=== Hardware scrolling ===
The start address can be manipulated to achieve horizontal and/or vertical hardware scrolling. However, it is a position in word (16 bits), not in byte. So , the screen will move two bytes at a time horizontally. Also, the precision will be one CRTC character vertically.
The first game that used R12/R13 for both horizontal and vertical hardware scrolling is [https://www.cpc-power.com/index.php?page=detail&num=1839 Roland on the Ropes] in 1984.