Memory access circuitry (26) enforces ownership rights for memory regions. A given memory region is associated with an owner realm specified from multiple realms, each realm corresponding to a portion of at least one software process executed by processing circuitry (8). In response to a first variant of an exception return instruction the processing circuitry (8) returns from processing of an exception while staying within the same realm. In response to a second variant of the exception return instruction the processing circuitry switches processing from a current realm to a destination realm.