XMS 05H: Local Enable A20

                                                     Compatibility: XMS 2.0+ 
 Expects: AH    05H
 Returns: AX    status: 0000H = successful
                        0001H = error and BL=XMS error code (if BL >= 80H)
    Info: This attempts to enable the A20 line.  It should only be used by
          programs which need direct access to the HMA.

          If you have used INT 2fH 4a02H to get an HMA suballocation from
          DOS, you must be certain that the A20 line is enabled before
          accessing that memory.  For performance reasons, use XMS 07H and
          then enable it onoly if it is not already enabled.  Use XMS 06H
          to disable it later (if necessary) to put it back the way it was.

    Note: If the HMA is owned by DOS, then it is highly likely that A20 is
          enabled all of the time.

See Also: XMS Functions
          Extended Memory Specification (XMS)
          DOS Functions