A computer memory access method includes: receiving external data with a prefetching length, the external data having an unmasked first data portion and a masked second data portion; writing the unmasked first data portion to a corresponding data storage unit of a computer memory by a writing unit and reading a third data portion corresponding to the masked second data portion from the data storage unit by a reading unit; producing modified external data by merging the unmasked first data portion and the third data portion in place of the masked second data portion; generating parity bits from the modified external data by an error correction code encoding circuit according to a given rule; and then writing the parity bits to a parity bit storage unit of the computer memory by the writing unit, in substitution of previous information in the parity bit storage unit.