1. A memory system for storing and retrieving data for a processing system, the memory system comprising: a plurality of memory devices; a memory controller for receiving and responding to memory access requests including memory update requests from a processor and for generating a memory update command in response to receiving a memory update request; one or more memory busses connected to the memory controller; and a memory hub device comprising: a first port in communication with the memory controller via one or more of the memory busses for transferring data and control information between the memory hub device and the memory controller; a second port in communication with one or more of the memory devices; and a control unit for decoding the memory update command from the data and control information and for accessing the memory devices via the second port to perform the memory update command local to the memory hub device as a logical read-modify-write sequence.