The diagram above shows a typical PC system. The processor or CPU communicates with ROM (BIOS) and RAM via the system bus, which comprises a data bus, an address bus and a set of control signals, such as Read/Write. The system bus also communicates with an Input/Output bridge that spits the system bus into two I/O busses - The Industry Standard Architecture (ISA) bus and Peripheral Component Interconnect (PCI) bus. These busses feed the I/O slots on the motherboard and other system components. The PCI bus feeds the IDE interfaces and the USB bus, while the ISA bus communicates with the Keyboard, Mouse, Floppy disc drive, serial and parallel ports. The reason why items such as the keyboard are connected to an I/O bus is that Radio Frequency Interference (RFI) is radiated from the copper tracks on the motherboard, which act as efficient aerials. As the frequency being carried on the copper tracks gets higher, the RFI gets worse, i.e. there is more of it, therefore designers try to keep the tracks carrying the higher frequencies around the processor. As items like the keyboard interface can operate at a slower frequency, they will work quite well on the ISA bus.