Editing CTIA and GTIA (section)

==History==

===2600 and TIA===
Atari had built their first display driver chip, the [[Television Interface Adaptor]] universally referred to as the TIA, as part of the [[Atari 2600]] console.<ref name=joe>Joe Decuir, [http://www.atariarchives.org/dev/CGEXPO99.html "3 Generations of Game Machine Architecture"], CGEXPO99</ref> The TIA display logically consisted of two primary sets of objects, the [[sprite (computer graphics)|"players" and "missiles"]] that represented moving objects, and the "playfield" which represented the static background image on which the action took place. The chip used data in [[memory register]]s to produce digital signals that were converted in realtime via a [[digital-to-analog converter]] and [[RF modulator]] to produce a television display.

The conventional way to draw the playfield is to use a [[bitmap]] held in a [[frame buffer]], in which each memory location in the frame buffer represents one or more locations on the screen. In the case of the 2600, which normally used a resolution of 160x192 pixels, a frame buffer would need to have at least 160x192/8 = 3840 bytes of memory. Built in an era where [[random access memory|RAM]] was very expensive, the TIA could not afford this solution.

Instead, the system implemented a display system that used a single 20-bit [[memory register]] that could be copied or mirrored on the right half of the screen to make what was effectively a 40-bit display. Each location could be displayed in one of four colors, from a palette of 128 possible colors. The TIA also included several other display objects, the "players" and "missiles". These consisted of two 8-bit wide objects known as "players", a single 1-bit object known as the "ball", and two 1-bit "missiles". All of these objects could be moved to arbitrary horizontal locations via settings in other registers.

The key to the TIA system, and the 2600's low price, was that the system implemented only enough memory to draw a single line of the display, all of which held in registers. To draw an entire screen full of data, the user code would wait until the television display reached the right side of the screen and update the registers for the playfield and player/missiles to correctly reflect the next line on the display. This scheme drew the screen line-by-line from program code on the [[ROM cartridge]], a technique known as "[[racing the beam]]".

===CTIA===
Atari initially estimated that the 2600 would have short market lifetime of three years when it was designed in 1976, which meant the company would need a new design by 1979.<ref name=joe/> Initially this new design was simply an updated 2600-like game console, and was built around a similar basic design, simply updated. Work on what would become the CTIA started in 1977, and aimed at delivering a system with twice the resolution and twice the number of colours. Moreover, by varying the number of colours in the playfield, much higher resolutions up to 320 pixels horizontally could be supported. Players and missiles were also updated, including four 8-bit players and four 2-bit missiles, but also allowing an additional mode to combine the four missiles into a fifth player.

Shortly after design began, 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.<ref name=joe/> In either role, a more complex playfield would be needed, especially support for [[character graphic]]s in the computer role. Design of the CTIA was well advanced at this point, so instead of a redesign a clever solution was provided by adding a second chip that would effectively automate the process of racing the beam. Instead of the user's programming updating the CTIA's registers based on its [[interrupt]] timing, the new [[ANTIC]] would handle this chore, reading data from a framebuffer and feeding that to the CTIA on the fly.

As a result of these changes, the new chips provide greatly improved number and selection of graphics modes over the TIA. Instead of a single playfield mode with 20 or 40 bits of resolution, the CTIA/ANTIC pair can display six text modes and eight graphics modes with various resolutions and color depths, allowing the programmer to choose a balance between resolution, colours, and memory use for their display.

===CTIA vs. GTIA===
[[File:Atari 8 bit GTIA NTSC palette.png|thumb|The 256 Color Palette used in the GTIA chip, 
(NTSC only)

with 16 hues, and 16 [[luminance]] values  
]]
[[File:Atari CTIA & TIA NTSC palette.png|thumb|233x233px|This is what the Atari TIA and CTIA used as a palette,
(NTSC only) with 16 hue, and only 8 luminance values,

making up 128 unique colors.
]]
The original design of the CTIA chip also included three additional color interpretations of the normal graphics modes. This feature provides alternate expressions of ANTIC's high-resolution graphics modes presenting 1 bit per pixel, 2 colors with one-half color clock wide pixels as 4 bits per pixel, up to 16 colors, two-color clock wide pixels. 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 extra color modes, were shipped until October–November 1981.<ref name="mmm"/><ref name="infoworld"/> From this point, all new Atari units were equipped with the new chip, now called GTIA, that supported the new color interpretation modes.<ref name="infoworld"/><ref name="creative"/>

The original Atari 800/400 operating system supported the GTIA alternate color interpretation modes from the start,<ref name="creative">{{cite book |last1=Small |first1=David |last2=Small |first2=Sandy |last3=Blank |first3=George |title=The Creative Atari |url=http://www.atariarchives.org/creativeatari/ |access-date=2011-01-26 |date=May 1983 |publisher=Creative Computing Press |isbn=978-0-916688-34-9 |chapter=Design Philosophy and GTIA Demos |chapter-url=http://www.atariarchives.org/creativeatari/Design_Philosophy_and_GTIA_Demos.php}}</ref> 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.<ref name="infoworld"/><ref name="compute-26"/>

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