CTIA and GTIA - Features

Features

The CTIA/GTIA is a television interface chip that performs the following functions:

  • It is responsible for interpreting the Playfield graphics data stream from ANTIC to apply color to the display.
  • The GTIA version includes three alternate color interpretation modes for the Playfield graphics.
    • 16 shades of a single hue from the 16 possible hues in the Atari palette. This is accessible in Atari BASIC as Graphics 9.
    • 16 hues in a single shade/luminance value. This is accessible in Atari BASIC as Graphics 11.
    • 9 colors per horizontal line in any hue and luminance from the entire Atari palette of 128 colors accomplished using all five playfield color registers, and the four player/missile color registers. This is accessible in Atari BASIC as Graphics 10.
  • It merges four Player and four Missile overlay objects (aka sprites) with the Playfield graphics.
  • It checks for collisions between the Players, Missiles, and the Playfield graphics.
  • It is responsible for reading the state of the joysticks' triggers (bottom buttons only in case of Atari 5200 controllers).
  • It contains four input/output pins that are used in different ways depending on the system:
    • In Atari 8-bit computers, three of the pins are used to read state of the console keys (Start/Select/Option).
    • The fourth pin controls the speaker built into the Atari 400/800, used to generate keyboard clicks. On later models there is no internal speaker, but the keyclick is still generated by GTIA and mixed with the regular audio output.
    • In the Atari 5200, the pins are used as part of the process that reads the state of the joysticks' keyboards.

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