Compatibility: XMS 2.0+
Expects: AH    07H
                                                                  
Returns: AX    status: 0000H = A20 is physically enabled
0001H = error and BL=XMS error code (if BL >= 80H)
                                                                  
Info: This checks the state of the a20 line by testing for segment-
wrap.  The A20 line must be enabled befor accessin the HMA.  See
INT 2fH 4axxH for a way to get access to a suballocation of the
HMA.

If your TSR toggles A20 on popup, you should first use this fn to
get the current state, and remember to restore that setting
before popping down.

See Also: XMS Functions
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XMS 07H: Query A20 State