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
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XMS 03H: Global Enable A20