The Mainboard contains a socket for the CPU chip (Intel or AMD), PCI/PCIe slots for adapter cards, and connectors for the USB, Firewire, audio, and SATA disks.
Most of the function in the mainboard is provided by two large chips that are collectively called “the chipset”. AMD CPUs and newer Intel CPUs have a built-in memory controller, so they connect directly to the memory. In older Intel CPU chips the CPU and memory communicate through the chipset.
Surprisingly, the less expensive mainboards often have an extra feature. They have an integrated video support and can drive one or two monitors without adding a separate video card. More expensive mainboards provide a larger number of high speed connectors for external adapter cards, including video cards used for gaming.
After the CPU chip, the mainboard or “motherboard” is the most important component of any personal computer. Intel makes a small number of its own boards, but most systems use motherboards from companies such as Abit, Asus, MSI, SuperMicro, Tyan, …
Socket
Intel has used an LGA775 CPU socket with 775 pins for the last four years. When it came out the socket supported Pentium 4 processors. Then came the Core and Core 2 generations with Dual or Quad core options. Newer generations of processors use lower voltage levels, but the newest LGA775 board today will still support an old Pentium 4 CPU chip if you still have one around. The LGA775 socket connects through a “Northbridge” chip on the mainboard, which in turn is connected to the memory and I/O devices. When the CPU has more than one core, the cores communicate with each other through the external Northbridge chip.
In 2003 AMD introduced the Opteron/Athlon CPU architecture. It had a memory controller that allowed memory to connect directly to the CPU. CPU cores could communicate with each other directly. There was now no need for a traditional Northbridge chip, but if you wanted two or three PCI-e x16 slots for video cards then you still needed a big chip where the Northbridge used to go.
Unfortunately, if you put more function into the CPU, the CPU and socket have to change when external technology changes. Migration from DDR to DDR2 and from one memory bus to two parallel buses forced AMD to change sockets from 939 pin to the AM2. Today there is an improved version called AM2+ that will support older Athlon chips but will also support the Phenom.
Intel finally accepted that AMD had a better design, so starting in Nov 2008 it introduced the Nehalem or Core i7 family with the memory controller integrated into the CPU chip and with internal communication between cores. The expensive versions use an LGA 1366 socket with three parallel memory buses, but in mid 2009 a mainstream chip will appear using an LGA1066 socket with two memory buses (as have been used for the last few years).
Power Distribution
Newer CPU chips use smaller circuits that would use less power. However, when two or four cores are added, and massively large cache memory, then the power requirements of the very largest and fastest CPU chip can go up.
All mainboards have one PCI-e slot for a video card. High end systems targeted at gamers can have two, three, sometimes even four such slots. Each card draws some of its power from the board.
One of the mainboard functions is power distribution. More power requires larger capacitors and extra distribution channels. Unless the manufacturer says explicitly in his technical documentation that he can handle the load, you should not expect a $50 to $100 mainboard to be able to handle a 140 watt top speed Quad core socket and heavy video load. Most of the difference between a $100 mainboard and a $300 mainboard goes to power management and distribution, although there may also be better engineered connections between the chips and sockets for increased stability, particularly if you overclock the system.
A few years back batches of counterfeit capacitors flooded the market. They failed after months of use, causing quite a problem for mainboard vendors. Today you will see a promise of “100% solid state capacitors” or “genuine Japanese capacitors” as a pledge that these vendors have learned their lesson.
Form Factor
The “standard size” board is called ATX. It typically has slots for seven adapter cards. There may be two x16 PCI-e slots for video, two small PCI-e slots for adapters, and three old PCI slots for old adapter cards. Boards range from around $100 up to $300, though at the top range you will get more video slots.
Less expensive, smaller desktop systems use a Micro ATX or MATX board. These boards range from $50 to $120. They have only four card sockets, only one of which will support a video card, but MATX boards typically have some video capability integrated into the mainboard. If you get a board with modern ATI or Nvidia capability, you typically will not need a video card.
A standard size midtower case will support both ATX and MATX boards. A smaller case, however, will only support MATX.
There are even smaller specialized boards for low power systems. Frequently they come with the CPU chip (an Intel Atom or VIA chip) already installed.
Video
Integrated video is typically an option only on MATX boards. Until recently it was always a poor performer, and most users were better off installing even a $30 video card. However, Vista and Blu-Ray forced vendors to improve things, and modern chip technology made it possible. Today you can get mainboards with ATI 4xxx series or Nvidia 8xxx or 9xxx series video chips. Then will run the OS and even decode HD TV or Blu-Ray movies. An external card is only needed by gamers, who probably will buy an ATX board anyway.
Once you get the right video chip, make sure the board has the right type of video connectors. Modern boards have DVI, HDMI, and analog VGA connectors. Certainly do not buy a board that doesn’t have the right type of connector for your monitor. If you have two monitors and both use DVI, you may need to get a video card anyway to drive them.
Attachments
The mainboard is attached to a tray in the bottom or side of the case by nine screws that screw into metal “standoffs” that keep the bottom of the mainboard a safe distance from the metal of the case. Typically you plug the CPU, heat sink (that goes on top of the CPU), memory, power, and front panel controls (power on, power light, disk light) into the mainboard.
A modern mainboard has mostly Serial ATA (SATA) connectors for disks. A single old Parallel ATA connector may exist for the DVD drive. You will find boards with 4 to 8 SATA connectors, and more is better unless you are absolutely sure you don’t need them.
Mainboards typically have the round PS/2 connectors for the keyboard and mouse, even though modern input devices typically have USB connectors.
Modern mainboards have some combination of PCI Express (PCI-e) slots and older PCI slots. PCI-e card slots come in sizes. Video always uses a long “x16” slot. Other types of cards can fit into smaller x1 and x4 slots. PCI-e cards have a size, but they can always be plugged into a larger mainboard slot. An x1 card will also plug into an x4 or x16 slot. However, a larger card will not fit into a smaller size slot. Other than video, PCI-e cards are still rather exotic devices. There are a few disk controllers and TV tuners. Look at the adapter cards you want to plug into the device and make sure the board has enough of the right type and size of slots.
Chip Set
The primary component of every mainboard is the Chip Set.
In what may be called the “classic design”, the CPU, memory, and video cards connect to a Northbridge high speed chip. The Northbridge then connects to a lower speed Southbridge chip that runs all the other devices (PCI slots, Ethernet, USB, SATA, ATA, keyboard/mouse, etc.).
The Northbridge (or whatever you call the chip to which the CPU and video connect) has to run very fast. It can generate a lot of heat, and more expensive mainboards have more elaborate heat sink and heat pipe systems to cool it.