XMS 07H: Query A20 State

                                                     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

          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
          Extended Memory Specification (XMS)
          DOS Functions