GCC嵌入汇编代码课件.doc

呆木舟 Posts - 8? Articles - 11? Comments - 0? 【转】GCC 嵌入汇编代码 ?4.?Basic Inline. The format of basic inline assembly is very much straight forward. Its basic form is 基本汇编嵌入格式如下: asm(assembly code); Example. asm(movl?%ecx?%eax);?/*?moves?the?contents?of?ecx?to?eax?*/?? __asm__(movb?%bh?(%eax));?/*moves?the?byte?from?bh?to?the?memory?pointed?by?eax?*/?? You might have noticed that here I’ve used?asm?and?__asm__. Both are valid. We can use__asm__?if the keywordasm?conflicts with something in our program. If we have more than one instructions, we write one per line in double quotes, and also suffix a ’\n’ and ’\t’ to the instruction. This is because gcc sends each instruction as a string toas(GAS) and by using the newline/tab we send correctly formatted lines to the assembler. 可能注意到了这里使用了 asm 和 __asm__. 都是有效的.如果关键字asm在程序中有冲突, 则可以使用__asm__. 如果我们需要使用一条以上的汇编指令, 我们应该每条占用一行, 用双引号括起,并加上\n和\t后缀. 这是因为gcc把用字符串的格式把汇编指令传给as(GAS), 然后利用换行符, 把它们转换成正确的汇编格式. ?Example. __asm__?(movl?%eax,?%ebx\n\t?? ?????????movl?$56,?%esi\n\t?? ?????????movl?%ecx,?$label(%edx,%ebx,$4)\n\t?? ?????????movb?%ah,?(%ebx));?? If in our code we touch (ie, change the contents) some registers and return from asm without fixing those changes, something bad is going to happen. This is because GCC have no idea about the changes in the register contents and this leads us to trouble, especially when compiler makes some optimizations. It will suppose that some register contains the value of some variable that we might have changed without informing GCC, and it continues like nothing happened. What we can do is either use those instructions having no side effects or fix things when we quit or wait for something to crash. This is where we want some extended functionality. Extended asm provides us with that functionality. 如果我们的代码里使用了寄存器, 并且在返回的时候没有还原它, 这将有坏的情况发生. 因为GCC并不知道寄存器的值改变了, 特别是编译器对代码进行优化的时候. 编译器会认为,那些存放变量的寄存器,我们并没有改变它,然后继续自己的优化. 为了避免这种情况, 要么, 我们不改变寄存器的值, 要么, 汇编函数返回之前, 还原寄存器使用前的值, 或者 等着代码崩溃(wait for something to crash