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PCI express basics


There seems to still be a lot of mystery surrounding PCI express, from version numbers (2.0, 2.1, 3.0 etc.) to bandwidth so I will fill you in on these and other important considerations in this article.

PCI Express lanes

To demystify PCI express somewhat we first need to touch on the subject of PCI Express lanes. Everybody has seen the jargon surrounding this, PCI Express x16, PCI Express x8, x4 and x1.|

What this means exactly is pretty simple actually. X16 means there are 16 lanes running to a particular PCI Express slot. All of these lanes carry data toward the PCI Express card (and back) at a certain speed. These lanes are physical electrical connections for transferring data. X8 means 8 lanes, x4 4 lanes and x1 is a single lane.

Because of this, a x16 PCI Express slot is 16 times as fast as a x1 PCI Express slot. A x16 PCI Express slot is twice as fast as a x8 PCI Express slot. Simple so far, right? Don't worry, it's about to get more complicated :-)

Slot size and actual number of lanes

There is a lot of confusion created in the specifications that various online retailers provide and this sometimes leads to specs appearing to be better than they actually are. This has to do with slot size vs. the actual number of lanes running to a PCI Express slot.

There are 4 types of physical PCI Express slots that can be on a mainboard, as detailed in the diagram here:

PCI Express physical slot sizes
x1PCI Express x1 slot.
x4PCI Express x4 slot.
x8PCI Express x8 slot.
x16PCI Express x16 slot.

The x16 and x1 slots are the most commonly found on consumer mainboards, the x4 and x8 slots not very often anymore.

Physical x16 slots don't always have the full 16 lanes running to (and from) them. A lot of times this physical slot size is used to cater to x8 and x4 connections. This is one reason you should always refer to a mainboard manual or official specifications on the manufacturer's site. Retailer's websites often refer to the physical slot size instead of how many lanes are really all wired up. This leads to misunderstanding as I've often seen mainboards listed as "dual x16 slots for graphics" while in reality the first slot will operate a card at x16 while the second full-sized slot will operate at x4!

One of the reasons for using a full sized slot for less lanes, is that PCI express cards that have 16 lanes can also operate with only 8 lanes available on the slot. Videocards are obviously the prime example of this. In fact, most videocards can even operate on 4 lanes. Those cards obviously do have pins for 16 lanes and so will only fit in a physical x16 slot!

It's worth noting that this works the other way around as well. If you have a x4 slot available on your mainboard you can plug a x1 capable network card into that slot and it will work. Obviously because the card has only 1 lane connected, the connection to the rest of your computer will be single lane as well. PCI Express is designed to scale both ways. To be sure if what you're attempting is possible though you should always consult both the mainboard manual and the manual of the device you are connecting.

Total number of physical lanes on a mainboard

As you probably have noticed, some mainboards accept only a single graphics card at a 16-lane connection (x16) while others can also cater to dual graphics cards, either in dual 16-lane configuration (x16/x16) or dual 8-lane configuration (x8/x8). Other boards can even do x16/x8/x8 for three videocards or x8/x8/x8/x8 for four videocards.

One of the factors that determine what a mainboard can support is the mainboard chipset (on AMD systems) or the CPU (on Intel Sandy Bridge systems). The difference between AMD and Intel in this regard is where the physical PCI express lanes (the copper lanes on your mainboard) originate.

AMD Systems (read if you want to understand the Intel section)

On AMD systems, the physical PCI Express lanes originate on the Northbridge chip and from there run to the various physical slots. How many total lanes you can have on a mainboard in this architecture depends on what northbridge chip is on that mainboard:
  • The AMD 990FX, 890FX and 790FX chipsets have support (physical pins!) for a total of 42 PCI Express lanes.
  • The AMD 880G, 870, 790GX, 785G chipsets have support (again physical pins) for a total of 22 PCI Express lanes.

There are ofcourse other chipsets for the AMD platform made by (among others) NVidia and VIA to name just a few. Those will have their own maximum number of PCI Express lanes that they support.

So, the chipset determines how many lanes (maximum) a mainboard can support. How these lanes are wired to the physical slots on the mainboard is largely up to the mainboard manufacturer. A mainboard manufacturer can for instance build a mainboard with an 890FX chipset that supports only 2 physical slots for videocards, both at x16. They could also choose to make a mainboard with this chipset that supports 3 cards at x16/x8/x8.

This is why you should always check the specs on the manufacturer's website if you want to be sure what your particular mainboard can do!

Now you might have noticed that the 890FX chipset has 42 lanes total, so why not make a mainboard that does x16/x16/x8. That would be faster right?

Well, yes and no. For one, the PCI Express lanes are not all used for the expansion slots. Internally, your onboard ethernet network controller, your extra onboard SATA controller and other stuff is also wired up using these PCI Express lanes. Part of these lanes are thus routed elsewhere than to the slots.
Secondly, there are PCI Express lanes originating from the Northbridge chip as well as from the Southbridge chip. The CPU only talks to the Northbridge, but if it wants to talk to the Southbridge it has to go through the Northbridge first! As a consequence, access to the PCI Express lanes running from the Southbridge is slower than access to the lanes running from the Northbridge.

Only the lanes running from the Northbridge are normally used to connect to graphics card slots. This is done because we need the maximum speed possible when talking to a graphics card.

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