INT 16 - PC Tools v5.1+ DESKTOP API - GET/SET???
        AX = FFB8h
        BH = subfunction
            00h get
                Return: BL = old value of ???
                        CL = old value of ??? (v6.0+)
                        CH = old value of ??? (v6.0+)
            nonzero set
                BL = new value for ???
                CL = new value for ??? (v6.0+)
                CH = new value for ??? (v6.0+)
                DH = ???
                Return: AL = old value replaced by CL (v6.0+)
                        AH = old value replaced by CH (v6.0+)