line's scan line height - 1. The maximum value range covers fine scrolling 16 scan lines from 0 to 15. When the fine scrolling limit is reached, scrolling continues by resetting the VSCROL value and performing a coarse vertical scroll of the scrolling region. Horizontal Coarse Scrolling Horizontal coarse scrolling requires a little more effort than vertical scrolling. While horizontal scrolling is expected to present the illusion of a view port moving left and right across a wide panoramic scene made of screen memory, ANTIC's automatic memory scan increment conflicts with this idea that the rows of screen data is wider than the display. Presenting screen memory as long horizontal lines requires an LMS modifier for every Display List Text or Map Mode instruction in the scrolling region. A horizontal step is accomplished by incrementing or decrementing all the LMS addresses of the scrolling region. Horizontal scrolling requires the same Display List implementation as the worst case example described for Vertical Coarse Scrolling. So, where Horizontal scrolling is implemented, vertical scrolling is also supported just by changing the value incrementing or decrementing the LMS addresses. The other benefit of this arrangement is that it is easy to manage screen memory around ANTIC's 4K screen memory boundary limit. See the Vertical Coarse Scrolling section for further discussion and time constraints when updating LMS modifier addresses. Horizontal Fine Scrolling Horizontal Fine Scrolling allows single color clock movement of the scrolling region for Text and Map modes. For all normal color interpretations this is by color clock, not pixel even when pixels are larger (or smaller) than the color clocks. ANTIC Modes using the alternate GTIA color interpretations must be scrolled by an entire GTIA pixel (two color clocks). Only even values should be used to ensure correct GTIA pixel scrolling. Odd values of HSCROL will shift the GTIA pixel stream into a different state that will be interpreted as different colors. Fine scrolling requires setup to define the horizontal scrolling region. The first step is to start with a Display List using the LMS instruction modifier to specify starting address of each Mode line. This is done to organize screen memory as long rows of data for the display. The next step is setting the Horizontal Scroll (HS) modifier bit in all Display List Mode line instructions for the scrolling region. This number of color clocks to shift is controlled with the