We present results from a numerical study of a coarse-grained model of di-block copolymer (BCP) thin films confined between two hard walls. One of these walls is neutral to the components of the BCP melt and the other one contains chemical inhomogeneities with a repeat spacing length scale comparable to the linear size of the BCP molecules. We find that the morphology of the BCP thin film is strongly influenced by the commensurability between the bulk unconstrained lamellar size lambda*, and the linear size of the surface inhomogeneities w. When w approximate to lambda*/2, the ordered morphology of the diblock copolymers has a strong overlap with the pre-assigned substrate chemical pattern throughout the film. However, for w approximate to lambda*, the overlap strongly depends on the distance from the substrate surface. Close to the substrate surface, the overlap of the morphology with the pre-assigned chemical pattern is large but the pattern becomes out-of-phase at a distance of approximately lambda*/2 from the substrate. For w approximate to 3 lambda*/4, the morphology of the diblock copolymers is either similar to that for w approximate to lambda*/2 or for w approximate to lambda*, depending on the film thickness. For some film thickness, the morphology shows a mixed characteristics of the morphologies for w approximate to lambda*/2 and w approximate to lambda*.