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A motherboard, also called system board, logic board, or mainboard, is a circuit board that either houses or is connected to all of the components operating in the computer. Choosing the correct motherboard requires attention regarding which features and configurations are available. Ensure that the board chosen is compatible with the system CPU and that there are enough compatible expansion and memory slots, keeping in mind future upgrading requirements.

A typical motherboard includes the following components:

Component Function / Characteristics
Processor interface Current motherboards have a socket that accepts the processor. Pins in the processor drop into the motherboard processor socket. The motherboard socket must match the socket type and design used by the processor (in other words, when choosing a motherboard, make sure it matches the processor you will use). Some motherboards support multiple processors and will have a socket for each processor.
Memory modules The motherboard contains slots for different types of memory. Memory modules must be compatible with the type supported by the motherboard, the total memory capacity, and the processor and chipset support.
Expansion slots Expansion slots allow you to add features to your computer by inserting expansion cards into the available slots. There are a number of different standard expansion slots including:
  • Industry Standard Architecture (ISA)
  • Peripheral Component Interconnect (PCI)
  • Accelerated Graphics Port (AGP)
Onboard components Many motherboards include onboard devices (such as network cards, audio cards, video cards, or USB and Firewire connections). Selecting a motherboard with onboard devices is typically cheaper than buying separate expansion cards for each feature. However, the quality of these onboard devices might not be as high as the quality you could get from devices through expansion cards.
Faceplate connectors A faceplate fits over the motherboard's ports to secure them and protect the motherboard from dust and debris. There are standard connectors for onboard I/O components that don’t require expansion cards. These connectors typically include the following:
  • PS/2 mouse and keyboard ports
  • USB ports
  • Serial ports (COM 1, 2, 3, and 4)
  • Parallel ports (LPT 1 and 2)
  • Mic in, line in, line out connectors
  • MIDI/Game port
Onboard internal connectors There are a number of connectors for components such as power supply, fans, and LED lights. In addition, many motherboards provide additional ports, such as USB or Infrared, by attaching adapter cards to the motherboard connectors (also called headers). These connectors are often labeled on the surface of the motherboard.
BIOS chip The BIOS chip is firmware (hardware hard-coded with software) attached to the motherboard and is essential in booting the computer.
CMOS battery The CMOS battery supplies power to the CMOS to retain system settings used by the BIOS during system boot.
Chipset The chipset is a group of chips that facilitate communication between the processor, memory components, and peripheral devices. The chipset controls the bus speed and also power management features. Chipsets are usually attached to the motherboard and are non-upgradeable. Most modern chipsets consist of the following:
  • The northbridge chip provides control for main and cache memory, the front side bus, and the AGP and PCIe graphics. The northbridge is closest to the CPU. The northbridge dictates the CPU and memory type supported by the motherboard. On some motherboards, the northbridge chip includes an integrated graphic processor. The northbridge often has a heat sink and sometimes a fan, especially if it includes built-in video.
  • The southbridge chip provides the real time clock, controls power management, and provides the controllers for the PCI bus and USB devices.
  • There are two other important chipsets on a motherboard: the keyboard controller and the I/O controller.

Recent developments for the chipset include:

  • Combining north- and southbridge functions into a single chipset.
  • Moving the memory controller from the northbridge to the CPU itself to improve memory access by the CPU.
Jumpers Jumpers are electrical connection points that can be set to control devices and functions attached to the motherboard. Some functions controlled by jumpers are:
  • Clearing the CMOS password
  • Clearing the CMOS settings
  • Setting the CPU bus speed on the motherboard
  • Enabling or disabling onboard components
Many functions previously performed by jumpers can now be configured in the CMOS or are configured automatically.
Documentation When selecting and working with motherboards, a good place for information is the motherboard documentation. Most motherboard documentation includes a diagram of the motherboard that identifies the components listed above and details any jumper settings. If you are missing the motherboard documentation, check the manufacturer's Web site.

Note: When selecting a motherboard, make sure the motherboard and system case use the same form factor.