Abstract: While a busy state is indicated in a status register by a first busy signal kept active while a command is stored in a parameter register group as a stored command, a free state is indicated while a second busy signal is kept inactive until later kept active while a parameter group is stored in the parameter register group as a stored parameter group. After lapse of the free state during which a command executing unit is operable without reference to the stored parameter group, the unit is operable by using a working register group alone. Furthermore, a like parameter group can be stored in the parameter register group, which is connected to a bus through a multiplexer while the second busy signal is inactive. During a short interval during which the first busy signal is inactive and before the second busy signal is activated, another command can be stored through the multiplexer. A logic circuit can be used instead of the status register.