A memory controller is provided for reading and writing to and from a memory module. The memory controller implements an error correction algorithm, which calculates error correction code for message data to be written to the memory module and checks the error correction code against the message data when the data is read out of the memory module. The memory controller spreads each codeword over at least four different beats sent over the interface with the memory module, with each beat comprising a symbol of error correction code. Bits of a particular symbol of message data occupy the same positions in different beats. Since the bits of the symbols occupy the same positions in different beat, the number of bits affected by a hardware error is minimised. With four symbols of error correction code available for use in the codeword.