No BMODE            BMODE
                     +-----+------+     +-----+------+
      R0 ---> area:  | 25  |  0   |     | 25  |  0   |
                     +-----+------+     +-----+------+
                     |   unused   |     |   unused   |
                     +------------+     +------------+
                     |    buf     |     |    buf     |
                     +------------+     +------------+
                     |    wcnt    |     |    wcnt    |
                     +------------+     +------------+
                     |     0      |     |mapping bits|
                     +------------+     +------------+
                                        |     0      |
                                        +------------+