CTIA and GTIA - GTIA Enhancements

GTIA Enhancements

The GTIA chip was backward compatible with the CTIA, and added 3 color interpretations for the 14 "normal" graphics modes. With the normal color interpretation of the CTIA chip the Atari was limited to a maximum of 4 colors in graphics and 5 colors in text, unless special programming techniques were used. The 3 new color interpretations in GTIA theoretically provided a total of 56 graphics modes (14 modes multiplied by four possible color interpretations). However, only the graphics modes based on half color clock pixels (that is, Antic text modes 2, 3, and graphics mode F) are capable of fully expressing the color palettes of these 3 new color interpretations. The three additional color interpretations use the information in two color clocks (four bits) to generate a pixel in one of 16 color values. This changes a mode F display from 2 colors per pixel, 320 pixels horizontally, one scanline per mode line, to 16 colors and 80 pixels horizontally. The additional color interpretations allowed the following:

• 16 shades of a single hue from the 16 possible hues in the Atari palette. This is also accessible in Atari BASIC as Graphics 9.
• 16 hues in a single shade/luminance value. This is accessible in Atari BASIC as Graphics 11.
• Finally, the last mode, accessible in Atari BASIC as Graphics 10, allows 9 colors per horizontal line in any hue and luminance from the entire Atari palette of 128 colors. This is accomplished using all five playfield color registers, and the four player/missile color registers.

Of these modes, Atari BASIC Graphics 9 is particularly notable. It enabled the Atari to display gray-scale digitized photographs, which despite their low resolution were very impressive at the time. Additionally, by allowing 16 shades of a single hue rather than the 8 shades available in other graphics modes, it increased the amount of different colors the Atari could display from 128 to 256. Unfortunately this feature was limited for use in this mode only, which due to its low resolution was not widely used.

The Antic 2 and 3 text modes are capable of displaying the same color ranges as mode F graphics when using the GTIA's alternate color interpretations. However, since the pixel reduction also applies and turns 8 pixel wide, 2 color text into 2 pixel wide, 16 color blocks these modes are unsuitable for actual text, and so these graphics modes are not popular outside of demos. Effective use of the GTIA color interpretation feature with text modes requires a carefully constructed character set treating characters as pixels. This method allows display of an apparent GTIA "high resolution" graphics mode that would ordinarily occupy 8K of RAM to instead use only about 2K (1K for the character set, and 1K for the screen RAM and display list.)

The GTIA also fixed an error in CTIA that caused graphics to be misaligned by "half a color clock". The side effect of the fix was that programs that relied on color artifacts in high-resolution monochrome modes would reverse their colors.

Atari owners can determine if their machine is equipped with the CTIA or GTIA by executing the BASIC command POKE 623,64. If the screen blackens after execution, the machine is equipped with the new GTIA chip. If it stays blue, the machine has a CTIA chip instead.