Memory modules, memory systems, and methods of operating memory modules

Abstract

A memory module includes semiconductor memory devices mounted on a circuit board and a control device mounted on the circuit board. The control device receives a command, an address, and a clock signal from an external device, and provides the command, the address, and the clock signal to the semiconductor memory devices. The control device, in a hidden training mode during a normal operation, performs a command/address training on at least one semiconductor memory device of the semiconductor memory devices by transmitting a first command/address and a first clock signal to the at least one semiconductor memory device and receiving a second command/address and a second clock signal in response to the first command/address and the first clock signal, from the at least one semiconductor memory device.