Realm execution context masking and saving

Memory access circuitry 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 realm switch from a source realm to a target realm at a more privileged exception level, state masking of a subset of architectural state associated with a source realm is performed to make the state inaccessible to a target realm. In response to a flush command following the realm switch, any of the subset of architectural state data not already saved to at least one realm execution context memory region is ensured to be saved.