A processor is programmed for accessing data-items from a matrix of rows and columns, access being constrained to a moving window. A cache memory caches data for the window. The cache memory makes a location used for a first data-item from an earliest row available for reuse when the window moves along the row direction, and retrieves a second data item for a latest row of the window into the cache memory. Data for the latest row may be written into the location just made available for reuse. The position of the first data-item along the row direction of the matrix trails the position of the second data-item along the row direction of the matrix at least by the width of the window.