宏操作_ _copy_user_zeroing()的定义在include/asm-i386/uacces.h中：
#define   _ _copy_user_zeroing(to,from,size)       
do{                                                              
int _ _d0, _ _d1;
_ _asm_ _     _ _volatile_ _(
                 "0:rep:movln"                             
                 "  movl  %3, %0n"                      
                 "1: rep:movsbn"                         
                 "2:n"                                           
                 ".section .fixup,"ax"n"              
                 "3: lea 0(%3,%0,4),%0n"           
                 "4: pushl %0n"                           
                 "    pushl %%eaxn"                    
                 "    xorl   %%eax,%%eaxn"        
                 "    rep; stosbn"                          
                 "    popl  %%eax n"                    
                 "    popl %0n"                             
                 "    jmp 2bn"                               
                 ".previousn"                               
                 ".section _ _ex_table, "a"n"    
                 "    .align 4n"                              
                 "    .long  0b,3bn"                       
                 "    .long  1b,4bn"                       
                 ".previous"                            
                 : "=&c"(size),  "=&D" (_ _d0),  "=&S" (_ _d1 )       
                 : "r"(size&3),  "0"(size/4),  "1"(to), "2"(from)        
                 : "memory");           
}while(0)
这段代码如何理解呢？同时高手给我提供一份GCC的手册，谢谢！

可以GOOGLE。
情景分析第三章系统调用一节做了非常详细的分析。