The heart of any motherboard is the various buses that carry signals between the components. A bus is a common pathway across which data can travel within a computer. This pathway is used for communication and can be established between two or more computer 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 front-side bus (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 processor to pass information to and from cache or main memory and the North Bridge of the chipset. The processor bus in a modern system runs at 66MHz, 100MHz, 133MHz, 200MHz, 266MHz, 400MHz, 533MHz, 800MHz, or 1066MHz and is normally 64 bits (8 bytes) wide.

• 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 for a bandwidth of up to 2133MBps. It is connected to the North Bridge or Memory Controller Hub of the chipset and is manifested as a single AGP slot in systems that support it. Newer systems are phasing out AGP slots in favor of PCI-Express.

• PCI-Express. The PCI-Express bus is a third-generation development of the PCI bus. 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 transfer rate in each direction (2Gbps effective speed). Thus a single-lane PCI-Express slot (known as x1) runs at 2.5Gbps in each direction. Some systems support PCI-Express x4, which provides 10Gbps in each direction. PCI-Express video cards generally use the x16 slot, which provides 40Gbps in each direction.

• PCI-X. PCI-X is a second-generation development of the PCI bus that provides faster speeds than PCI but is backward-compatible with PCI. It is used primarily in workstation and server installations. PCI-X supports 64-bit slots that are backward-compatible with 64-bit and 32-bit PCI cards. PCI-X version 1 runs at 133MHz, whereas PCI-X 2.0 supports operation at up to 533MHz. Typically, PCI-X 2.0's bandwidth is subdivided among multiple PCI-X and PCI slots. Although a few South Bridge chips can generate the PCI-X bus, most chipsets that support PCI-X use a separate PCI-X bus chip.

• 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 versionmostly 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-bit slots, normally white in color and numbering from four to six on most motherboards. High-speed peripherals, such as SCSI adapters, network cards, video cards, and more, can be plugged into PCI bus slots. PCI-X and PCI-Express are faster developments of the PCI bus. PCI-Express motherboards and systems began to appear in mid-2004.

• ISA bus. This is an 8MHz 16-bit bus that has disappeared from recent systems after first appearing in the original PC in 8-bit, 5MHz form and in the 1984 IBM AT in full 16-bit 8MHz form. It is a very slow-speed bus, but it was ideal for certain slow-speed or older peripherals. It has been used in the past for plug-in modems, sound cards, and various other low-speed peripherals. The ISA bus is created by the South Bridge part of the motherboard chipset, which acts as the ISA bus controller and the interface between the ISA bus and the faster PCI bus above it. The Super I/O chip usually was connected to the ISA bus on systems that included ISA slots.

Some motherboards feature a special connector called an Audio Modem Riser (AMR) or a Communications and Networking Riser (CNR). These are dedicated connectors for cards that are specific to the motherboard design to offer communications and networking options. They are not designed to be general-purpose bus interfaces, and few cards for these connectors are offered on the open market. Usually, they're offered only as an option with a given motherboard. They are designed such that a motherboard manufacturer can easily offer its boards in versions with and without communications options, without having to reserve space on the board for optional chips. Normal network and modem options offered publicly, for the most part, will still be PCI based because the AMR/CNR connection is somewhat motherboard specific

Several hidden buses exist on modern motherboardsbuses that don't manifest themselves in visible slots or connectors. I'm talking about buses designed to interface chipset components, such as the Hub Interface and the LPC bus. The Hub Interface is a quad-clocked (4x) 66MHz 8-bit bus that carries data between the MCH and ICH in hub architecture chipsets made by Intel. It operates at a bandwidth of 266MBps and was designed as a chipset component connection that is faster than PCI and yet uses fewer signals for a lower-cost design. Some recent workstation/server chipsets and the latest 9xx-series desktop computer chipsets from Intel use faster versions of the hub interface. The most recent chipsets from major third-party vendors also bypass the PCI bus with direct high-speed connections between chipset components.

In a similar fashion, the LPC bus is a 4-bit bus that has a maximum bandwidth of 16.67MBps; it was designed as an economical onboard replacement for the ISA bus. In systems that use LPC, it typically is used to connect Super I/O chip or motherboard ROM BIOS components to the main chipset. LPC is faster than ISA and yet uses far fewer pins and enables ISA to be eliminated from the board entirely.

The system chipset is the conductor that controls the orchestra of system components, enabling each to have its turn on its respective buses.

Note that many of the buses use multiple data cycles (transfers) per clock cycle to achieve greater performance. Therefore, the data transfer rate is higher than it would seem for a given clock rate, which allows for an easy way to take an existing bus and make it go faster in a backward-compatible way.