Port 008  -  8237A-5 Status/Command register

        All PCs except PCjr: Read returns status, write sets
        command.

        Status register bits:

          1xxx xxxx  Channel 3 request pending
          x1xx xxxx  Channel 2 request pending
          xx1x xxxx  Channel 1 request pending
          xxx1 xxxx  Channel 0 request pending
          xxxx 1xxx  Channel 3 ended
          xxxx x1xx  Channel 2 ended
          xxxx xx1x  Channel 1 ended
          xxxx xxx1  Channel 0 ended


        Command register bits:

          1xxx xxxx  0=DACK sense active low
          x1xx xxxx  0=DREQ sense active high
          xx1x xxxx  0=select late write (do not change)
          xxx1 xxxx  0=fixed priority (do not change)
          xxxx 1xxx  0=normal timing (do not change)
          xxxx x1xx  0=controller enabled
          xxxx xx1x    unused on PC - should be zero
          xxxx xxx1  0=memory to memory disabled
                       unused on PC - should be zero