转载请注明出处： http://blog.csdn.net/weifenghai/article/details/52794872   概述： 内核中分配文件描述符时找第一个0的位置的一个底层函数，了解Linux内核嵌入式汇编知识参见《LINUX内核源代码情景分析(上)》中的1.5节《1.5Linux内核源代码中的汇编语言代码》，内核汇编指令参见《汇编语言程序设计(美)布鲁姆_着,马朝晖_等译》，代码摘自2.6.11.1内核。 函数代码： /**  *find_first_zero_bit - find the first zero bit in a memory region  *@addr: The address to start the search at  *@size: The maximum size to search  *  *Returns the bit-number of the first zero bit, not the number of the byte  *containing a bit.  */ static inline int find_first_zero_bit(constunsigned long *addr, unsigned size) {          intd0, d1, d2;          intres;            if(!size)                    return0;          /*This looks at memory. Mark it volatile to tell gcc not to move it around */          __asm__ __volatile__(                    "movl$-1,%%eax "                    "xorl%%edx,%%edx "                    "repe;scasl "                    "je1f "                    "xorl-4(%%edi),%%eax "                    "subl$4,%%edi "                    "bsfl%%eax,%%edx "                    "1: subl%%ebx,%%edi "                    "shll$3,%%edi "                    "addl%%edi,%%edx"                    :"=d"(res), "=&c" (d0), "=&D" (d1), "=&a"(d2)                    :"1"((size + 31) >> 5), "2" (addr), "b" (addr) :"memory");          returnres; } 代码分析： 输出部： 代码：:"=d" (res), "=&c" (d0), "=&D"(d1), "=&a" (d2) 解释： %0: res放入edx %1: d0放入ecx %2:d1放入edi %3:d2放入eax 输入部： 代码：:"1" ((size + 31) >> 5), "2" (addr),"b" (addr)     %4:与$1相同，(size + 31) >>5放入ecx     %5:与$2相同，addr放入edi     %6:addr 放入ebx 损坏部：: "memory" 指令部：                    "movl$-1,%%eax "：把eax的所有位都置成1                    "xorl%%edx,%%edx "：把edx置0                    "repe;scasl "：在edi（即addr）中搜索与eax不匹配的（即不全为1的）                    "je1f " 没有找到                    "xorl-4(%%edi),%%eax "  把eax中edi为0的位置1，其它位置0                    "subl$4,%%edi "  edi存当前找到的位置                    "bsfl%%eax,%%edx " edx存放eax中从右边第一个为1的位的索引，从0开始，edx存放目标字节内的偏移                    "1: subl%%ebx,%%edi "   现在edi中存放字节偏移位置                    "shll$3,%%edi "    edi左移3位，存偏整体移位数                    "addl%%edi,%%edx"   计算偏移位数   转载请注明出处： http://blog.csdn.net/weifenghai/article/details/52794872