The illustration above shows how the various buses connect to the CPU.

System Bus vs. PCI Bus

Twenty or 30 years ago, the processors were so slow that the processor and the bus were synchronized -- the bus ran at the same speed as the processor, and there was one bus in the machine. Today, the processors run so fast that most computers have two or more buses. Each bus specializes in a certain type of traffic.

A typical desktop PC today has two main buses:

  • The second one­ is a slower bus for communicating with things like hard disks and sound cards. One very common bus of this type is known as the PCI bus. These slower buses connect to the system bus through a bridge, which is a part of the computer's chipset and acts as a traffic cop, integrating the data from the other buses to the system bus.

Technically there are other buses as well. For example, the Universal Serial Bus (USB) is a way of connecting things like cameras, scanners and printers to your computer. It uses a thin wire to connect to the devices, and many devices can share that wire simultaneously. Firewire is another bus, used today mostly for video cameras and external hard drives.

Next, learn about the history of PCI buses.