Port 00B  -  8237A-5    Mode register

        All PCs except PCjr: PC uses "single" mode for all channels.
        Initialization values: Channel 0=58h, 1=41h, 2=42h, 3=43h.

          11xx xxxx  00=demand mode
                     01=single mode
                     10=block mode
                     11=cascade mode
          xx1x xxxx  0=address increment
                     1=address decrement
          xxx1 xxxx  1=automatic reinitialize
          xxxx 11xx  00=verify
                     01=write
                     10=read
          xxxx xx11  00 = channel 0
                     01 = channel 1
                     10 = channel 2
                     11 = channel 3