APPENDIX ELEVEN



     After this is done, POKE 53769,0 ($D209). Now enable the inter-
     rupt: POKE 16 with PEEK(16) plus the number of the interrupt
     you're using (1 = timer 1 interrupt, 2 = timer 2, 4 = timer 4--
     there's no timer 3!). POKE the same value into 53774. Your inter-
     rupt routine will begin; it will generate an interrupt when the
     timer counts down to zero. The timer is reloaded with the orig-
     inal value you POKEd there, and the process begins all over
     again.

     There are several problems to watch for: First, the OS pushes
     the A register onto the stack before jumping through the vector
     address. If you need the X and Y registers, push them on as
     well. Before you return from the interrupt, pull the X and Y back
     off, PLA, and clear the interrupt with CLI.

     If you don't need the screen display, POKE 559,0 to turn it off;
     DMA steals clock cycles from the timer, This means you'll have
     to make any commands which deal with shadow registers (like
     SETCOLOR and GRAPHICS) first. DMA also turns off the keyboard
     repeat and realtime clock. Disable the keyboard to gain a bit
     more time if necessary.

     Refer to Micro and ROM, December 1984, for more information
     about POKEY timers.

555            22B            SRTIMR
     Each time you read this location, you get a different number.
     That's because it's counting down from when a key is de-
     pressed to time the delay before repeating the key.

558            22E            CDTMF5
     Set when location 544,545 ($220,$221) counts down to zero. From
     Joe Gelman, Atari SIG, CompuServe.

570            23A            CDEVIC
     The current SIO bus ID (device) number.

632            278            STICK0
     The pins on the joystick port are mapped as follows:

     _______________________________
     \                             /
      \ 1     2     3     4     5 /
       \                         /
        \  6     7     8     9  /
         \_____________________/

     1 Stick forward
     2 Stick back
     3 Stick left