An image data decompression apparatus for decoding blocks of motion compensated non-intra coded data uses a memory (14) storing reference picture data. A decoding processor (12) decodes a current block of a generated picture using lines of previously decoded image data from the memory (14) that are selected in dependence upon a motion vector (V.sub.1) for the current block. In order to improve access efficiency to the memory (14) the decoding processor (12) concatenates fetches into bursts for different sections of lines of previously decoded data that lie within a predetermined range within the memory addresses of the memory (14).