No BMODE, CMODE     BMODE or CMODE
                     +-----+------+     +-----+------+
      R0 ---> area:  | 26  |  0   |     | 26  |  0   |
                     +-----+------+     +-----+------+
                     |  reserved  |     |  reserved  |
                     +------------+     +------------+
                     |    buf     |     |    buf     |
                     +------------+     +------------+
                     |    wcnt    |     |    wcnt    |
                     +------------+     +------------+
                     |    crtn    |     |mapping bits|
                     +------------+     +------------+
                                        |    crtn    |
                                        +------------+