Hardware Reference
In-Depth Information
System Bus Types, Functions, and Features
The heart of any motherboard is the various buses that carry signals between the compon-
ents. A
bus
is a common pathway across which data can travel within a computer. This
pathwayisusedforcommunicationandcanbeestablishedbetweentwoormorecomputer
elements.
The PC has a hierarchy of different buses. Most modern PCs have at least three buses;
some have four or more. They are hierarchical because each slower bus is connected to
the faster one above it. Each device in the system is connected to one of the buses, and
some devices (primarily the chipset) act as bridges between the various buses.
The main buses in a modern system are as follows:
•
Processor bus
—Also called the FSB, this is the highest-speed bus in the system and
is at the core of the chipset and motherboard. This bus is used primarily by the pro-
cessor to pass information to and from cache or main memory and the North Bridge
of the chipset. This is generally the fastest bus in the system, and the speed and width
depend on the specific processor and chipset combination.
•
AGP bus
—This is a 32-bit bus designed specifically for a video card. It runs at
66MHz (AGP 1x), 133MHz (AGP 2x), 266MHz (AGP 4x), or 533MHz (AGP 8x),
which allows forabandwidth ofupto2,133MBps. Itisconnected tothe NorthBridge
or Memory Controller Hub of the chipset and is manifested as a single AGP slot in
systems that support it. Current systems have phased out AGP slots in favor of PCI
Express.
•
PCI Express
—The PCI Express (PCIe) bus is a third-generation development of the
PCI bus that began to appear in mid-2004. PCI Express is a differential signaling bus
that can be generated by either the North Bridge or South Bridge. The speed of PCI
Express is described in terms of lanes. Each bidirectional dual-simplex lane provides
a 2.5Gbps or 5Gbps (PCIe version 2) transfer rate in each direction (250MBps or
500MBps effective speed). PCI Express video cards generally use the x16 slot, which
provides 4,000MBps or 8,000MBps in each direction. Most chipsets released after
2008 include PCIe version 2.0.
•
PCI bus
—This is usually a 33MHz 32-bit bus found in virtually all systems since the
days of the Intel 486 CPU. Some newer systems include an optional 66MHz 64-bit
version (mostly workstations or server-class systems). This bus is generated by either
the chipset North Bridge in North/South Bridge chipsets or the I/O Controller Hub in
chipsets using hub architecture. This bus is manifested in the system as a collection of
32-bitslots,normallywhiteincolorandnumberingfromonetothreeonmostmother-
