When storing an array of data in memory, the data array is divided into a plurality of blocks, and for respective groups of the blocks that the data array has been divided into, a set of data representing the group of blocks that includes: for each block of the group of blocks, a set of data for that block of the group of blocks; and a size indication for each of one or more of the blocks of the group of blocks, the size indication for a block of a group of blocks indicating the size in memory of the set of data for that block of the group included in the stored set of data representing the group of blocks, is stored. A set of header data is also stored separately for each group of blocks of the data array.