XMS 03H: Global Enable A20

                                                     Compatibility: XMS 2.0+ 
 Expects: AH    03H
 Returns: AX    status: 0000H = successful
                        0001H = error and BL=XMS error code (if BL >= 80H)
    Info: This enables the A20 line, making the HMA accessible, starting
          at FFFF:0000.

          Only the owner of the HMA should mess with this; see XMS 01H
          (request HMA).

   Notes: ■ Toggling A20 is relatively slow on some machines.

          ■ TSR owners of the HMA who toggle A20 on popup, should first get
            the current state via XMS 07H, and restore that setting before
            popping down.

            Setting A20 disables segment wrap-around--on which a few
            poorly-written programs depend.

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