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
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XMS 04H: Global Disable A20