The lower four bits (D3-D0) of the SSCR define a horizontal delay of between 0 and 15 bits i.e. high resolution (mode 2) pixels. This shifts the screen image to the right by the value programmed , "losing" pixels behind the right border and instead displaying random data on the left. It is left to the programmer to ensure that the delay value is always a multiple of the number of bits per pixel.
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, . The effect is immediate and can be turned "losingon" what would otherwise be and "off" multiple times on the first lines same line. Note however to be displayedget the expected result over the whole screen (so that the display is shifted up), the value must equal 6845's R9 when the 6845's Horizontal Counter matches 6845's R1. When it is active at this time, the current MA is stored and instead appending extra lines to used on the bottom of next scanline. This allows the screento be scrolled correctly. 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 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.