The disk subsystem uses two ports on the system's  8520 CIA  chips, and
several registers in the Paula chip:

   CIAAPRA    ($BFE001)    four input bits for disk sensing
   CIABPRB    ($BFD100)    eight output bits for disk selection, control
                             and stepping
   ADKCON     ($DFF09E)    control bits (write only register)
   ADKCONR    ($DFF010)    control bits (read only register)
   DSKPTH     ($DFF020)    DMA pointer (32 bits)
   DSKLEN     ($DFF024)    length of DMA
   DSKBYTR    ($DFF01A)    Disk data byte and status read
   DSKSYNC    ($DFF07E)    Disk sync finder; holds a match word

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