ANTIC - Playfield Graphics Modes

Playfield Graphics Modes

The ANTIC chip allows a variety of different Playfield modes and widths. However, the original Atari Operating System included with the Atari 800/400 computers provides easy access to a limited subset of these graphics modes. ANTIC Playfield modes 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 can be freely mixed without CPU intervention by changing instructions in the Display List.

The ANTIC screen geometry is not fixed. A hardware register can direct ANTIC to display narrow playfield (128 color clocks/256 hi-res pixels wide), normal width playfield (160 color clocks/320 hi-res pixels wide), and wide, overscan playfield (192 color clocks/384 hi-res pixels wide). While the Operating System's default height for graphics modes is 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 NTSC display methods and limitations. 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 time needed to guarantee correct and consistent color regardless of the pixel's horizontal location on the screen. The result is accurate pixel size and coloring that does not display color "strobing" defects when horizontally scrolled. (Color strobing is unsightly hue changes in pixels based on horizontal position which is caused when signal changes do not align with the color clock and so do not provide the TV/monitor hardware adequate time to reach the correct color.)