modifier option on every line that does scroll. Vertical scrolling on this display requires updating one LMS modifier address for each Mode line in the scrolling region. When the display is managed at this level of detail ANTIC's 4K screen RAM boundary can be easily circumvented. Even in this worst case example the CPU effort expended is a tiny fraction of the processing needed to bulk move screen data. One other limit to be aware of is that the two byte LMS modifier address must not be updated when ANTIC is processing the LMS. If ANTIC reads the LMS address while it is partially updated ANTIC will display the incorrect section of memory for that Mode line. When the scrolling region of a display requires multiple LMS address updates the possibility of a partial update increases, and the possibility of having two subsequent lines display the same data also increases. Programs solve this in a number of ways—performing updates during the Vertical blank, monitoring the VCOUNT before updates, or doing the updates in Display List Interrupts executed during sections of the display away from the scrolling region. Vertical Fine Scrolling Vertical Fine Scrolling allows single scan line movement of the scrolling region for Text and Map modes which are greater than one scan line tall. Vertical Fine Scrolling for Map modes only one scan line tall is logically not practical. The effect of vertical "fine" scrolling for single scan line Map modes can be performed using the easier coarse scrolling method. Fine scrolling requires setup to define the scrolling region. This is done by setting the Vertical Scroll (VS) modifier bit in sequential Display List instructions. The first Mode line instruction without the VS bit set becomes the end of the scrolling region and is used as a buffer line to supply the new scan lines to scroll up into the bottom of the scrolling region. ANTIC's process of vertical fine scrolling works by simply directing ANTIC to skip a number of scan lines for the first mode line of the scrolling region. Subsequent Mode lines in the scrolling region are not affected. The last line of the region supplies the bottom border of the scrolling region displaying the number of scan lines that were removed from the first line. This number of scan lines is controlled with the VSCROL register. The value ranges from 0 to the ANTIC Mode