So we're getting virtual address of the boot_params structure and pass it to the copy_bootdata function, where we copy real_mod_data to the boot_params which is declared in the arch/x86/include/asm/setup.h

Improvement, fix typo and error(as for the current code, maybe it is not a error before):

So we're getting virtual address of variable boot_params which come along from real mode, and pass it to the copy_bootdata function, where we copy real_mode_data to the boot_params which is defined in the arch/x86/kernel/setup.c

And, of course

    extern struct boot_params boot_params;

should be changed to:

    struct boot_params boot_params;