Classic Computer Magazine Archive COMPUTE! ISSUE 41 / OCTOBER 1983 / PAGE 10

Atari Machine Language Graphics

I own an Atari 400 and would like to know how to change graphics modes in machine language. Also, I'd like to know how to change the background colors in the GTIA modes.

Bim Feysteryga

All the graphics routines you find in BASIC (GRAPHICS, PLOT, COLOR, DRAWTO, etc.) can be accessed from the operating system. Refer to Bill Wilkinson's "Insight: Atari" in the February 1982 issue. For complete information on the GTIA, you will find a series of illuminating articles in COMPUTE!'s First Book of Atari Graphics.

The background color in GTIA modes 9 and 11 is, as usual, in color register four (SETCOLOR 4, POKE 712). In mode 9 you can only control the color, not the luminance, and the reverse is true for mode 11. The background color in GTIA mode GRAPHICS 10 is controlled by memory location 704. POKE it with the color multiplied times 16 and then add the luminance. Instead of SETCOLOR n, 4, 6 use POKE 704, 4 * 16 + 6.

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