A data processor includes a cache memory having a plurality of cache rows each row storing a cache line of data values, a memory management unit responsive to a page table entry to control access to a corresponding group of memory addresses forming a memory page, and a cache controller coupled to said cache memory and responsive to a cache miss to trigger a line fill operation to store data values into a cache row. The cache controller is responsive to a cache line size specifier associated with at least one page table entry to vary the number of data values within a cache line fetched in a line fill operation in dependence upon said cache line size specifier. Controlling cache line size on a page basis is more efficient than controlling cache line size on a cache row or virtual address basis.