Abstract: A SIMM is provided which adds ECC onto a computer system which system does not have ECC capabilities. The SIMM has ECC on board, and does ECC on each byte of a data word. Thus, individual bytes of multiple data words can be written without the need for read-modify-write cycles. Also, parity of the written data bytes is checked, and check bits used for the ECC are manipulated to check parity on the read cycle. Hence, additional or dynamically changeable wait states need not be present in the system. Therefore, the SIMM is usable on a great number of different computer systems without modification.