A lithographic apparatus comprises an array of individually controllable elements that modulate a beam of radiation, a compressed-pattern memory that stores a compressed representation of a requested dose pattern to be formed on a substrate by the modulated beam, and a dictionary decompressor that at least partially decompresses the compressed representation. The dictionary decompressor comprises an external dictionary memory, a first dictionary decoder that at least partially decompresses at least a part of the compressed representation by extracting, from the external dictionary memory, portions of pattern data corresponding to references to the external dictionary memory contained in the compressed representation, and a second dictionary decoder having an internal dictionary memory, the second dictionary decoder that at least partially decompresses at least a part of the compressed representation by extracting, from the internal dictionary memory, portions of pattern data corresponding to references to the internal dictionary memory contained in compressed representation.