A memory system, a memory controller and an operating method are disclosed. When a target command which instructs an operation of writing target data to a memory device is received from a host, the target data is divided into data units, and a first data unit among the data units is controlled such that the entire first data unit is written to the memory device or none of the first data unit is written to the memory device. As a consequence, it is possible to write data in specific units in a memory system using a multi-core.