Load Memory Scan
Bit6: position value $40, loads a new address for retrieving screen memory before retrieving screen memory for the current Playfield mode line.
The LMS option loads ANTIC's screen memory scan address with the 16-bit value in the two bytes following the instruction. The Character or Map mode specified will begin displaying bytes from that address. The LMS option must appear in the first Playfield Mode Instruction line (not Blank Line) in the Display List to initialize ANTIC to begin reading at the start of screen memory. Most of the Operating System-generated full screen modes will have only that one occurrence of the LMS modifier present in the Display List. ANTIC will automatically increment its LMS address at the end of each mode line in preparation for the next line. As long as screen memory does not cross over a 4K boundary the LMS modifier is not needed on subsequent Playfield Mode Instructions.
Full screen displays using Playfield Map Modes E or F will have a second occurrence of the LMS modifier on a Map Instruction near the middle of the screen, since the full display requires more than 4K of screen memory. (Mode F displays include high res 320 pixel lines, and the GTIA display modes.)
Combining the Map and Character Mode instructions with the LMS bit makes it possible to set the screen memory address freely within the 64K address space independently for each display line. In other words, the screen memory does not have to be completely contiguous memory scanned sequentially towards higher addresses - only that a single Mode line must be provided with adequate sequential bytes of memory to meet the requirements of that Mode, width of the display, and Scrolling feature.
Read more about this topic: ANTIC, The Display List, Instruction Modifiers, LMS Modifer
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