Port 3CF, 03  -  Data Rotate Register

        111x xxxx  Reserved
        xxx1 1xxx  Function select
                     Data written to the video buffer can be operated
                     on logically by data already in the system latches.
                     Data can by any of the choices selected by the
                     write mode bits except system latches, which
                     cannot be modified. If rotated data is selected
                     also, the rotate is performed before the logical
                     operation. The logical operations selected are shown
                     below.
        xxxx x111  Rotation count
                     In write mode 0, these bits select the number of
                     positions the system data is rotated to the right
                     during a system memory write operation. To write
                     data that is not rotated in mode 0, all bits are
                     set to 0.


                Function select
                Bits
                4 3   Function                 
                0 0   Data unmodified
                0 1   Data ANDed with latched data
                1 0   Data ORed with latched data
                1 1   Data XORed with latched data