1. A writing method of a memory module, comprising: temporarily storing a piece of 2 m -byte data as p characters, wherein each character comprises q bits, and m, p and q are positive integers; rearranging the 2 m -byte data to obtain K symbols, wherein each symbol has m bits and K is a positive integer smaller than 2 m ; encoding the K symbols into a codeword according to a Reed-Solomon (RS)-code algorithm, wherein the codeword comprises N symbols, the codeword has a parity code, the parity code comprises (2T=N−K) symbols, and N and T are positive integers; rearranging and converting the N symbols into a piece of (2 m +(2T×m/8))-byte data, wherein the (2T×m/8) bytes are the parity code; and writing the p characters and the parity code into a memory-cell array, wherein all the symbols belong to a finite field GF(2 m ).