PCIe lanes serve as the information highway through which bits of data are transferred from the PCIe device connected on the motherboard to the CPU for processing. In the following text we read more about what are PCIe lanes.
PCIe is a high-speed standard that allows you to add extra functionalities to your motherboard with the help of expansion cards. PCIe is one of two primary methods, the other being SATA, that allows you to add components to your PC.
Knowing what PCIe lanes are and how many you have is an essential part of your building a PC that meets your current needs and is also future proof.
The PCIe expansion cards get attached to PCIe slots found on the motherboard and thus add functionality such as video graphic processing, network capability, storage expansion and more.
Different expansion cards have different PCIe lane requirement. The more demanding an expansion card is, the more PCIe lanes it requires for catering to the higher throughput rate of the expansion card.
A card can have 1, 4, 8 or 16 lane requirement, 1 lane being used by weaker card like WiFi network cards and 16 lanes used by demanding cards that produce a lot of data like a graphics card.
What is PCIe?
As mentioned earlier, PCIe is an interface on the motherboard t hat allows you to add high speed devices like graphics cards, network cards, NVMe SSDs etc.
PCIe stands for Peripheral Component Interconnect Express and it is one of the primary interfaces that is used to add internal components to your, the other one being SATA.
What Are PCIe Lanes?
Image: PCIe Lanes illustrated. Source: HowStuffWorks.com
A PCIe express lane is basically a connection that contains 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 at least. The actual speed at which the data is transferred across a lane is determined by the CPU as well as the PCIe version.
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.
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 traffic lanes. When you have data moving across the highway, a single bit can be perceived as a vehicle traversing a lane.
The more lanes connected to a device, the more traffic it can handle.
In addition to that, just because the highway has the potential to carry certain amount of traffic, does not mean it will always be 100% occupied or saturated.
Similarly, sometimes a device connected does not use the entire capacity. For instance a WiFi 6 network card has a max throughput of 300 MB/s. This is an x1 devices meaning it requires the smallest x1 slot with single lane to operate.
However, a single PCIe 3.0 lane has a throughput rate potential of 985 MB/s second. Hence the WiFi 6 card will not saturate even a single PCe 3.o lane.
You Don’t Have Unlimited Lanes
It goes without saying that you do not have unlimited PCIe lanes on your PC. A typical desktop computer may only have 20 PCIe lanes.
The amount of lanes available to you for expansion are determined by the CPU and the motherboard chipset.
This is important to note because, 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. Read the Note on SLI and Crossfire below.
Image: Intel Core i7-10700K has 16 PCIe lanes.
You can find out the amount PCIe lanes you have through the specsheet.
A typical processor gives you about 16 user accessible PCIe lanes for plugging expansion cards into.
Depending upon the generation the make and model of your CPU, the amount of PCIe lanes differ. For instance an average Intel Core series processor from 10th and lower generation offers 16 lanes. All 16 lanes connect to the x16 slot.
On the 11th Gen Intel Core series CPU, you have 20 PCIe lanes. 16 of these connect to an x16 slot and the rest of 4 lanes connect to an M.2 slot (typically).
A typical AMD Ryzen processor has 24 lanes. 16 of these connect to an x16 slot, 4 of them connect to an M.2 slot (typically) and the rest of the 4 connect to the motherboard chipset.
An average 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 CPUs boast a lot more PCIe lanes. As such you can add a large number of expansion cards or graphics card working at x16 mode.
Image: Intel Core i9-9980XE has 44 PCIe lanes.
A single workstation grade processor like the Intel Core X-Series processor has enough PCIe lanes to operate two graphics cards at full X16 bandwidth with PCIe lanes to spare.
AMD Ryzen Threadripper Pro 3995WX workstation grade processor offers 128 PCIe lanes!
These processors require specialized motherboards.
The following table shows how many PCIe Lanes contemporary and new gen CPU offers:
|Processor||Generation||PCIe Lanes||Version Supported|
|Intel Core||11th |
|AMD Ryzen||5000 Series|
|AMD Ryzen||3000 Series (Non-G Suffix)|
|AMD Ryzen||3000 Series (G-Suffix)|
|AMD Ryzen||2000 Series|
|AMD Threadripper||3000 Series|
|AMD Threadripper |
|Intel Core X||Cascade Lake||48||V3.0|
|Intel Xeon||Cascade Lake||48||V3.0|
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 Z490 chipset has 24 PCIe lanes. Most of these are reserved for other critical components including SATA, USB ports, built in LAN Card, PCI bridge etc. Only a few remaining PCIe Lanes are user accessible in the shape of an x4 and a couple of x1 slots on the motherboard.
Again, how many lanes you have available from the motherboard chipset all depends upon how it is configured i.e how many SATA ports it has, how many M.2 slots it has, does it have WiFi card built in or not etc.
Image: Intel Z490 Chipset with a single DMI link which accounts for a single x4 slot.
The following tables show the amount of PCIe lanes as found on different popular motherboard chipsets today.
|B460||16||Mid Range /
|B560||12||Mid Range /
Also Read: Why is a Motherboard Important?
PCIe Version and the Impact on Transfer Speeds
The version of the PCIe has a huge impact on how fast the transfer speed for each PCIe lane is.
Every subsequent PCIe veneration doubles the transfer speed per lanes compared to the previous 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.
PCIe generation is one of the critical aspect that determines the performance of your PC. A newer version of PCIe can support newer and faster cards.
For instance, an NVMe SSD built for PCIe V4.0 is twice as fast in both read and write speed compared to the same built for PCIe V3.0. Compare Samsung 970 Pro (PCIe v3.0 SSD) vs Samsung 980 Pro (PCIe v4.0 SSD). The former has 3500 MB/s read speed, the later has 7000 MB/s read speed.
In order to cater to higher speeds of the newer version of PCIe, a new and a faster processor is required. For example, the newer 3000 series AMD Ryzen processors supports PCIe v4.0. The older 2000 series AMD Ryzen processors do not.
Also Read: How to Check How Many PCIe Slots Do I Have?
Does the Version of PCIe Have an Impact on the GPU Performance? – PCIe 4.0 vs 3.0
TechSpot conducted a thorough study on the impact of PCIe v4.0 vs v3.0 on several games.
The used the NVIDIA RTX 3080 for this purpose – one of the flagship GPU with some of the highest bandwidth requirement.
Here is there conclusion:
In a nutshell, right now PCIe 4.0 does little to improve performance with the RTX 3080. It’s possible that could change with future games, but for now it’s a non-issue. – TechSpot.com
As such, with the current generation of GPUs and the game engines, there is little to no performance impact on either of the PCIe versions.
You Can Have Different PCIe Versions on the Same Motherboard
Even on the same motherboard, you can have PCIe slots conforming to different PCIe version.
For instance, you can have the slot connected to the CPU lanes conforming to PCIe v4.0 and the slots connected to Motherboard Chipset Lanes conforming to PCIe 3.0 lanes.
This is all determined the by the motherboard chipset. The following ROG B550 Strix Motherboard has the primary x16 slot conforming to PCIe v4.0 and the rest of the chipset connect slots conforming to Version 3.0
The following tables summarizes how the version of the PCIe differs in the AMD 500 series motherboards.
PCIe Slot Configuration (x1, x2, x4 and x16) and Their Corresponding Lanes
PCIe slot configuration is offered in x1, x4, x8, and x16 sizes.
The number after the x prefix DOES NOT always indicate the amount of PCIe lanes the slot has, particularly in the case of x16. More on this below.
x1 PCIe Slot
This configuration has just one lane. This is used for low demanding devices like sound cards or network cards.
x4 PCIe Slot
This one has four lanes, so quadruple the number of lanes compared to the x1 slot.
This is used for moderately demanding expansion cards like NVMe Expansion cards, 4k video capture cards etc.
Note than an x4 slot on a motherboard has the same size an x16 slot. Therefore, just by simply looking at the slot, you cannot judge how many lanes it has.
Take for instance the motherboard above, the second PCIe x16 slot is in fact an x4 slot. Hence, it is ONLY suited for cards that require 4 lanes such as 10G network cards, 4K video capture cards etc.
x16 PCIe Slot
The x16 slot is the most confusing to understand. As mentioned and seen above, the “16” here does not always indicate the lane count the slot has. Instead, this only refers to the size of the slot.
If your motherboard only has a single x16 slot, it will almost definitely have 16 lanes.
If you have multiple x16 slots, then the top x16 slot is the primary x16 slot with 16 lanes is almost always used by the graphics cards.
If your motherboard has multiple x16 slots, then you need to refer to the manual because some x16 slots are hardwired to only 8 or even just 4 lanes.
x8 PCIe Slot
X8 PCIe Slot is often found on motherboards designed for dual graphics cards. So if you plan to build an NVIDIA SLI system or AMD Crossfire system with two graphics card, then an you will need to have an x8 slot on your board.
An x8 slot also has the same physical size an x16 slot, but it has half the lanes.
GPUs from both AMD and NVIDIA ideally require 16 lanes, however, they can also work on 8 lanes well for the purpose of SLI and Crossfire configuration.
More on this in the section below:
Also Read: Can You Plug A PCIe X1 Into an X16 Slot?
Note on SLI and CrossFire Lane Requirement and Configuration
Recall from earlier that you do not have unlimited PCIe lanes. Therefore, your overall build depends highly upon the availability of dedicated PCIe lanes.
NVIDIA SLI requires at least x8 bandwidth or 8 PCIe lanes for each card. AMD Crossfire, however, can work with 4 PCIe lanes.
To clear this point, let us look at an example:
Image: MSI MPG Z490 Gaming Carbon WiFi
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 x4 chipset lanes 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
- Single GPU: x16/0/x4
- Multie GPU: x8/x8/x4
A few things to note here.
Firstly, in the case of NVIDIA NVIDA SLI configuration, the third x16 slot cannot be occupied with an NVIDIA graphics card since it operates only at 4 lanes and NVIDIA requires at least 8 lanes for their cards as mentioned earlier.
Secondly, if you have NVIDIA graphics cards in the SLI configuration installed in the top and middle slots, then they will both operate at x8 speed instead of x16. This is due to the limited availability of PCIe lanes coming from the CPU, which is 16 in this case.
If you have a single graphics card, always plug it on the top PCIe x16 slot to allow it use all 16 lanes.
Thirdly, if you have AMD graphics cards, you can install 3 of them in Crossfire Mode on this motherboard. 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 speed and performance.
Does Installing a Graphics Card in an x8 Slot Affect Its Performance?
There have been studies done by PudgetSystem and GamerNexus that prove that in certain situations the performance may be impacted when using a GPU in an x8 vs an x16 slot, whereas in others, it may not.
GamerNexus Performance Report
According to a study by Gamernexus.net where they tested the performance of an MSI GTX 1080 on several games, they found that the difference in performance was almost negligible.
On Metro Last Light, for instance, the difference amounted to only 1%
Note that this study did not consider dual GPUs. Instead they tested a single MSI GTX 1080 on an x16 and then again on an x8 slot separately.
Read full GamerNexus report here: PCIe 3.0 x8 vs. x16: Does It Impact GPU Performance?
Pudget Systems Performance Report: Mixed Results
Pudget Systems also conducted a similar study. They tested an NVIDIA Titan X separately on an x8 and x16 slot as well as tested dual NVIDIA Titan X.
The following were their observations.
Negligible Difference When Using One Graphics Card On Either Slot
When using a Single Titan X on either an x16 or an x8 slot, the performance difference was negligible.
On certain applications there was no difference at all, and on others there was a difference of no more than 5%.
Upto 30% Performance Difference When Using Dual Titan X
This finding came as a surprise. On certain applications, upto 30% performance impact was observed when using a GPU in x16/x16 vs x8/x8.
However, this impact wasn’t true for ALL application and games. For instance on GRID Autosport, There was only a marginal impact on whether you are using an x16/x16 or an x8/x8 configuration.
Surprising, and quite counter intuitively, in certain instances, x8/x8 even OUTPERFORM the x16/x16 configuration.
I highly recommend reading this study in order to learn about about the impact of using a GPU in x8 or an x16 slot: Pudget Systems full report on Titan X Performance: PCI-E 3.0 x8 vs x16
PCIe Lane Cross Compatibility
What happens when you plug a PCIe x1 device into a PCIe x16 slot? Will the device malfunction?
Not it will not malfunction thanks to the cross compatibility of the PCIe standard.
If you plug a small expansion card into a slot with more 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, putting a smaller card into a larger slot isn’t advisable since you will have potentially wasted the potential of larger slot that can be used for more demanding expansion cards.
Hence it is for this reason that almost all motherboards come with a variety of large and small slots with a varying range of lane count.
The amount of slots you have available will largely depend upon the PCIe lanes you have, as discussed earlier, as well as on the motherboard form factor.
For instance an ATX motherboard offers plenty of PCIe slots due to its large form factor. A mini-ITX, on the other hand, is very limited as it only has a single PCIe x16 slot.
The Reverse is Also True
You can just as well plug a larger 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 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.
Also Read: PCIe 3.0 vs 2.0 – What is the Difference?
Here we basically looked at what PCIe lanes are 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.
And finally, the amount of PCIe lanes you have are limited and are determined by the CPU and motherboard chipset.
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