Anatomy of a Motherboard-Dissecting the Tech Behind PC Components

You might have a desktop PC at work, school, or home. You might use one to work out tax returns or play the latest games; you might even be into building and tweaking computers. But how well do you know the components that make up a PC? Take the humble motherboard -- it sits there, quietly keeping everything running, and rarely gets the same attention as the CPU or graphics card.

Motherboards are remarkably important though, and full of really cool technology. So let's go all Grey's Anatomy, and dissect the motherboard -- breaking down its various parts and seeing what each bit does!

A simple overview to start with...

Let us begin with the main role of a motherboard. In essence, it serves two purposes:

Provide electrical power to the individual componentsProvide a route to allow the components to communicate with each other

There are other things a motherboard does (e.g. holds the components in place, or provides feedback as to how well everything is functioning) but the aforementioned aspects are critical to how a PC operates, that almost every other part that makes up the motherboard, is related to these two things.

Nearly every motherboard used in a standard desktop PC today will have sockets for the central processing unit (CPU), memory modules (nearly always a type of DRAM), add-in expansion cards (such a graphics card), storage, input/ouputs, and a means to communicate with other computers and systems.

The only problem with the picture (other than the motherboard being quite... umm... well, used) is that there are a lot of visible components, making it trickier to spot everything clearly.

Let's strip it all away and look at a simplified diagram to begin with (below).

That's better, but there is still a lot of sockets and connectors to talk about! Let's start near the top, with the most important one of all.

Wiring up the brains of a PC

The diagram has a structure labelled LGA1150. This is the name used by Intel to describe the socket used to hold many of their CPUs. The letters, LGA, stand for Land Grid Array, a common type of packaging technology for CPUs and other integrated circuits.

LGA systems have lots of little pins in the motherboard, or in a socket on the board, to provide power and communications to the processor. You can see them in the picture below:

The metal bracket holds the CPU in place but it's getting in the way of seeing the pins clearly, so let's move it to one side.

Remember the name for this? LGA1150. The number is for how many pins there are in this socket. We'll explore the connections for a CPU in another article, but for now we'll just point out that motherboards for other CPUs will have more or fewer pins.

In general, the more capable the CPU (in terms of number of cores, amount of cache, etc), the more pins will be found in the socket. A large number of these connections will be used to send and receive data to the next important feature on a motherboard.

Big brains need big memory

The sockets or slots that are always the closest to the CPU are those that hold DRAM modules, aka system memory. These are connected directly to the CPU and nothing else on the motherboard. The number of DRAM slots depend mostly on the CPU, as the controller for the memory is built into the central processor.

In the example we're looking at, the CPU that fits into this motherboard has 2 memory controllers, with each one handling 2 sticks of memory - hence there are 4 sockets in total. You can see that, on this motherboard, the memory sockets are colored in way to let you know which ones are managed by which controller. They're commonly called memory channels, so channel #1 handles two of the slots and channel #2 handles the other two.

For this particular motherboard, the colors of the slots can be a little confusing (and it certainly confused this author!): the two black slots are actually one each for the two memory controllers (and same for the grey ones). So the black slot closest to the CPU socket is channel #1, and the next black one is channel #2.

It's colored like this to encourage you use the motherboard in what is called dual memory channel mode - by using both controllers at the same time, the overall performance of the memory system is increased. So let's say you had two RAM modules, each one 8 GB in size. No matter what slots you put them in, you'll always have a total of 16 GB of available memory.

However, if you put both modules into both of the black slots (or both of the grey slots), the CPU will essentially have double the routes possible to access that memory. Do it the other way (one module in each color) and the system will be forced to access the memory with just the one memory controller. Given that it can only manage one route at a time, it's not hard to see how this doesn't help performance.

This CPU/motherboard combination uses DDR3 SDRAM (double data rate version 3, synchronous dynamic random access memory) chips and each socket holds one SIMM or DIMM. The 'IMM' part stands for Inline Memory Module; the S and D refers to where the module has one side filled with chips or both sides (single or dual).

Along the bottom edge of the memory module are lots of gold plated connectors, and this type of memory has 240 of them in total (120 each side). These provide the power and data signals for the chips.

A single DIMM of DDR3 SDRAM. Image: Crucial

Bigger modules would allow you to have more memory, but the whole setup is limited by the pins on the CPU (almost half of the 1150 pins in this example are dedicated to handle these memory chips) and space for all of the traces or electrical wires in the motherboard.

The computer industry has stuck with using 240 pins on memory modules since 2004 and shows no signs of changing any time soon. To improve memory performance, the chips simply run faster with each new version released. In the example we're looking at, the CPU's memory controllers can each send and receive 64 bits of data per clock cycle. So with two controllers, the memory sticks will having 128 pins dedicated to transferring information. So why 240 pins?

Each memory chip on the DIMM (16 in total, 8 per side) can transfer 8 bits per clock cycle. That means each chip needs 8 pins, just for data transfers; however, two chips share the same data pins, so only 64 of the 240 are data ones. The remaining 176 pins are required for timing and reference purposes, transmitting the addresses of the data (location of where the data is on the module), controlling the chips, and providing electrical power.

So you can see that having more than 240 pins won't necessarily make things better!

RAM isn't the only thing that's hooked up to the CPU

System memory is connected directly to the central processor to boost performance, but there are other sockets on the motherboard that are wired a bit like this (and for the same reason). They use a connection technology called PCI Express (PCIe, for short) and every modern CPU has a PCIe controller built into it.

These controllers can handle multiple connections (typically referred to as lanes), even though it is a 'point-to-point' system, meaning that the lanes in the socket aren't shared with any other device. In our example, the CPU's PCI Express controller has 16 lanes.

The image below shows 3 sockets: the top two are PCI Express, while the bottom one is a much older system called PCI (related to PCIe, but a lot slower). The little one at the top is labelled PCIEX1_1 because it is a single lane socket; the one below it is a 16 lane socket

If you scroll back up and look at the whole motherboard again, you can see that there are:

2x PCI Express 1 lane sockets3x PCI Express 16 lane sockets2x PCI sockets

But if the CPU's controller only has 16 lanes, what's going on? First of all, only PCIEX16_1 and PCIEX16_2 are connected to the CPU - the third one, and the two single lane sockets are connected to another processor on the motherboard (more about that in a moment). Secondly, if both sockets were filled with devices that use 16 PCIe lanes, then the CPU will only dedicate 8 lanes to each.

This is the case of all CPUs today; they have a limited number of lanes, so as more devices get connected to the CPU, each one gets a smaller number of lanes to work with.

Different CPU and motherboard configurations have their own way of handling of this. For example, Gigabyte's B450M Gaming motherboard has one PCIe 16 lane socket, one PCIe 4 lane socket and a M.2 socket that uses 4 PCIe lanes. With only 16 lanes available from the CPU, using any two sockets will force the larger x16 one to be capped to 8 lanes.

So what kind of things use those sockets? The most common choices are:

16 lanes = graphics card

4 lanes = solid state drives (SSD storage)

1 lane = sound cards, network adapters

You can see the difference between the connectors in the image above. The graphics card sports the longer 16 lane one, compared to the sound card's little 1-lane setup. The latter has far less data to transfer than the former, so it doesn't need all those extra lanes.

In our motherboard example, like all others, has lots more sockets and connections to manage, and so the CPU gets a helping hand from another processor.

Let's head south and cross the bridge

If we go back 15 years or so, and look at motherboards from that era, there were two additional chips built into them to support the CPU. Together, they were called a chip set (usually concatenated to chipset), and individually they were called the Northbridge (NB) and Southbridge (SB) chips.

The former handled the system memory and graphics card, the latter processed the data and instructions for everything else.

The above image, of an ASRock 939SLI32 motherboard, clearly shows the NB/SB chips - they're both hidden under aluminum heatsinks, but the one closest to the CPU socket in the middle of the image is the Northbridge. A few years after this product was around, both Intel and AMD released CPUs that had the NB integrated into the central processor.

The Southbridge, though, has remained separate and is likely to be so for the foreseeable future. Interestingly, both CPU manufacturers have stopped calling it the SB and often refer to it as the chipset (Intel's proper name for it is the PCH, platform controller hub), even though it's just a single chip!

On our more modern example from Asus, the SB is also covered with a heatsink, so let's pop it off and have a look at the extra processor.

This chip is an advanced controller, handling multiple types and numbers of connections. Specifically, it's an Intel Z97 chipset and offers the following features:

8 PCI Express lanes (version 2.0 PCIe)14 USB ports (6 for version 3.0, 8 for version 2.0)6 Serial ATA ports (version 3.0 SATA)

It also has an integrated network adapter, an integrated sound chip, a VGA display output, and a whole host of other timing and controlling systems. Other motherboards will have more basic/advanced chipsets (providing more PCIe lanes, for example) but in general, most chipsets offer the same kind of features.

For this particular motherboard, this is the processor that handles the single lane PCIe slots, the third 16 lane slot, and the M.2 slot. Like many newer chipsets, it handles all of these different connections by using a set of high speed ports that can be switched to PCI Express, USB, SATA, or networking, depending on what is connected at the time. This, unfortunately, places a limit on how many devices plugged into the motherboard, despite all those sockets.

In the case of our Asus motherboard, the SATA ports (used to attach hard drives, DVD burners, etc) are grouped as shown above because of this limitation. The block of 4 ports in the middle use the chipset's standard USB connections, whereas the two on the left use some of these high speed connections.

So if you use the ones on the left, then the chipset will have fewer connections for other sockets. The same is true for the USB 3.0 ports. There is support for up to 6 devices, but 2 of these ports will also eat into the high speed connections.

The M.2 socket, used to connect SSD storage, uses the fast system, too (along with the third 16 lane PCI Express slot on this motherboard); however, on some CPU/motherboard combinations, the M.2 sockets connect directly to the CPU, as many newer products have more than 16 PCIe lanes to distribute and use.

Along the left hand side of our motherboard, there is a row of connectors generally called the I/O set (input/output) and in this instance, the Southbridge chip (or chipset) only handles a few of them:

PS/2 connector - for keyboards/mice (top left) VGA connector - for older/cheaper monitors (top middle)USB 2.0 ports - black in color (bottom left)USB 3.0 ports - blue in color (bottom middle)

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