XMS 06H: Local Disable A20

                                                     Compatibility: XMS 2.0+ 
 Expects: AH    06H
 Returns: AX    status: 0000H = successful
                        0001H = error and BL=XMS error code (if BL >= 80H)
    Info: This cancels a previous call to XMS 05H (local enable A20).

          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, and put back to to its former state before
          exiting.  Use XMS 07H to learn the state of A20 before enabling
          it (XMS 05H) and use this function (XMS 06H), if necessary, to
          put it back the way it was.

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