Editing Atari 8-bit computers (section)

=== ANTIC ===

[[ANTIC]] is a microprocessor which processes a sequence of instructions known as a [[display list]]. An instruction adds one row of the specified graphics mode to the display. Each mode varies based on whether it represents text or a bitmap, the resolution and number of colors, and its vertical height in [[scan line]]s. An instruction also indicates if it contains an interrupt, if fine scrolling is enabled, and optionally where to fetch the display data from memory.<ref>{{Cite book |last1=Small |first1=David |url=https://books.google.com/books?id=O4Y_AQAAIAAJ |title=The Creative Atari |last2=Small |first2=Sandy |last3=Blank |first3=George |date=1983 |publisher=Creative Computing Press |isbn=978-0-916688-34-9 |language=en |access-date=18 December 2023}}</ref>

Since each row can be specified individually, the programmer can create displays containing different text or bitmapped graphics modes on one screen, where the data can be fetched from arbitrary, non-sequential memory addresses.<ref>{{Cite book |url=https://books.google.com/books?id=9tsnHAAACAAJ |title=De Re Atari Anno Domini MCMLXXXI: A Guide to Effective Programming of the Atari 400/800 Home Computer |date=1981 |publisher=[[Atari, Inc.]] |language=en |access-date=18 December 2023}}</ref>

ANTIC reads this display list and the display data using DMA (Direct Memory Access), then translates the result into a pixel data stream representing the playfield text and graphics. This stream then passes to [[CTIA and GTIA|GTIA]] which applies the playfield colors and incorporates Player/Missile graphics ([[sprite (computer graphics)|sprite]]s) for final output to a TV or composite monitor. Once the display list is set up, the display is generated without any CPU intervention.

There are 15 character and bitmap modes. In low-resolution modes, 2 or 4 colors per display line can be set. In high-resolution mode, one color can be set per line, but the luminance values of the foreground and background can be adjusted. High resolution bitmap mode (320x192 graphics) produces NTSC [[composite artifact colors]]; these colors do not occur on PAL machines.

For text modes, the [[character set]] data is pointed to by a register. It defaults to an address in ROM, but if pointed to RAM then a programmer can create custom characters. Depending on the text mode, this data can be on any 1K or 512 byte boundary. Additional registers flip all characters upside down and toggle inverse video.

[[File:Atari8.png|thumb|A [[Moiré pattern]] in 320 horizontal pixel graphics mode creates [[artifact colors]] from displaying hi-res pixels which are half the size of the NTSC color clock.]]

The [[ANTIC]] chip allows a variety of Playfield modes and widths, and the original Atari Operating System included with the Atari 800/400 computers provides easy access to a 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. The later version of the OS used in the XL/XE computers added support for most of these "missing" graphics modes.

ANTIC text modes support soft, redefineable character sets. ANTIC has four different methods of glyph rendering related to the text modes: Normal, Descenders, Single color character matrix, and Multiple colors per character matrix.

The ANTIC chip uses a display list and other settings to create these modes. Any graphics mode in the default [[CTIA and GTIA|CTIA/GTIA]] color interpretation can be freely mixed without CPU intervention by changing instructions in the display list.

The actual ANTIC screen geometry is not fixed. The hardware can 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) by setting a register value. The operating system's default height for creating graphics modes is 192 scan lines, and ANTIC can display vertical overscan up to 240 TV scan lines tall by creating a custom display list.

The display list capabilities provide [[ANTIC#Scrolling|horizontal and vertical coarse scrolling]] requiring minimal CPU direction. Furthermore, the ANTIC hardware supports horizontal and vertical fine scrolling—shifting the display of screen data incrementally by single pixels (color clocks) horizontally and single scan lines vertically.

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.