A realm management unit (RMU) maintains an ownership table specifying ownership entries for corresponding memory regions defining ownership attributes specifying, from among a plurality of realms, an owner realm of the corresponding region. Each realm corresponds to at least a portion of at least one software process. The owner realm has a right to exclude other realms from accessing data stored in the corresponding region. Memory access is controlled based on the ownership table. In response to a region fuse command specifying a fuse target address indicative contiguous regions of memory to be fused into a fused group of regions, a region fuse operation updates the ownership table to indicate that the ownership attributes for the fused group of regions are represented by a single ownership entry. This provides architectural support for enabling improvement of TLB performance.