The individual components of the system board are interconnected by a multiple connection system known as a bus. The system bus is divided into three logical functions; the address bus, the data bus and the control bus. The address bus specifies the memory locations (addresses) for the data transfers and the data bus holds the data transfered. The data bus is bidirectional, it allows data to be read into the CPU and written from the CPU. The control bus contains various lines used to transfer timing and control signals throughout the system. Many flavours of bus exist, for example ISA and PCI buses are popular ways of connecting peripherals to the system.