We propose a theoretical explanation of the parallel and perpendicular lamellar orientations in free surface films of symmetric polystyrene-block-polybutadiene diblock copolymers on silicon substrates (with a native SiOx layer). Two approaches are developed: A correction to the strong segregation theory and a qualitative analysis of the intermediate segregation regime. We show that the perpendicular orientation of the lamellae formed by the molecules of high molecular weight is stabilized by A-B interfacial interactions. They are weaker in the case of the perpendicular orientation of the lamellae, whereas the surface tension coefficient of the A-B interface decreases with the increase of the molecular weight.