1. A method of data processing in a data processing system having a system memory and a cache hierarchy that caches contents of said system memory, said method comprising: receiving a storage modifying operation having a cacheable target real memory address; determining whether or not said storage modifying operation has an associated bypass indication; in response to determining that said storage modifying operation has an associated bypass indication, bypassing the cache hierarchy and performing an update indicated by said storage modifying operation in said system memory; and in response to determining that said storage modifying operation does not have an associated bypass indication, performing said update indicated by said storage modifying operation in said cache hierarchy.