depending on the resolution, the starting address must align to a 1K or 2K boundary (also see DMACTL). Bugs and Border Conditions Certain combinations of values in the Display List can produce unexpected results. Also, some of ANTIC's activities have critical timing. Changes to registers outside of proper time windows may result in delayed effect or unexpected results. Some of these situations are exploitable for interesting effects and others produce display corruptions. Exploitable Behaviors Graphics 9++ One method of using the enhanced GTIA color modes without the large memory requirements of ANTIC mode F is to direct ANTIC to repeat each line of data one or more times by using Mode line instructions with the LMS modifier to redisplay the same screen data on multiple scan lines. This produces a lower vertical resolution where each pixel is 2 or more scan lines tall, but has higher DMA overhead due to ANTIC needing to re-read the same bytes from screen memory. Rather than using repeated LMS modifiers, a quirk of vertical scrolling can be exploited to create a four scan-line mode that displays pixels using the hi-res or GTIA color interpretation modes. Vertical Scrolling would ordinarily not be considered useful for ANTIC Mode F which is one scan line tall. However, if Vertical Scrolling is enabled for the Mode F Display List instruction, and the VSCROL register is set to 13, then ANTIC's line counter will count from 13 to 0 (four scan lines—13, 14, 15, 0). In doing so ANTIC outputs the same line of graphics for each scan line from its internal buffer without re-reading the same screen memory again. The end of the vertical scrolling region needs to have VSCROL reset to 3 allowing ANTIC to count from 0 to 3 (four scan lines). The end result is a Mode F display where the lines are four scan lines tall rather than one, but with the DMA overhead of only one screen memory data read occurring on the first scan line. Unexploitable Behaviors Mode 8/Mode 9 Horizontal Scroll Corruption ANTIC Map Modes 8 and 9 will be corrupted when immediately following a horizontally scrolled line of a different mode using normal or wide playfield width. This occurs if the Mode 8 or 9 line is horizontally scrolled or not, but the corruption results are different in each case. The problem appears for certain HSCROL values causing ANTIC to