Disclosed herein is a system for controlling the process of performing a command in an HDD. The HDD comprises a register for holding the command control information supplied from a host system, a command register for holding a specific command. The HDD further comprises a CPU. The CPU fetches the command control information from the register when the specific command held in the command register is one for setting the command control information. The CPU controls the process of performing a command in accordance with the command control information, thereby to finish performing the command within a period between the time when the host system issued the command and the time when the host system issues the next command.