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
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XMS 05H: Local Enable A20