624



     and circles!

     GTIA modes are cleared on the OPEN command. How can you
     tell if you have the GTIA chip? Try POKE 623,64. If you have the
     GTIA, the screen will go all black. If not, you don't have it. Here
     is a short routine, written by Craig Chamberlain and Sheldon
     Leemon for COMPUTE!, which allows an Atari to test itself for the
     presence of a CTIA or GTIA chip. The routine flashes the answer
     on the screen, hut can easily be modified so a program will
     "know" which chip is present so it can adapt itself accordingly:

     10 POKE 66,1:GRAPHICS 8:POKE 709,0:PO
        KE 710,0:POKE 66,0:POKE 623,64:P0K
        E 53248,42:POKE 5326l,3:PUT#6,1
     20 POKE 53278,0:FOR K=1 TO 300:NEXT K
        :GRAPHICS 18:POKE 53248,0:POSITION
         8,5:? #6;CHR$(71-PEEK(53252));"TI
        A"
     30 POKE 708,PEEK(20):GOTO 30

     How can you get the GTIA if you don't have one? Ask your local
     Atari service representative or dealer, or write directly to Atari in
     Sunnyvale, California.

     See the GTIA/CTIA introduction at location 53248 ($D000) for
     more discussion of the chip. See BYTE, May 1982, COMPUTE!,
     July through September 1982, and De Re Atari for more on the
     GTIA chip, and the GTIA Demonstration Diskette from the Atari
     Program Exchange (APX).

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Locations 624 to 647 ($270 to $287) are used for game controllers:
paddle, joystick and lightpen values.

624            270            PADDL0
     The value of paddle 0 (paddles are also called pots, short for
     potentiometer); PEEK 624 returns a number between zero and
     228 ($E4), increasing as the knob is turned counter-clockwise.
     When used to move a player or cursor (i.e., PLOT
     PADDLE(0),0), test your screen first. Many sets will not display
     locations less than 48 ($30) or greater than 208 ($D0), and in
     many GRAPHICS modes you will get an ERROR 141 -- cursor
     out of range. Paddles are paired in the controller jacks, so paddle
     0 and paddle 1 both use jack one. PADDL registers are shadows
     for POKEY locations 53760 to 53767 ($D200 to $D207).

625            271            PADDL1
     This and the next six bytes are the same as 624, but for the other
     paddles.

626            272            PADDL2