1.2.2.3 Connection: PCI and PCI Express

The PCI (peripheral component interconnect) interface is the most standard (though slow) method of adding I/O peripherals to a PC. The main drawback of PCI is that it is a ``parallel'' bus. This makes it more difficult to design motherboards and maintain a high speed connection to I/O peripherals.

PCI Express is a new standard that is based on PCI from the software perspectives. However, PCI Express is a serial interface that supports 1, 4, 8 and 16 bits per clock of transfer. Due to the ``serial'' nature of PCI Express, it makes motherboards easier to design, and it also improves the speed of data transfer rate.

Due to the high data transfer rate of PCI Express, it is now replacing AGP as the most popular interface for connecting a video controller.

Note that PCI peripheral cards are not upward compatible with PCI Express slots in a PC. It is also true that PCI Express cards are not downward compatible with PCI slots.