声明 
         </div> 
         <div>
           global _start&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ; 指定入口函数 
         </div> 
         <div>
           _start:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; ; 在屏幕上显示一个字符串 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; mov edx, len&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 参数三：字符串长度 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; mov ecx, msg&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 参数二：要显示的字符串 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; mov ebx, 1&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 参数一：文件描述符(stdout) 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; mov eax, 4&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 系统调用号(sys_write) 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int 0x80&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 调用内核功能 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 退出程序 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; mov ebx, 0&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 参数一：退出代码 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; mov eax, 1&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 系统调用号(sys_exit) 
         </div> 
         <div>
           &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int 0x80&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;; 调用内核功能 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       上面两个汇编程序采用的语法虽然完全不同，但功能却都是调用 Linux 内核提供的 sys_write 来显示一个字符串，然后再调用 sys_exit 退出程序。在 Linux 内核源文件 include/asm-i386/unistd.h 中，可以找到所有系统调用的定义。 
     </div> 
     <div> 
      <a style="text-decoration:underline; color:rgb(185,76,0)" rel="nofollow"><span style="color:#000000; word-wrap:normal; word-break:normal">四、系统调用</span></a> 
     </div> 
     <div>
       即便是最简单的汇编程序，也难免要用到诸如输入、输出以及退出等操作，而要进行这些操作则需要调用操作系统所提供的服务，也就是系统调用。除非你的程序只完成加减乘除等数学运算，否则将很难避免使用系统调用，事实上除了系统调用不同之外，各种操作系统的汇编编程往往都是很类似的。 
      <br> 
     </div> 
     <div>
       在 Linux 平台下有两种方式来使用系统调用：利用封装后的 C 库（libc）或者通过汇编直接调用。其中通过汇编语言来直接调用系统调用，是最高效地使用 Linux 内核服务的方法，因为最终生成的程序不需要与任何库进行链接，而是直接和内核通信。 
      <br> 
     </div> 
     <div>
       和 DOS 一样，Linux 下的系统调用也是通过中断（int 0x80）来实现的。在执行 int 80 指令时，寄存器 eax 中存放的是系统调用的功能号，而传给系统调用的参数则必须按顺序放到寄存器 ebx，ecx，edx，esi，edi 中，当系统调用完成之后，返回值可以在寄存器 eax 中获得。 
      <br> 
     </div> 
     <div>
       所有的系统调用功能号都可以在文件 /usr/include/bits/syscall.h 中找到，为了便于使用，它们是用 SYS_&lt;name&gt; 这样的宏来定义的，如 SYS_write、SYS_exit 等。例如，经常用到的&nbsp;write 函数是如下定义的： 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           ssize_t write(int fd, const void *buf, size_t count); 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       该函数的功能最终是通过 SYS_write 这一系统调用来实现的。根据上面的约定，参数 fb、buf 和 count 分别存在寄存器 ebx、ecx 和 edx 中，而系统调用号 SYS_write 则放在寄存器 eax 中，当 int 0x80 指令执行完毕后，返回值可以从寄存器 eax 中获得。 
      <br> 
     </div> 
     <div>
       或许你已经发现，在进行系统调用时至多只有 5 个寄存器能够用来保存参数，难道所有系统调用的参数个数都不超过 5 吗？当然不是，例如 mmap 函数就有 6 个参数，这些参数最后都需要传递给系统调用 SYS_mmap： 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px; width:550px; height:51px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           void * mmap(void *start, size_t length, int prot , int flags, int fd, off_t offset);c 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       当一个系统调用所需的参数个数大于 5 时，执行int 0x80 指令时仍需将系统调用功能号保存在寄存器 eax 中，所不同的只是全部参数应该依次放在一块连续的内存区域里，同时在寄存器 ebx 中保存指向该内存区域的指针。系统调用完成之后，返回值仍将保存在寄存器 eax 中。 
      <br> 
     </div> 
     <div>
       由于只是需要一块连续的内存区域来保存系统调用的参数，因此完全可以像普通的函数调用一样使用栈(stack)来传递系统调用所需的参数。但要注意一点， Linux 采用的是 C 语言的调用模式，这就意味着所有参数必须以相反的顺序进栈，即最后一个参数先入栈，而第一个参数则最后入栈。如果采用栈来传递系统调用所需的参数，在执行 int 0x80 指令时还应该将栈指针的当前值复制到寄存器 ebx中。 
      <br> 
     </div> 
     <div> 
      <a style="text-decoration:underline; color:rgb(185,76,0)" rel="nofollow"><span style="color:#000000; word-wrap:normal; word-break:normal">五、命令行参数</span></a> 
     </div> 
     <div>
       在 Linux 操作系统中，当一个可执行程序通过命令行启动时，其所需的参数将被保存到栈中：首先是 argc，然后是指向各个命令行参数的指针数组 argv，最后是指向环境变量的指针数据 envp。在编写汇编语言程序时，很多时候需要对这些参数进行处理，下面的代码示范了如何在汇编代码中进行命令行参数的处理： 
      <br> 
     </div> 
     <div>
       例3. 处理命令行参数 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           # args.s 
          <br> 
         </div> 
         <div>
           .text 
          <br> 
         </div> 
         <div>
           .globl _start 
          <br> 
         </div> 
         <div>
           _start: 
          <br> 
         </div> 
         <div>
           popl %ecx # argc 
          <br> 
         </div> 
         <div>
           vnext: 
          <br> 
         </div> 
         <div>
           popl %ecx # argv 
          <br> 
         </div> 
         <div>
           test %ecx, %ecx # 空指针表明结束 
          <br> 
         </div> 
         <div>
           jz exit 
          <br> 
         </div> 
         <div>
           movl %ecx, %ebx 
          <br> 
         </div> 
         <div>
           xorl %edx, %edx 
          <br> 
         </div> 
         <div>
           strlen: 
          <br> 
         </div> 
         <div>
           movb (%ebx), %al 
          <br> 
         </div> 
         <div>
           inc %edx 
          <br> 
         </div> 
         <div>
           inc %ebx 
          <br> 
         </div> 
         <div>
           test %al, %al 
          <br> 
         </div> 
         <div>
           jnz strlen 
          <br> 
         </div> 
         <div>
           movb $10, -1(%ebx) 
          <br> 
         </div> 
         <div>
           movl $4, %eax # 系统调用号(sys_write) 
          <br> 
         </div> 
         <div>
           movl $1, %ebx # 文件描述符(stdout) 
          <br> 
         </div> 
         <div>
           int $0x80 
          <br> 
         </div> 
         <div>
           jmp vnext 
          <br> 
         </div> 
         <div>
           exit: movl $1,%eax # 系统调用号(sys_exit) 
          <br> 
         </div> 
         <div>
           xorl %ebx, %ebx # 退出代码 
          <br> 
         </div> 
         <div>
           int $0x80 
          <br> 
         </div> 
         <div>
           ret 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> <br> </p> 
     <div> 
      <a style="text-decoration:underline; color:rgb(185,76,0)" rel="nofollow"><span style="color:#000000; word-wrap:normal; word-break:normal">六、</span></a>GCC 内联汇编 
      <br> 
      <br> 
     </div> 
     <div>
       用汇编编写的程序虽然运行速度快，但开发速度非常慢，效率也很低。如果只是想对关键代码段进行优化，或许更好的办法是将汇编指令嵌入到 C 语言程序中，从而充分利用高级语言和汇编语言各自的特点。但一般来讲，在 C 代码中嵌入汇编语句要比"纯粹"的汇编语言代码复杂得多，因为需要解决如何分配寄存器，以及如何与C代码中的变量相结合等问题。 
      <br> 
     </div> 
     <div>
       GCC 提供了很好的内联汇编支持，最基本的格式是： 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           __asm__("asm statements"); 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       例如： 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           __asm__("nop"); 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       如果需要同时执行多条汇编语句，则应该用"\\n\\t"将各个语句分隔开，例如： 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           __asm__( "pushl %%eax \\n\\t" 
          <br> 
         </div> 
         <div>
           "movl $0, %%eax \\n\\t" 
          <br> 
         </div> 
         <div>
           "popl %eax"); 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       通常嵌入到 C 代码中的汇编语句很难做到与其它部分没有任何关系，因此更多时候需要用到完整的内联汇编格式： 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div>
           __asm__("asm statements" : outputs : inputs : registers-modified); 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       插入到 C 代码中的汇编语句是以":"分隔的四个部分，其中第一部分就是汇编代码本身，通常称为指令部，其格式和在汇编语言中使用的格式基本相同。指令部分是必须的，而其它部分则可以根据实际情况而省略。 
      <br> 
     </div> 
     <div>
       在将汇编语句嵌入到C代码中时，操作数如何与C代码中的变量相结合是个很大的问题。GCC采用如下方法来解决这个问题：程序员提供具体的指令，而对寄存器的使用则只需给出"样板"和约束条件就可以了，具体如何将寄存器与变量结合起来完全由GCC和GAS来负责。 
      <br> 
     </div> 
     <div>
       在GCC 内联汇编语句的指令部中，加上前缀''%''的数字(如%0，%1)表示的就是需要使用寄存器的"样板"操作数。指令部中使用了几个样板操作数，就表明有几个变量需要与寄存器相结合，这样GCC和GAS在编译和汇编时会根据后面给定的约束条件进行恰当的处理。由于样板操作数也使用'' %''作为前缀，因此在涉及到具体的寄存器时，寄存器名前面应该加上两个''%''，以免产生混淆。 
      <br> 
     </div> 
     <div>
       紧跟在指令部后面的是输出部，是规定输出变量如何与样板操作数进行结合的条件，每个条件称为一个"约束"，必要时可以包含多个约束，相互之间用逗号分隔开就可以了。每个输出约束都以''=''号开始，然后紧跟一个对操作数类型进行说明的字后，最后是如何与变量相结合的约束。凡是与输出部中说明的操作数相结合的寄存器或操作数本身，在执行完嵌入的汇编代码后均不保留执行之前的内容，这是GCC在调度寄存器时所使用的依据。 
      <br> 
     </div> 
     <div>
       输出部后面是输入部，输入约束的格式和输出约束相似，但不带''=''号。如果一个输入约束要求使用寄存器，则GCC在预处理时就会为之分配一个寄存器，并插入必要的指令将操作数装入该寄存器。与输入部中说明的操作数结合的寄存器或操作数本身，在执行完嵌入的汇编代码后也不保留执行之前的内容。 
      <br> 
     </div> 
     <div>
       有时在进行某些操作时，除了要用到进行数据输入和输出的寄存器外，还要使用多个寄存器来保存中间计算结果，这样就难免会破坏原有寄存器的内容。在GCC内联汇编格式中的最后一个部分中，可以对将产生副作用的寄存器进行说明，以便GCC能够采用相应的措施。 
      <br> 
     </div> 
     <div>
       下面是一个内联汇编的简单例子： 
      <br> 
     </div> 
     <div>
       例4.内联汇编 
      <br> 
     </div> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <table style="margin:0px; padding:0px"> 
      <tbody> 
       <tr> 
        <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
         <div> 
          <br> 
         </div> 
         <div>
           int main() 
          <br> 
         </div> 
         <div>
           { 
          <br> 
         </div> 
         <div>
           int a = 10, b = 0; 
          <br> 
         </div> 
         <div>
           __asm__ __volatile__("movl %1, %%eax;\\n\\r" 
          <br> 
         </div> 
         <div>
           "movl %%eax, %0;" 
          <br> 
         </div> 
         <div>
           :"=r"(b) 
          <br> 
         </div> 
         <div>
           :"r"(a) 
          <br> 
         </div> 
         <div>
           :"%eax"); 
          <br> 
         </div> 
         <div>
           printf("Result: %d, %d\\n", a, b); 
          <br> 
         </div> 
         <div>
           } 
          <br> 
         </div> </td> 
       </tr> 
      </tbody> 
     </table> 
     <p style="margin-top:0px; margin-bottom:5px; padding-top:0px; padding-bottom:0px; border:0px; list-style:none; word-wrap:normal; word-break:normal"> </p> 
     <div>
       上面的程序完成将变量a的值赋予变量b，有几点需要说明： 
      <br> 
     </div> 
     <ul style="margin:0px; padding:0px; border:0px; list-style:none"> 
      <li style="margin:0px 0px 0px 30px; padding:0px; border:0px; list-style:disc">变量b是输出操作数，通过%0来引用，而变量a是输入操作数，通过%1来引用。<br> </li> 
      <li style="margin:0px 0px 0px 30px; padding:0px; border:0px; list-style:disc">输入操作数和输出操作数都使用r进行约束，表示将变量a和变量b存储在寄存器中。输入约束和输出约束的不同点在于输出约束多一个约束修饰符''=''。<br> </li> 
      <li style="margin:0px 0px 0px 30px; padding:0px; border:0px; list-style:disc">在内联汇编语句中使用寄存器eax时，寄存器名前应该加两个''%''，即%%eax。内联汇编中使用%0、%1等来标识变量，任何只带一个''%''的标识符都看成是操作数，而不是寄存器。<br> </li> 
      <li style="margin:0px 0px 0px 30px; padding:0px; border:0px; list-style:disc">内联汇编语句的最后一个部分告诉GCC它将改变寄存器eax中的值，GCC在处理时不应使用该寄存器来存储任何其它的值。<br> </li> 
      <li style="margin:0px 0px 0px 30px; padding:0px; border:0px; list-style:disc">由于变量b被指定成输出操作数，当内联汇编语句执行完毕后，它所保存的值将被更新。<br> </li> 
     </ul> 
     <div>
       在内联汇编中用到的操作数从输出部的第一个约束开始编号，序号从0开始，每个约束记数一次，指令部要引用这些操作数时，只需在序号前加上''%''作为前缀就可以了。需要注意的是，内联汇编语句的指令部在引用一个操作数时总是将其作为32位的长字使用，但实际情况可能需要的是字或字节，因此应该在约束中指明正确的限定符： 
      <br> 
     </div> 
     <div> 
      <table style="margin:0px; padding:0px"> 
       <tbody> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>限定符</strong> 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div> 
           <strong>意义</strong> 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "m"、"v"、"o" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            内存单元 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "r" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            任何寄存器 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "q" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            寄存器eax、ebx、ecx、edx之一 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "i"、"h" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            直接操作数 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "E"和"F" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            浮点数 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "g" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            任意 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "a"、"b"、"c"、"d" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            分别表示寄存器eax、ebx、ecx和edx 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "S"和"D" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            寄存器esi、edi 
           <br> 
          </div> </td> 
        </tr> 
        <tr> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            "I" 
           <br> 
          </div> </td> 
         <td style="margin:0px; padding:0px; font-family:Verdana,宋体,sans-serif; line-height:18px"> 
          <div>
            常数（0至31） 
           <br> 
          </div> </td> 
        </tr> 
       </tbody> 
      </table> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

解释下，关于gcc的汇编中，

# [ Explanation of .cfi\_def\_cfa\_offset][Explanation of .cfi_def_cfa_offset] #

# I would like an explanation for the values used with the .cfi\_def\_cfa\_offset directives in assembly generated by GCC. I know vaguely that the .cfi directives are involved in call frames and stack unwinding, but I would like a more detailed explanation of why, for example, the values 16 and 8 are used in the assembly outputted by GCC in compiling the following C program on my 64-bit Ubuntu machine.  The C program:  `#include <stdio.h> int main(int argc, char** argv) { printf("%d", 0); return 0; }`  I invoked GCC on the source file test.c as follows: `gcc -S -O3 test.c`. I know that -O3 enables nonstandard optimization, but I wanted to limit the size of the generated assembly for the sake of brevity.  The generated assembly:  `.file "test.c" .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "%d" .text .p2align 4,,15 .globl main .type main, @function main: .LFB22: .cfi_startproc subq $8, %rsp .cfi_def_cfa_offset 16 xorl %edx, %edx movl $.LC0, %esi movl $1, %edi xorl %eax, %eax call __printf_chk xorl %eax, %eax addq $8, %rsp .cfi_def_cfa_offset 8 ret .cfi_endproc .LFE22: .size main, .-main .ident "GCC: (Ubuntu/Linaro 4.5.2-8ubuntu4) 4.5.2" .section .note.GNU-stack,"",@progbits`  Why are the values 16 and 8 used for the .cfi\_def\_cfa\_offset directives in the generated assembly? Also, why is the number 22 used for the local function begin and function end labels?  看看老外的解释： #

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     <a title="This answer is not useful" style="margin:0px auto 10px; padding:0px; border:0px; font-size:1px; color:rgb(0,119,204); overflow:hidden; display:block; text-indent:-9999em; width:40px; height:30px" rel="nofollow">down vote</a> 
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    <div style="margin:0px 0px 5px; padding:0px; border:0px; font-size:15px; width:660px; word-wrap:break-word; line-height:1.3"> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> As the&nbsp;<a href="http://dwarfstd.org/doc/DWARF4.pdf" style="margin:0px; padding:0px; border:0px; text-decoration:none; color:rgb(0,89,153)" rel="nofollow">DWARF spec</a>&nbsp;says in section 6.4:</p> 
     <blockquote style="margin:0px 0px 10px; padding:10px; border-width:0px 0px 0px 2px; border-left-style:solid; border-left-color:rgb(255,235,142); quotes:none; background-color:rgb(255,248,220)"> 
      <p style="margin-top:0px; margin-bottom:0px; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> [...] The call frame is identified by an address on the stack. We refer to this address as the <span style="color:#ff0000">Canonical Frame Address or CFA</span>. Typically, the CFA is defined to be <span style="color:#ff0000">the value of the stack pointer at the call site in the previous frame</span> (which may be different from its value on entry to the current frame).</p> 
     </blockquote> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> <code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">main()</code>&nbsp;is called from somewhere else (in the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">libc</code>&nbsp;C runtime support code), and, at the time the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">call</code>&nbsp;instruction is executed,&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">%rsp</code>&nbsp;will point to the top of the stack (which is the lowest address - the stack grows downwards), whatever that may be (exactly what it is doesn't matter here):</p> 
     <pre style="margin-top:0px; margin-bottom:1em; padding:5px; border:0px; font-size:13px; overflow:auto; width:auto; max-height:600px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; word-wrap:normal; background-color:rgb(238,238,238)"><code style="margin:0px; padding:0px; border:0px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:inherit">:                :                              ^
|    whatever    | &lt;--- %rsp                    | increasing addresses
+----------------+                              |
</code></pre> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> The value of&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">%rsp</code>&nbsp;at this point is the "value of the stack pointer at the call site", i.e. the CFA as defined by the spec.</p> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> As the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">call</code>&nbsp;instruction is executed, it will push a 64-bit (8 byte) return address onto the stack:</p> 
     <pre style="margin-top:0px; margin-bottom:1em; padding:5px; border:0px; font-size:13px; overflow:auto; width:auto; max-height:600px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; word-wrap:normal; background-color:rgb(238,238,238)"><code style="margin:0px; padding:0px; border:0px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:inherit">:                :
|    whatever    | &lt;--- CFA
+----------------+
| return address | &lt;--- %rsp == CFA - 8
+----------------+
</code></pre> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> Now we are running the code at&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">main</code>, which executes&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">subq $8, %rsp</code>&nbsp;to reserve another 8 bytes of stack for itself:</p> 
     <pre style="margin-top:0px; margin-bottom:1em; padding:5px; border:0px; font-size:13px; overflow:auto; width:auto; max-height:600px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; word-wrap:normal; background-color:rgb(238,238,238)"><code style="margin:0px; padding:0px; border:0px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:inherit">:                :
|    whatever    | &lt;--- CFA
+----------------+
| return address |
+----------------+
| reserved space | &lt;--- %rsp == CFA - 16
+----------------+
</code></pre> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> The change of stack pointer is declared in the debugging information using the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">.cfi_def_cfa_offset</code>directive, and you can see that the CFA is now at an offset of 16 bytes from the current stack pointer.</p> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> At the end of the function, the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">addq $8, %rsp</code>&nbsp;instruction changes the stack pointer again, so another&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">.cfi_def_cfa_offset</code>&nbsp;directive is inserted to indicate that the CFA is now at an offset of only 8 bytes from the stack pointer.</p> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> (The number "22" in the labels is just an arbitrary value. The compiler will generate unique label names based on some implementation detail, such as its internal numbering of basic blocks.)</p> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

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     <span style="margin:8px 0px; padding:0px; border:0px; font-size:20px; display:block; color:rgb(119,119,119)">40</span> 
     <a title="This answer is not useful" style="margin:0px auto 10px; padding:0px; border:0px; font-size:1px; color:rgb(0,119,204); overflow:hidden; display:block; text-indent:-9999em; width:40px; height:30px" rel="nofollow">down vote</a> 
     <span title="loading when this answer was accepted..." style="margin:0px auto; padding:0px; border:0px; font-size:1px; overflow:hidden; display:block; text-indent:-9999em; width:40px; height:40px; color:rgb(119,119,119)">accepted</span> 
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    <div style="margin:0px 0px 5px; padding:0px; border:0px; font-size:15px; width:660px; word-wrap:break-word; line-height:1.3"> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> As the&nbsp;<a href="http://dwarfstd.org/doc/DWARF4.pdf" style="margin:0px; padding:0px; border:0px; text-decoration:none; color:rgb(0,89,153)" rel="nofollow">DWARF spec</a>&nbsp;says in section 6.4:</p> 
     <blockquote style="margin:0px 0px 10px; padding:10px; border-width:0px 0px 0px 2px; border-left-style:solid; border-left-color:rgb(255,235,142); quotes:none; background-color:rgb(255,248,220)"> 
      <p style="margin-top:0px; margin-bottom:0px; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> [...] The call frame is identified by an address on the stack. We refer to this address as the Canonical Frame Address or CFA. Typically, the CFA is defined to be the value of the stack pointer at the call site in the previous frame (which may be different from its value on entry to the current frame).</p> 
     </blockquote> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> <code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">main()</code>&nbsp;is called from somewhere else (in the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">libc</code>&nbsp;C runtime support code), and, at the time the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">call</code>&nbsp;instruction is executed,&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">%rsp</code>&nbsp;will point to the top of the stack (which is the lowest address - the stack grows downwards), whatever that may be (exactly what it is doesn't matter here):</p> 
     <pre style="margin-top:0px; margin-bottom:1em; padding:5px; border:0px; font-size:13px; overflow:auto; width:auto; max-height:600px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; word-wrap:normal; background-color:rgb(238,238,238)"><code style="margin:0px; padding:0px; border:0px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:inherit">:                :                              ^
|    whatever    | &lt;--- %rsp                    | increasing addresses
+----------------+                              |
</code></pre> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> The value of&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">%rsp</code>&nbsp;at this point is the "value of the stack pointer at the call site", i.e. the CFA as defined by the spec.</p> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> As the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">call</code>&nbsp;instruction is executed, it will push a 64-bit (8 byte) return address onto the stack:</p> 
     <pre style="margin-top:0px; margin-bottom:1em; padding:5px; border:0px; font-size:13px; overflow:auto; width:auto; max-height:600px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; word-wrap:normal; background-color:rgb(238,238,238)"><code style="margin:0px; padding:0px; border:0px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:inherit">:                :
|    whatever    | &lt;--- CFA
+----------------+
| return address | &lt;--- %rsp == CFA - 8
+----------------+
</code></pre> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> Now we are running the code at&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">main</code>, which executes&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">subq $8, %rsp</code>&nbsp;to reserve another 8 bytes of stack for itself:</p> 
     <pre style="margin-top:0px; margin-bottom:1em; padding:5px; border:0px; font-size:13px; overflow:auto; width:auto; max-height:600px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; word-wrap:normal; background-color:rgb(238,238,238)"><code style="margin:0px; padding:0px; border:0px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:inherit">:                :
|    whatever    | &lt;--- CFA
+----------------+
| return address |
+----------------+
| reserved space | &lt;--- %rsp == CFA - 16
+----------------+
</code></pre> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> The change of stack pointer is declared in the debugging information using the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">.cfi_def_cfa_offset</code>directive, and you can see that the CFA is now at an offset of 16 bytes from the current stack pointer.</p> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> At the end of the function, the&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">addq $8, %rsp</code>&nbsp;instruction changes the stack pointer again, so another&nbsp;<code style="margin:0px; padding:1px 5px; border:0px; font-size:13px; font-family:Consolas,Menlo,Monaco,'Lucida Console','Liberation Mono','DejaVu Sans Mono','Bitstream Vera Sans Mono','Courier New',monospace,sans-serif; white-space:pre-wrap; background-color:rgb(238,238,238)">.cfi_def_cfa_offset</code>&nbsp;directive is inserted to indicate that the CFA is now at an offset of only 8 bytes from the stack pointer.</p> 
     <p style="margin-top:0px; margin-bottom:1em; padding-top:0px; padding-bottom:0px; border:0px; clear:both"> (The number "22" in the labels is just an arbitrary value. The compiler will generate unique label names based on some implementation detail, such as its internal numbering of basic blocks.)</p> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

[Explanation of .cfi_def_cfa_offset]: http://stackoverflow.com/questions/7534420/gas-explanation-of-cfi-def-cfa-offset