PCIe is a high-speed standard that allows you to add extra functionalities to your motherboard with the help of expansion cards.
These cards get attached to a PCIe slots found the motherboard and thus add functionality such as Video Graphic Processing, network capability or storage expansion.
These devices need to communicate and share data with the CPU to carry out their tasks.
To facilitate this connection, we have PCIe lanes. These lanes serve as the information highway through which bits of data are transferred from the PCIe device to the CPU for processing. In the following text we read more about what are PCIe lanes.
Understanding PCIe Lanes: An Analogy
To better understand what PCIe lanes are, let’s use a real-world example that can serve as an analogy to what exactly these lanes are and how they work.
PCIe lanes can be likened to a highway with lanes. When you have data moving across the highway, a single bit can be perceived as a vehicle traversing a lane.
As we will see, moving from one PCIe configuration to the next, the number of lanes increases.
This can be seen as increasing the number of lanes on a highway to accommodate more vehicles at a given point at any given time. Similarly, more PCIe lanes means higher transfer rate.
What Are PCIe Lanes?
A PCIe express lane is basically a connection that contain two pairs of wires. One for receiving data from the CPU and another for sending data to the CPU.
Each lane moves the data at 1 bit per cycle. The speed at which the data is transferred across a lane is determined by the CPU as well as the PCIe version.
For example PCIe V2.0 is twice as fast as the PCIe V1.0. Similarly, PCIe V3.0 is twice as fast as PCIe V2.0. So on an so forth.
Similarly, a processor with a faster clock speed cycle would move the data across the lanes faster.
X1 is the smallest connection as it donates a single lane. This connection is suitable for devices that require very little data transfer like sound cards.
More Lanes More Speed
Since PCIe lanes are scalable, more lanes connected to a device will multiply the data transfer speed.
Therefore if an X1 device works at 1 bit per cycle, an X4 device will work at 4 bits per cycle but will also occupy 4 PCIe lanes.
A typical graphics card is an X16 device and hence occupies 16 PCIe lanes.
You Don’t Have Unlimited Lanes
It goes without saying that you do not have unlimited PCIe lanes. Therefore, if your motherboard has two PCIe X16 slots but your overall build only has 20 available lanes, that would not mean that you can install two graphics card working at X16 mode as that would require 32 lanes.
The amount of lanes available to you for expansion are determined by the CPU and the motherboard chipset.
A typical processor gives you about 16 lanes of dedicated PCIe lanes. You can find the number of lanes available to a CPU in the relevant spec sheet.
A typical processor has enough PCIe lanes for a single X16 Graphics Card working at full X16 mode or two Graphics Cards working at X8 / X8 mode.
Extreme or server grade processor boast a lot more PCIe lanes. As such you can add a large number of expansion cards or graphics card working at X16 mode.
A single high grade processor like the Intel Core extreme processor has enough PCIe lanes to operate two graphics cards at full X16 bandwidth with PCIe lanes to spare.
Motherboard Chipset Lanes
Other than the CPU, you also get dedicated PCIe lanes from the motherboard chipset. However, the amount of PCIe lanes available to you depend on the type of chipset the motherboard uses.
Also, not all motherboard PCIe lanes are dedicated. For example, the Intel Z170 chipset has 20 PCIe lanes. 16 of these are reserved for other critical components including SATA, USB ports, built in LAN Card, PCI bridge etc. While the rest of the 4 PCIe Lanes will be dedicated for use via a X4 expansion slot on the motherboard.
PCIe slot configuration is offered in X1, X4, X8, and X16 bandwidth sizes. The number after the X prefix denoted the amount of lanes are available to the slot.
X1 PCIe Slot
This configuration has just one lane. This means that only one path for the data bits to be transmitted in to and from the CPU. This is like a single-lane road.
With each generation, the lane-speed of a PCIe interface is usually the same and so to get faster data transfers, we need to increase the number of lanes.
PCIe X4 Slot
This one has four lanes, so quadruple the number of lanes compared to the X1 slot. Since the data rate would be the same per lane, you would, in effect, have 4 bits of data getting transferred to and from the devices in every cycle. This means more bandwidth and hence more data transfer. But it doesn’t stop there.
PCIe X8 Slot
This slot has twice the number of lanes that the X4 slot has. This means that you get four more lanes through which data can be transferred with every clock cycle.
This, of course, has additional bandwidth and is, therefore, better than the X4 slot under applications that can make use of all the available lanes.
This brings us to the largest widely available largest configuration.
PCIe X16 Slot
With this configuration, you get 16 data lanes that can be used to transmit 16 bits of data with each cycle.
This results in faster transfers as a result of the larger bandwidths and is ideal for bandwidth-intensive devices like a graphics card.
Note on SLI and CrossFire
Recall from earlier that you do not have unlimited PICe lanes. Therefore, your overall build depends highly upon the availability of dedicated PCIe lanes.
NVIDIA SLI requires at least X8 bandwidth for each card. AMD Crossfire, however, can work at X4 bandwidth.
To clear this point, let us look at an example:
The motherboard above has a total of THREE PCIe slots with the PHYSICAL size of an X16. (Two silver ones and One black at the bottom)
The top and middle slots directly use the CPU PCIe lanes where as the bottom on uses the DMI X4 from the motherboard.
As such, this motherboard can support either 2 way SLI or 3 way Crossfire.
Here is how the configuration will look like
- SLI: X8 / X8 / 0
- CrossFire: X8 / X8 / X4
A few things to note here.
Firstly, in the SLI configuration, the third PCIe slot cannot be occupied with a graphics card since it operates at X4 speed and NVIDIA requires at least X8 for their cards.
Secondly, note that both cards in the SLI configuration operate at X8 speed instead of X16. This is due to the availability of maximum PCIe lanes coming from the CPU, which is 16 in this case. Had you used a single graphics card, it would operate at full X16 speeds.
Thirdly, if you have AMD graphics cards, you can install 3 of them in Crossfire Mode. While the first two cards will work similarly to the SLI at X8 speeds, the third will occupy the X4 slot and thus operate at lower speeds and performance.
PCIe Lane Compatibility
Some motherboards come with more than one PCIe slot and most of the times, these slots have different PCIe lane capacities.
What happens when you plug a PCIe X1 device into a PCIe X16 slot? Will the device malfunction?
Since the lanes only provide the data connection to the CPU, if you plug a device into a slot with many lanes than the device needs, your device will still work. However, it will not gain any sort of performance boost from the larger slot.
Also you will have potentially wasted a larger slot that can be used for more demanding expansion cards.
Instead, only the necessary lanes will be put to use while all the other lanes remain unused.
For this reason, many motherboard manufacturers provide multiple slots with different lane counts so that you do not end up sacrificing too many lanes for a device that only needs a few.
The amount of slots you have available will largely depend upon the motherboard form fact, chipset as well as the amount of PCIe lanes available to you by the CPU.
The Reverse is Also True
You can just as well plug a device into a slot with fewer lanes than it requires. For example, you can plug a PCIe X16 graphics card into an X8 PCIe slot.
The overall bandwidth will be theoretically be halved but in many cases it won’t result in a 50% drop in performance. That depends a lot upon the ideal transfer rate required by the graphics card. If a certain graphics card can satisfy its transfer rate on an X8 slot, then it won’t benefit from an X16 slot.
All this is made possible by the backward compatibility that PCIe devices and slots have. This means that you can also use a PCIe 3.0 expansion card on a PCIe 2.0 slot without much of an issue besides the fact that the entire setup will operate at the speed of the PCIe 2.0 slot.
Here we basically looked at What are PCIe lanes from a consumer perspective. Knowing what a PCIe lane is and how it defines your PC builds is crucial for any enthusiast.
In technical terms, PCIe lanes are vital for communication between a computer’s CPU and the peripheral devices attached to the motherboard.
The important thing to note about the lanes is that the more the lanes on a slot, the more the bandwidth you can have if you connect a suitable device to the slot.
Another important thing to note is that different devices require different amount of lanes. Powerful devices like graphics card require a very fast transfer speeds and thus require larger 16 lane PCIe slots. Networks cards, on the other hand, can be fitted on a single lane slot.