hi all How to understand some codes in the function of __FD_SET?

[lx]

hi all:
       the codes of functions is:

 51 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L51>
#undef  __FD_SET <http://lxr.oss.org.cn/ident?v=2.6.16;i=__FD_SET>
 52 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L52>
static __inline__ void __FD_SET
<http://lxr.oss.org.cn/ident?v=2.6.16;i=__FD_SET>(unsigned long __fd,
__kernel_fd_set
<http://lxr.oss.org.cn/ident?v=2.6.16;i=__kernel_fd_set> *__fdsetp)
 53 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L53> {
 54 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L54>
        unsigned long __tmp = __fd / __NFDBITS
<http://lxr.oss.org.cn/ident?v=2.6.16;i=__NFDBITS>;
 55 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L55>
        unsigned long __rem = __fd % __NFDBITS
<http://lxr.oss.org.cn/ident?v=2.6.16;i=__NFDBITS>;
 56 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L56>
        __fdsetp->fds_bits[__tmp] |= (1UL<<__rem);
 57 <http://lxr.oss.org.cn/source/include/asm-sh/posix_types.h?v=2.6.16#L57> }


I can't understand the usage of __rem,How to understand it? thank you.

PS:


 21 <http://lxr.oss.org.cn/source/include/linux/posix_types.h?v=2.6.16#L21>
#undef __NFDBITS <http://lxr.oss.org.cn/ident?v=2.6.16;i=__NFDBITS>
 22 <http://lxr.oss.org.cn/source/include/linux/posix_types.h?v=2.6.16#L22>
#define __NFDBITS <http://lxr.oss.org.cn/ident?v=2.6.16;i=__NFDBITS>
    (8 * sizeof(unsigned long))
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