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[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System Buses | System Bus Functions and Features ]

Bus Hierarchy

The PC has a hierarchy, in a way, of different buses. Most modern PCs have at least four buses. I consider them a hierarchy because each bus is to some extent further removed from the processor; each one connects to the level above it, integrating the various parts of the PC together. Each one is also generally slower than the one above it (for the pretty obvious reason that the processor is the fastest device in a modern PC):

  • The Processor Bus: This is the highest-level bus that the chipset uses to send information to and from the processor.
  • The Cache Bus: Higher-level architectures, such as those used by the Pentium Pro and Pentium II, employ a dedicated bus for accessing the system cache. This is sometimes called a backside bus. Conventional processors using fifth-generation motherboards and chipsets have the cache connected to the standard memory bus.
  • The Memory Bus: This is a second-level system bus that connects the memory subsystem to the chipset and the processor. In some systems the processor and memory buses are basically the same thing.
  • The Local I/O Bus: This is a high-speed input/output bus used for connecting performance-critical peripherals to the memory, chipset, and processor. For example, video cards, disk storage devices, high-speed networks interfaces generally use a bus of this sort. The two most common local I/O buses are the VESA Local Bus (VLB) and the Peripheral Component Interconnect Bus (PCI).
  • The Standard I/O Bus: Connecting to the above three buses is the "good old" standard I/O bus, used for slower peripherals (mice, modems, regular sound cards, low-speed networking) and also for compatibility with older devices. On almost all modern PCs this is the Industry Standard Architecture (ISA) bus.

The system chipset is the conductor that controls this orchestra of communication, and makes sure that every device in the system is talking properly to every other one.

Some newer PCs actually use an additional "bus" that is specifically designed for graphics communications only. The word "bus" is in quotes because it isn't actually a bus, it's a port: the Accelerated Graphics Port (AGP). The distinction between a bus and port is that a bus is generally designed for multiple devices to share the medium, while a port is only for two devices.

Next: Data and Address Buses

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