Port 00A  -  8237A-5 Mask register

        All PCs except PCjr: Select DMA channel masks to enable or
        disable the channel.

          xxxx x1xx  1=set mask bit (disable), 0=clear mask bit (enable)
          xxxx xx11  00 = channel 0
                     01 = channel 1
                     10 = channel 2
                     11 = channel 3