hi about C and asm

hi all: They are some asm codes in the kernel like: #define set_bit(nr,addr) ({\ register int res __asm__("ax"); \ __asm__ __volatile__("btsl %2,%3\n\tsetb %%al": \ "=a" (res):"0" (0),"r" (nr),"m" (*(addr))); \ res;}) But, almost I can't understand this. So I want to understand asm in C, which book or website I should learn ? Thank you