1. A memory checker comprising: a parity checker, a bit storage connected to said parity checker, and a parity generator connected to said bit storage, and installed in a memory module of a computer system for checking data error, wherein: the parity checker receives a data bus from the computer system and an input parity bit generated by a parity generator of the computer system, and is controlled by a read/write control signal from the computer system to provide a corresponding bit value output to the bit storage as the parity checker determines whether a relation between the data bus and the input parity bit is in conformity with a parity checking protocol; the bit storage receives the bit value output of the parity checker and the read/write control signal from the computer system, and is controlled by the read/write control signal to output the stored bit value to the parity generator and simultaneously to clear the stored bit value from the bit storage; the parity generator receives an output data bus of the memory module and the read/write control signal of the computer system, and is controlled to output a bit signal to the parity checker according to the parity checking protocol, causing the parity checker to determine whether there is an error in data which has been fetched from the memory module and then to provide an interrupt signal to the computer system upon determining that an error exists.