INT 11 - BACK&FORTH (before v1.62) API
        AX = FFFEh
        CX = FFFEh
        BX = function
            00h installation check
                Return: AX = 0001h BNFHIGH and BNFLOW both loaded
                           = 0003h only BNFHIGH loaded
                           else neither loaded
            01h ???
                Return: DX:AX -> ???
            02h ???
            03h ???
            04h ???
            05h ??? switches current PSP segment and stack if BNFLOW has not
                    yet announced itself installed
            06h ???
                Return: AX = ???