We numerically study morphology of symmetric diblock copolymer films with a thickness smaller than the bulk equilibrium period. These films are cast on either flat or corrugated substrates. Our results show that the formation of uniformly sized lateral domains that appear randomly on a flat surface can be controlled by using topographically patterned substrates. This is in excellent agreement with recent experimental studies. Control of lateral pattern fails if (i) the distance between the steps of the substrate is smaller than the bulk lamellar wavelength, or (ii) the lateral size of the corrugation is larger than the bulk lamellar wavelength.