<'
struct structs_units3 { 
  addr : byte; 
  // True it is write
  rd_wr: bool; 
  // Write data is present only during write operation
  when TRUE'rd_wr structs_units3 {
    data : byte;
  };
}; 

extend sys {
  obj : structs_units3;
  // This method shows a sub type can accessed
  print_obj () is {
    if (obj.rd_wr == TRUE) {
      out ("Access Type    is : Write");
      outf("Access Address is : %x\n",obj.addr);
      // To access subtype object is bit complicated, as it does not exit
      // in nomal struct
      outf("Access Data    is : %x\n",obj.as_a(TRUE'rd_wr structs_units3).data);
    } else {
      out ("Access Type    is : Read");
      outf("Access Address is : %x\n",obj.addr);
      // Below code if you uncomment it will compile, but will give run time
      // error, as data does not exist when rd_wr is false
      //outf("Access Data    is : %x\n",obj.as_a(TRUE'rd_wr structs_units3).data);
    };
  }; 
  // Just generate the obj and print it
  run() is also {
    for {var i : int = 0; i < 4; i = i + 1} do {
      gen obj;
      print_obj(); 
    };
  };
};
'>