receiving a command from a host;

determining an operation mode of a buffer allocation circuit in a memory controller as one of a first operation mode and a second operation mode;

when the buffer allocation circuit is in the first operation mode, setting an allocation ratio between a main buffer memory arranged in the memory controller and a sub-buffer memory arranged outside the memory controller to a predefined ratio; and

when the buffer allocation circuit is in the second operation mode, variably setting the allocation ratio based on whether a type of the command is a sequential write command,

wherein the variably setting of the allocation ratio comprises:

when the type of the command is the sequential write command, setting an allocation to the main buffer memory to be equal to an allocation to the sub-buffer memory; and

when the type of the command is remaining commands excluding the sequential write command, setting the allocation to the sub-buffer memory to be greater than the allocation to the main buffer memory.