INT 2F - MS Windows - "V86MMGR" VIRTUAL DEVICE API
        AX = 1607h
        BX = 0006h (VxD identifier of "V86MMGR")
        CX = 0000h
Return: AX = status
            0000h if local A20 state changed
            1607h if A20 unchanged
            other if global A20 state changed

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