what's the meaning of "%%fs" ?

[lx]

hi all:
       the codes is:

extern inline unsigned char get_fs_byte(const char * addr)
{
        unsigned register char _v;

        __asm__ ("movb %%fs:%1,%0":"=r" (_v):"m" (*addr));
        return _v;
}

What's the meaning of "%%fs" ?

Is "  __asm__ ("movb %1,%0":"=r" (_v):"m" (*addr));" not ok?


Thank you.
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