simobj DiskImage(SimObject):
    abstract = True
    image_file = Param.String("disk image file")
    read_only = Param.Bool(false, "read only image")

simobj RawDiskImage(DiskImage):
    pass

simobj CowDiskImage(DiskImage):
    child = Param.DiskImage("child image")
    table_size = Param.Int(65536, "initial table size")
    image_file = ''