Port 020  -  8259A Command Port

        Send 20h here to signal end-of-interrupt (EOI).

          1xxx xxxx  Rotate Mode
          x1xx xxxx  Set Interrupt Level
          xx1x xxxx  End of Interrupt Mode
          xxx1 1xxx  Reserved - must be set to 0
          xxxx x111  Interrupt level (when bit 6=1)

          Bits 7-5:  These bits define the rotate mode, end of interrupt
                     mode, or a combination of the two.
                       000  Reserved
                       001  Nonspecific End of Interrupt Command
                       010  No operation
                       011  Specific End of Interrupt Command
                            (bits 2-0 select interrupt to end).
                       100  Reserved
                       101  Rotate on nonspecific End of Interrupt Command
                       110  Set prioriry Command
                            (bits 2-0 select interrupt to act on).
                       111  Rotate on specific end of interrupt command