This reg is the audio channel x (x=0,1,2,3) DMA
         data buffer. It contains 2 bytes of data (each
         byte is a twos complement signed integer) that
         are outputed sequentially (with digital to analog
         conversion)to the audio output pins. With maximum
         volume, each byte can drive the audio outputs
         with 0.8 volts(peak to peak,typ). The audio DMA
         channel controller automatically transfers data
         to this reg from RAM. The processor can also
         write directly to this reg. When the DMA data is
         finished (words outputted=length) and the data in
         this reg has been used, an audio channel interrupt
         request is set.
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