53760-54015



     lower value (both are set to 240 above). To change the tone
     duration, you POKE 1538 with a lower value (it is set to 255 in the
     routine above). Do these before you do your USR call or alter the
     DATA statements to permanently change the values in your own
     program. Turn off DMA (see location 559) to get clearer tones.

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Locations 53280 to 53503 ($D020 to $D0FF) are repeats of locations
53248 to 53279 ($D000 to $D01F). You can't use any of the repeated
locations; consider them "filler." They maybe used for other purposes
in any Atari OS upgrade.

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Locations 53504 to 53759 ($D100 to $D1FF) are unused. These loca-
tions are not empty; you can PEEK into them and find out what's
there. They cannot, however, be user-altered.

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POKEY

53760-54015    D200-D2FF
     POKEY is a digital I/O chip that controls the audio frequency and
     control registers, frequency dividers, poly noise counters, pot
     (paddle) controllers, the random number generator, keyboard
     scan, serial port I/O, and the IRQ interrupts.

     The AUDF# (audio frequency) locations are used for the pitch for
     the corresponding sound channels, while the AUDC# (audio
     control registers) are the volume and distortion values for those
     same channels. To POKE sound values, you must first POKE zero
     into locations 53768 ($D208) and a three into 53775 ($D20F).

     Frequency values can range from zero to 255 ($FF), although the
     value is increased by the computer by one to range from one to
     256. Note that the sum of the volumes should not exceed 32, since
     volume is controlled by the least four bits. It is set from zero as no
     volume to 15 ($F) as the highest. A POKE with 16 ($10) forces
     sound output even if volume is not set (i.e., it pushes the speaker
     cone out. A tiny "pop" will be heard). The upper four bits control
     distortion: 192 ($C0) is for pure tone; other values range from 32 to
     192. Note that in BASIC, the BREAK key will not turn off the
     sound; RESET will, however. See De Re Atari and BYTE, April
     1982, for more information on sound generation.

     The AUDF registers are also used as the POKEY hardware timers.
     These are generally used when counting an interval less than one
     VBLANK. For longer intervals, use the software timers in locations
     536 to 545 ($218 to $221). You load the AUDCTL register with the