do_copy() , do_compare(), do_byte_pack() , do_is_vaild() , do_allocate() & do_byte_unpack() !!!

do_*  methods overrides the default implementation of the vmm_data methods that is created by the shorthand macros. If the do_* methods are defined, this method is used instead of the default implementation. do_* methods are very useful can work well with short hand macros example you can just override the default implementation of  byte_pack() & byte_unpack() methods using do_byte_pack and do_byte_unpack and rest of the vmm_data methods can be auto generated by short hand macros.