Port 009  -  8237A-5 Request register

        Provided to generate DMA block mode request by software.
        Unused on PC - Not supported by PS.

          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