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

A New Atari Graphics Mode?

I have been programming with my Atari 400 for almost a year now and have had a lot of fun with it. And recently, I have noticed advertisements in magazines about programs written in graphics mode 7.5. Is this a new graphics mode? If so, what does it and the other " .5" modes have over the others or GTIA?

David Brundage

There are no ".5" modes. GRAPHICS 7 is a 4-color mode with a horizontal resolution of 160 and a vertical resolution of 96. GRAPHICS 8 is the high-resolution, one-color (except through artifacting) mode with a resolution of 320h x l92v.

Built into the ANTIC chip is another mode that is not supported by the 400/800 operating system. It is a four-color mode with a resolution of l59h x 192v, the same vertical resolution as GRAPHICS 8. Since it is "halfway" between modes 7 and 8, it has been called GRAPHICS 7½ or GRAPHICS 7.5. You can turn a GRAPHICS 8 screen into this new mode by replacing all the 15's and 79's in the display list with 14's and 78's. (The hexadecimal code for the number 14, which is this mode's ANTIC number, is E, so GRAPHICS 7½ is sometimes called GRAPHICS E.) This will do the trick:

10 GRAPHICS 8 + 16 : DLIST = PEEK (560) + 256 * PEEK (561) + 4
20 FOR I = - 1 TO 200 : A = PEEK (DLIST + I) : IF A = 15 OR A = 79 THEN POKE DLIST + I, A - 1
30 NEXT I : POKE 87, 7

If you want a text window, just remove the " + 16" from line 10. The POKE on line 30 fools the operating system into thinking that you are in GRAPHICS 7. This lets you use only the top half of the screen with BASIC PLOTs and DRAWTOs, since the OS considers vertical numbers greater than 95 to be in error for GRAPHICS 7. The Atari 1200XL and all the new 600XL, 800XL, 1400XL, and 1450XL computers fully support this mode in the operating system and BASIC.