CTIA and GTIA - History

History

The CTIA was designed in 1977 as part of the chipset for use in an improved successor of the Atari 2600 console. The 2600 used a chip known as the Television Interface Adaptor (TIA). In terms of graphics support, the design of the CTIA followed that of the TIA - it had a "Playfield" layer for background graphics along with several "Players" and "Missiles" (today known as sprites) for moving foreground objects. CTIA improved the number of Player/Missile objects, from two players and two missiles to four of each. TIA's fifth object called the "ball" was removed. In CTIA the four missiles could be combined to form a fifth Player of a different color, by setting a hardware register.

During development, the home computer revolution started in earnest in the later half of 1977. In response, Atari decided to release two versions of the new machine, a low-end model as a games console, and a high-end version as a home computer. In either role, a more complex playfield would be needed, especially support for character graphics in the computer role. For this purpose the new ANTIC chip was introduced to handle the storage and interpretation of a bitmap framebuffer, which the TIA did not support. Under the new model, the ANTIC would feed the CTIA with data, which would then be colored and sent into the video circuitry. This had the added advantage of greatly reducing programming complexity, compared to the "racing the beam" system used in the 2600.

As a result of these changes, the number and selection of graphics modes on the new models was greatly improved over the TIA. Instead of a single playfield mode with 20 or 40 bits of resolution, the CTIA/ANTIC could display six text modes and eight graphics modes with various resolutions and color depths, allowing the programmer to select a mode with the minimum memory needs they required for their display.

The original design of the CTIA circuit also included support for three additional color interpretations of the normal graphics modes. This featured provided alternate expressions of the Antic Mode F graphics (2 color, 320 pixel wide) as 16 color, 80 pixel wide displays. This feature was ready before the computers' November 1979 debut, but was delayed so much in the development cycle that Atari had already ordered a batch of about 100,000 CTIA chips with the graphics modes missing. Not wanting to throw away the already-produced chips, the company decided to use them in the initial release of the Atari 400 and 800 models in the US market. The CTIA-equipped computers, lacking the 3 color modes, were shipped until October–November 1981. From this point, all new Atari units were equipped with the new chip, now called GTIA, that supported the new color interpretation modes.

The original Atari 800/400 Operating System supported the GTIA alternate color interpretation modes from the start, which allowed for easy replacement of the CTIA with the GTIA once it was ready. Atari authorized service centers would install a GTIA chip in CTIA-equipped computers free of charge if the computer was under warranty; otherwise the replacement would cost $62.52.

GTIA was also mounted in all later Atari XL and XE computers and Atari 5200 consoles.