[ 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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