Abstract: The present invention provides a computer system having a backplane with a plurality of subsystems which can be individually switched on or off. The computer system comprises a backplane having a plurality of power zones, a power control circuit installed in the backplane for each of the power zones for controlling electric power supplied to the power zone from a power supply, and a subsystem installed in each of the power zones, each of the subsystems comprising an I/O (input/output) bus, at least one I/O slot connected to the I/O bus for connecting an I/O device, and a processor connected to the I/O bus for controlling operations of the subsystem. Each of the subsystems can be switched on or off by using the power control circuit of the power zone of the subsystem.