Bit mask               index 08h  GRAPH

Layout  b7-b0  BM   Bit mask

        The bit mask register is used to mask certain bit positions
        from being modified.

        0 : All bits set to 0 mask the corresponding bits in the
            display memory; thus, these pixels cannot be modified
            during a host write operation.
        1 : All bits set to 1 allow the corresponding bits in the
            display memory to be modified by a write operation.