Playfield Graphics Capabilities
While the ANTIC chip allows a variety of different playfield modes and widths, the original Atari Operating System included with the Atari 800/400 computers provides easy access to a limited subset of these graphics modes. These are exposed to users through Atari BASIC via the "GRAPHICS" command, and to some other languages, via similar system calls. Oddly, the modes not directly supported by the original OS and BASIC are modes most useful for games. The later version of the OS used in the Atari 8-bit XL/XE computers added support for most of these "missing" graphics modes.
The ANTIC chip uses a Display List and other settings to create these modes. Any graphics mode in the default GTIA color interpretation could be freely mixed without CPU intervention by changing instructions in the Display List.
The actual ANTIC screen geometry is not fixed. The hardware could be directed to display a narrow playfield (128 color clocks/256 hi-res pixels wide), the normal width playfield (160 color clocks/320 hi-res pixels wide), and a wide, overscan playfield (192 color clocks/384 hi-res pixels wide) using a register value. While the Operating System's default height for graphics modes was 192 scan lines Antic can display vertical overscan up to 240 TV scan lines tall by creating a custom Display List.
The video display system was designed with careful consideration of the NTSC video timing for color output. The system CPU clock and video hardware are synchronized to one-half the NTSC clock frequency. Consequently, the pixel output of all display modes is based on the size of the NTSC color clock which is the minimum size needed to guarantee correct and consistent color regardless of the pixel location on the screen. The fundamental accuracy of the pixel color output allows horizontal fine scrolling without color "strobing" -- unsightly hue changes in pixels based on horizontal position caused when signal timing does not provide the TV/monitor hardware adequate time to reach the correct color.
Read more about this topic: Atari 8-bit Family
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