Changes
/* Soft scroll facility */
The next three bits (D6-D4) will be added to the least significant three bits of the scan line address, thus determining which of the eight 2k blocks contains the data for the first scan line on the screen. the effect of this is to shift the display up by the number of scan lines programmed, "losing" what would otherwise be the first lines to be displayed, and instead appending extra lines to the bottom of the screen.
The RA output of the 6845 is defined by the current raster count plus the soft scroll value(probably ANDed with &1f). Note that this value is compared against R9 to determine when MA should be updated to indicate the next char line, it doesn't seem to effect the 6845's internal char line counter and raster counter, this explains why it behaves strangely when R9<7 is used, but works ok when R9>=8.
The most significant bit (D7), when set, causes the border to extend over the first two bytes (16 high resolution pixels) of each scan line, masking out the bad data caused by the horizontal soft scroll. Software which intends to use horizontal soft scroll should have this bit always set, so that the screen width does not keep changing.
