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                                    <p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; background-color: rgb(226, 226, 226);"><span style="word-wrap: normal; word-break: normal; background-color: rgb(255, 255, 255); font-size: 12px; line-height: 18px;"><font color="#494949" face="Verdana, Lucida Grande, Arial, Helvetica, sans-serif">原文：http://blog.sina.com.cn/s/blog_67ed0acc01014lp2.html</font></span></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><span style="word-wrap: normal; word-break: normal; line-height: 18px; font-family: Verdana, 'Lucida Grande', Arial, Helvetica, sans-serif; font-size: 12px; background-color: rgb(255, 255, 255);"><br></span></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><span style="word-wrap: normal; word-break: normal; line-height: 18px; font-family: Verdana, 'Lucida Grande', Arial, Helvetica, sans-serif; font-size: 12px; background-color: rgb(255, 255, 255);">B或BL</span><span style="font-family: Arial, Helvetica, sans-serif; line-height: 1.5; background-color: rgb(255, 255, 255);">指令引起处理器转移到“子程序名”处开始执行。两者的不同之处在于BL指令在转移到子</span></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><span style="color: rgb(73, 73, 73);">程序执行之前，将其下一条指令的地址拷贝到R14（LR,链接寄存器）。</span><font color="#ff0000">由于BL指令保存了下条指令的地</font></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><font color="#ff0000">址，因此使用指令“MOV PC ,LR”即可实现子程序的返回</font><span style="color: rgb(73, 73, 73);">。</span><font color="#ff0000">而B指令则无法实现子程序的返回，只能实</font></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><font color="#ff0000">现单纯的跳转</font><span style="color: rgb(73, 73, 73);">。用户在编程的时候，可根据具体应用选用合适的子程序调用语句。</span></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">&nbsp;<wbr>&nbsp;<wbr>&nbsp;AREA Init,CODE,READONLY&nbsp;<wbr></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">;该伪指令定义了一个代码段，段名为Init，属性只读<br>ENTRY&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;;程序的入口点标识</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">.</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">.</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">bl delay&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;;调用延迟</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">.</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">.</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">mov pc,lr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;;返回</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">下面的在BLOG中看到觉得讲得比较详细就拷过来了</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">ARM汇编指令的一些总结<br>ARM汇编指令很多，但是真正常用的不是很多，而且需要认真琢磨的又更少了。<br>比较有用的是MOV B BL LDR STR<br>还是通过具体汇编代码来学习吧。<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;@ disable watch dog timer&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;mov&nbsp;<wbr>&nbsp;<wbr>&nbsp;r1, #0x53000000&nbsp;<wbr>&nbsp;<wbr>&nbsp;//立即数寻址方式&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;mov&nbsp;<wbr>&nbsp;<wbr>&nbsp;r2, #0x0&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;str&nbsp;<wbr>&nbsp;<wbr>&nbsp;r2, [r1]&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr><br>立即数寻址方式，立即数要求以“#”作前缀，对于十六进制的数，还要求在#后面加上0x或者&amp;。STR是</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">比较重要的指令了，跟它对应的是LDR。ARM指令集是加载/存储型的，也就是说它只处理在寄存器中的</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">数据。那么对于系统存储器的访问就经常用到STR和LDR了。STR是把寄存器上的数据传输到指定地址的</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">存储器上。它的格式我个人认为很特殊：<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;STR(条件) 源寄存器，&lt;存储器地址&gt;<br>比如 STR R0, [R1] ，意思是R0-&gt; [R1]，它把源寄存器写在前面，跟MOV、LDR都相反。<br>LDR应该是非常常见了。LDR就是把数据从存储器传输到寄存器上。而且有个伪指令也是LDR，因此我有</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">个百思不得其解的问题。看这段代码：<br>mov r1, #GPIO_CTL_BASE&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;add&nbsp;<wbr>&nbsp;<wbr>&nbsp;r1, r1, #oGPIO_F&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;r2,=0x55aa&nbsp;<wbr>&nbsp;<wbr>&nbsp;// 0x55aa是个立即数啊，前面加个=干什么？&nbsp;<wbr><br>对于当中的ldr 那句，我就不明白了，如果你把=去掉，是不能通过编译的。我查了一些资料，个人感</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">觉知道了原因：这个=应该表示LDR不是ARM指令，而是伪指令。作为伪指令的时候，LDR的格式如下：<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;LDR 寄存器， =数字常量/Label<br>它的作用是把一个32位的地址或者常量调入寄存器。嗬嗬，那大家可能会问，<br>“MOV r2,#0x55aa”也可以啊。应该是这样的。不过，LDR是伪指令啊，也就是说编译时编译器会处理</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">它的。怎么处理的呢？——规则如下：如果该数字常量在MOV指令范围内，汇编器会把这个指令作为MOV</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">。如果不在MOV范围中，汇编器把该常量放在程序后面，用LDR来读取，PC和该常量的偏移量不能超过</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">4KB。<br>然后说一下跳转指令。ARM有两种跳转方式。<br>（1） mov pc &lt;跳转地址〉<br>这种向程序计数器PC直接写跳转地址，能在4GB连续空间内任意跳转。<br>（2）通过 B BL BLX BX 可以完成在当前指令向前或者向后32MB的地址空间的跳转（为什么是32MB呢？</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">寄存器是32位的，此时的值是24位有符号数，所以32MB）。<br>B是最简单的跳转指令。要注意的是，跳转指令的实际值不是绝对地址，而是相对地址——是相对当前</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">PC值的一个偏移量，它的值由汇编器计算得出。<br>BL非常常用。它在跳转之前会在寄存器LR(R14)中保存PC的当前内容。BL的经典用法如下：<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;bl NEXT&nbsp;<wbr>&nbsp;<wbr>&nbsp;; 跳转到NEXT&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;……&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;NEXT&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;……&nbsp;<wbr><br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;mov pc, lr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;; 从子程序返回。&nbsp;<wbr><br>最后提一下Thumb指令。ARM体系结构还支持16位的Thumb指令集。Thumb指令集是ARM指令集的子集，它</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">保留了32位代码优势的同时还大大节省了存储空间。由于Thumb指令集的长度只有16位，所以它的指令</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">比较多。它和ARM各有自己的应用场合。对于系统性能有较高要求，应使用32位存储系统和ARM指令集；</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">对于系统成本和功耗有较高要求，应使用16位存储系统和ARM指令集。&nbsp;<wbr><br>对ARM异常（Exceptions）的理解<br>分类:技术笔记<br>毕设笔记<br>1．对ARM异常（Exceptions）的理解<br>所有的系统引导程序前面中会有一段类似的代码，如下：<br>.globl _start&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；系统复位位置<br>_start: b&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;reset&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；各个异常向量对应的跳转代码<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _undefined_instruction ；未定义的指令异常<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _software_interrupt&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；软件中断异常<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _prefetch_abort&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；内存操作异常<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _data_abort&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；数据异常<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _not_used&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；未使用<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _irq&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；慢速中断异常<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _fiq&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；快速中断异常</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">从中我们可以看出，ARM支持7种异常。问题时发生了异常后ARM是如何响应的呢？第一个复位异常很好</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">理解，它放在0x0的位置，一上电就执行它，而且我们的程序总是从复位异常处理程序开始执行的，因</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><font color="#494949">此复位异常处理程序不需要返回。那么怎么会执行到后面几个异常处理函数呢？</font><br><font color="#494949">看看书后，明白了ARM对异常的响应过程，于是就能够回答以前的这个疑问。</font><br><font color="#ff0000">当一个异常出现以后，ARM会自动执行以下几个步骤：<br>（1）把下一条指令的地址放到连接寄存器LR(通常是R14)，这样就能够在处理异常返回时从正确的位置</font></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><font color="#ff0000">继续执行。<br>（2）将相应的CPSR(当前程序状态寄存器)复制到SPSR（备份的程序状态寄存器）中。从异常退出的时</font></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);"><font color="#ff0000">候，就可以由SPSR来恢复CPSR。<br>(3) 根据异常类型，强制设置CPSR的运行模式位。<br>（4）强制PC（程序计数器）从相关异常向量地址取出下一条指令执行，从而跳转到相应的异常处理程</font></p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">序中。<br>至于这些异常类型各代表什么，我也没有深究。因为平常就关心reset了，也没有必要弄清楚。<br>ARM规定了异常向量的地址：<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;b&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;reset&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;； 复位 0x0<br>ldr pc, _undefined_instruction ；未定义的指令异常 0x4<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _software_interrupt&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；软件中断异常&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;0x8<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _prefetch_abort&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；预取指令&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;0xc<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _data_abort&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；数据&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;0x10<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _not_used&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；未使用&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;0x14<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _irq&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；慢速中断异常&nbsp;<wbr>&nbsp;<wbr>&nbsp;0x18<br>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;ldr&nbsp;<wbr>&nbsp;<wbr>&nbsp;pc, _fiq&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;；快速中断异常&nbsp;<wbr>&nbsp;<wbr>&nbsp;<wbr>&nbsp;0x1c<br>这样理解这段代码就非常简单了。碰到异常时，PC会被强制设置为对应的异常向量，从而跳转到相应的</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">处理程序，然后再返回到主程序继续执行。<br>这些引导程序的中断向量，是仅供引导程序自己使用的，一旦引导程序引导Linux内核完毕后，会使用</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">自己的中断向量。<br>嗬嗬，这又有问题了。比如，ARM发生中断(irq)的时候，总是会跑到0x18上执行啊。那Linux内核又怎</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">么能使用自己的中断向量呢？原因在于Linux内核采用页式存储管理。开通MMU的页面映射以后，CPU所</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">发出的地址就是虚拟地址而不是物理地址。就Linux内核而言，虚拟地址0x18经过映射以后的物理地址</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">就是0xc000 0018。所以Linux把中断向量放到0xc000 0018就可以了。<br>MMU的两个主要作用：<br>(1）安全性：规定访问权限<br>(2) 提供地址空间：把不连续的空间转换成连续的。<br>第2点是不是实现页式存储的意思？</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">.globl _start ；系统复位位置<br>_start: b reset ；各个异常向量对应的跳转代码<br>ldr pc, _undefined_instruction ；未定义的指令异常</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">……</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">_undefined_instruction :<br>.word undefined_instruction</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">也许有人会有疑问，同样是跳转指令，为什么第一句用的是 b reset；<br>而后面的几个都是用ldr？</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">为了理解这个问题，我们以未定义的指令异常为例。</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">当发生了这个异常后，CPU总是跳转到0x4，这个地址是虚拟地址，它映射到哪个物理地址<br>取决于具体的映射。<br>ldr pc, _undefined_instruction&nbsp;<wbr><br>相对寻址，跳转到标号_undefined_instruction，然而真正的跳转地址其实是_undefined_instruction</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">的内容——undefined_instruction。那句.word的相当于：<br>_undefined_instruction dw undefined_instruction (详见毕设笔记3）。<br>这个地址undefined_instruction到底有多远就难说了，也许和标号_undefined_instruction在同一个</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">页面，也许在很远的地方。不过除了reset，其他的异常是MMU开始工作之后才可能发生的，因此</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">undefined_instruction 的地址也经过了MMU的映射。<br>在刚加电的时候，CPU从0x0开始执行，MMU还没有开始工作，此时的虚拟地址和物理地址相同；另一方</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">面，重启在MMU开始工作后也有可能发生，如果reset也用ldr就有问题了，因为这时候虚拟地址和物理</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">地址完全不同。</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">因此，之所以reset用b，就是因为reset在MMU建立前后都有可能发生，而其他的异常只有在MMU建立之</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">后才会发生。用b reset，reset子程序与reset向量在同一页面，这样就不会有问题（b是相对跳转的）</p><p style="margin: 5px auto; padding: 0px; border: 0px; list-style: none; word-wrap: normal; word-break: normal; line-height: 21px; color: rgb(73, 73, 73); font-family: simsun; font-size: 14px; background-color: rgb(226, 226, 226);">。如果二者相距太远，那么编译器会报错的</p>
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