XMS 04H: Global Disable A20

                                                     Compatibility: XMS 2.0+ 
 Expects: AH    04H
 Returns: AX    status: 0000H = successful
                        0001H = error and BL=XMS error code
    Info: This disables the A20 line, making the HMA inaccessible
          (FFFF:0000 will point to someplace in low memory).

          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.

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