The CPU may be the brain of a computer, but by itself it has no way to interact with the PC's other components—it needs a nervous system. That would be the motherboard. In many ways, it's little more than the component to which all the other ones are attached, and therefore it has the least measurable impact on a system's performance in most situations. But without it, you couldn't interact with your computer and the other components couldn't interact with each other, so it deserves maximum care and consideration when you're scoping out hardware for a build.
Proper motherboard shopping isn't too complicated, but it requires a thorough knowledge of either what other components you already have, or which ones you're planning to buy—you need to make sure everything is compatible with everything else. As long as you know what to look for, you won't have much trouble finding the motherboard that will make the ideal base for your next computer.
A motherboard is defined chiefly by what sort of processor it's designed to hold, so that's the most important decision you'll make. Most current motherboards use one of five sockets: the AM2 or AM3 for AMD CPUs; or the LGA1155, LGA1156, and LGA1366 for Intel CPUs. Under most circumstances, any processor designed for that type of socket will work. (There are rare exceptions, but these can generally be fixed with a BIOS upgrade from the motherboard manufacturer's Web site.) The good news if you're going with AMD is that AM3 sockets are backwards compatible with AM2 and AM2+ CPUs. But LGA1155, LGA1156, and LGA1366 chips are interchangeable only within their families, not with each other. Once you buy a motherboard, you're limited to other processors of that type. Luckily, both Intel and AMD offer a variety of chips at all price and performance levels for all sockets (though the highest-end LGA1366 will be the most expensive overall), so you'll always be able to find something that works for your budget.
Note: The LGA1155 socket is the mainstream replacement for Intel's older LGA1156. Because the new socket was just introduced in early January, there are still lots of LGA1156 motherboards and CPUs to be found. But you should expect LGA1155 hardware to gradually replace LGA1156 in the market.
A motherboard's size and shape will determine its capabilities, and how much room it has for expansion—the different styles are called form factors. Though there's some variation here, and a new form factor will appear (and vanish) every year or two, the two main ones you're likely to come across are ATX and microATX. ATX motherboards measure approximately 12 by 9.6 inches, and typically offer the largest number of expansion slots (usually seven), RAM bays (four to six), and data connectors. Motherboards using the microATX form factor measure 9.6 by 9.6 inches and have fewer expansion slots, and often fewer RAM bays and other amenities. With the recent trend toward desktop miniaturization, microATX motherboards are much more common now than they used to be. On the other hand, just about every high-end gaming desktop will have a full-size and full-capability ATX motherboard at its core.
Random Access Memory (RAM for short, or colloquially "memory") is the primary "workspace" for a computer, where data is processed after being removed from the hard drive, solid-state drive (SSD), and other media, and before being output. In almost every circumstance, the more RAM you have, the more efficient your computing will be, and you add RAM by installing it in special bays on your motherboard. Most motherboards have between two and six, each of which holds one memory module (or DIMM) of a certain capacity and speed. Knowing what your motherboard can handle is vital in order to obtain the best performance—if your RAM is faster than your motherboard, the extra potential is wasted; if it's slower, you'll run into processing bottlenecks when your processor tries to crunch more data than the RAM can store. In addition to the maximum amount of memory your motherboard can hold (probably between 8GB and 24GB), look out for the type and speed of memory the bay supports. The two most common types of RAM now are DDR2 and the faster DDR3, but one kind won't work in the other's bay, so be sure you get the right one. RAM speed is measured in megahertz (MHz), or millions of operations per second; common values are 1,066 MHz and 1,333 MHz. As long as both values match up with your motherboard's specifications, you should be fine.
Note: Most motherboards support "dual-channel" or "triple-channel" technology, which offers a performance boost if a certain number of DIMMs are installed in the proper bays. Both AMD and Intel make some motherboards that support dual-channel; triple-channel is strictly a feature of Intel LGA1366 motherboards. To get this benefit, you'll need to buy either two- or three-stick memory "kits" and install the DIMMs according to the motherboard manual's instructions. The speed increase isn't huge, but it is noticeable, so it's well worth doing.
Though low-end and midrange motherboards have long offered built-in video capabilities, traditionally they haven't been very good. They've let you run Windows, basic apps, and (very) light games with few problems, just by plugging your monitor into the back of your computer, but if you wanted to do anything more challenging you had to spring for a discrete (or standalone) video card. The latest releases from Intel and AMD are slowly changing this, however, with video- and media-processing powers beyond what we've seen before. Activities like transcoding video and editing large photos is a lot smoother process now than it was even six months ago. You'll still run into trouble playing games, so if that's what you're interested in, a discrete card is a must (see "Expansion Slots" below). Cards range wildly in price, from under $60 to over $600. Find out what you need, and what your motherboard has, before you buy. (Almost all motherboards aimed at gamers and power users don't include integrated video systems, and thus require separate cards.)
A motherboard's expansion slots give you the chance to add additional capabilities to your computer by way of add-in cards—these days, most often video cards or sound cards. Most ATX motherboards contain six or seven slots, and microATX boards four. The most commonly utilized slot these days is PCI Express 2.0 (frequently abbreviated to PCIe), which comes in four flavors: x16, x8, x4, and x1. The slot types look different (the x16 is the largest, for example, and is the de facto standard for video cards), and are downward-compatible—in other words, a PCIe x1 card will work in a PCIe x16 slot, but not vice versa. Some motherboards have more than one PCIe x16 slot, letting you use two, three, or even more video cards at once, but not all of the slots will run equally fast. The older slot standard, PCI, is rapidly being phased out, but you'll still find a slot or two on most motherboards.
Hard drives and SSDs are most frequently connected to your motherboard today by Serial ATA (SATA) ports, running at speeds of either 3 gigabits per second (Gbps) or 6 Gbps, the latter being the most recent standard introduced within the last 18 months. Every motherboard will have between two or eight of these connectors; some will even have special SATA jacks devoted to RAID, for setting up larger and/or more secure storage systems across multiple disks (these will be clearly marked). SATA is a relatively new technology, however, and there are many drives out there that still use the old standard, Parallel ATA (PATA), sometimes also known as IDE. These connectors are much larger than SATA connectors, and frequently found in different places on a motherboard. Because slower, lower-capacity PATA drives are uncommon these days, connectors for them are becoming harder to find, but many motherboards will still have at least one that can control two drives—if you have an old PATA hard drive you'll want to use in your new computer, this is something to watch out for.
Like every other component in your PC your motherboard needs power, and how it gets it can be a bigger issue than you may expect. Just about every motherboard will have two power connectors on it: a larger one, located along the right edge, with 24 pins; and a smaller connector, closer to the CPU, with either four or eight. If you don't know what connections your power supply has, you could be in for a nasty surprise when it comes time to hook everything up. Because older motherboards had 20-pin connectors, power supply manufacturers have developed built-in adapters that let their products work with either 20- or 24-pin motherboards. The smaller connectors can be much harder: Some power supplies have both, but many will have just one or the other, so check to see that your power supply and motherboard are compatible ahead of time.
Rear Panel Ports
We've already discussed integrated video, but there are a lot of other jacks that your motherboard may give you access to. You're probably familiar with Ethernet, for connecting a network cable, and USB 2.0, for hooking up your mouse, keyboard, iPod, and many other devices. Others you may run into include:
USB 3.0. This newer and much faster USB standard is currently only lightly supported, but is slowly gaining traction from external device manufacturers. USB 3.0 ports are blue in color, to differentiate them from black USB 2.0 ports, and are backwards compatible with USB 2.0 devices.
FireWire. Another transfer standard, FireWire (sometimes marked as IEEE 1394) is overall faster than USB 2.0 and conveys power more easily to devices that need it. FireWire ports look like thick rectangles with two corners chopped off. Many high-end motherboards still include these for legacy reasons, but they have become increasingly uncommon in recent years as External SATA (see below) and USB 3.0 have gained prominence.
External SATA (eSATA). A data-transfer standard specifically for external storage, designed to run faster than either USB 2.0 or FireWire. These ports are a bit longer and thinner than USB ports, and look more like the SATA jacks inside your computer. (Note: Some computers have combo USB-eSATA ports that can use either kind of devices; these pretty much just look like eSATA ports, but will be labeled differently.)
Audio. Integrated audio is even more common these days than integrated video; practically every motherboard will have speaker, microphone, and headphone jacks, and some (particularly high-end gaming models) will even support 5.1 or 7.1 surround sound. You may also find S/PDIF optical or coaxial jacks for hooking up digital audio systems.
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