Peripherals are real devices, such as graphics cards or disks controlled by controller chips on the system board or cards plugged into it. The IDE disks are controlled by the IDE controller chip and the SCSI disks by the SCSI disk controller chips and so on. These controllers are connected to the CPU and to each other by a variety of buses. Most systems built now use PCI and ISA buses to connect the main system components. The controllers are processors like the CPU itself. They can be viewed as intelligent helpers to the CPU. The CPU is in overall control of the system.