Introduction to PCIe
Adding peripherals to a computer is a simple task. Need to fit a GPU? Slot it right into the dedicated PCIe slot that is designed on the motherboard. The same goes for any storage, wi-fi cards, and RAID cards. But how do these components have the capability to read, write, and communicate with the motherboard and CPU/Socket? The answer is simple, through the PCIe interface. This blog will explore the PCIe interface, what they do, and how they have evolved throughout the years.
What is the PCIe interface?
PCIe, or Peripheral Component Interconnect Express, is a standard interface that connects components to a computer. This interface allows for the host (motherboard) to communicate with whichever peripheral that is slotted into them (endpoint). You may recognize PCIe slots very easily by the lines on a motherboard that typically resemble Lego blocks. PCIe was designed to replace older bus standards (PCI, PCI-X, etc.). The architecture of a PCIe differs from past serial buses due to its topology. Where older interfaces like PCI utilize parallel bus interface, a PCIe uses serial interface that allows for full-duplex communication between endpoints. This makes PCIe more reliable, faster, and much cheaper to produce compared to PCI’s parallel interface. This also means that communication of the data can happen in both directions, simultaneously. This is similar to a telephone call, where both parties can speak and be heard at the same time.
PCIe Slots and PCIe Lanes Explained
When examining PCIe slots on a computer, it may be confusing to understand that the physical PCIe “slot” doesn’t necessarily equate to the number of available PCIe “lanes” that allows for the transfer of data. Sometimes, the physical slots do not even resemble the numbers. A PCIe x4 or x8 slot resembles a PCIe x16, yet it only houses 4 - 8 lanes. This odd caveat can prove to be challenging, so in order to differentiate the two words being used interchangeably, many will differentiate the two by using mechanical and electrical. The mechanical representation of the PCIe connector on the motherboard dictates the physical slot in which peripherals socket in. PCIe slots come in a variety of physical sizes: x1, x2, x4, x8, and x16. A PCIe lane is the electrical representation. A single electrical PCIe lane consists of two pairs of copper wires that allow for bidirectional transfer of data, one pair to send and the other pair to receive. These lanes dictate how much data can be transferred to and from the add on cards. Visually think of these lanes on a highway - the more lanes, the more vehicles can fit within the highway and drive down the lanes without having to worry about traffic. So, the more lanes, the more data can travel, giving faster data transfer speeds. PCIe slots can support 1, 4, 8, 16, and even 32 lanes for data transferring, although 32 lanes are very rare in consumer products.
In the most ideal setting, the PCIe slot should represent the number of lanes that it also has. However, this is not the case. As mentioned prior, a great example is the PCIe x16 slots that are normally found on a standard desktop motherboard. While these slots all look like a mechanical x16 slot, the number of lanes can be x4 or x8 instead of x16. When examining a motherboard, the top PCIe slot will usually be the primary x16 lane, that is almost always used for graphics cards.
However, based upon the motherboard, the number of slots and lanes vary, and the manufacturer of the board is responsible for designating the number of lanes. This means that there is not an infinite number of lanes that allow you to have send and receive data within the given mechanical slots. The lanes will be based upon the capabilities of the CPU and motherboard chipset. A standard desktop computer usually houses an average of 20 PCIe lanes, but the actual number can vary. This is very important to know, because, while you may have two x16 PCIe slots open on your motherboard, you are not able to, for example, run two x16 graphics cards that require x16 lanes each. It is good practice to check how the motherboard divides up the available PCIe lanes and see how many PCIe lanes a particular CPU supports from the motherboard manufacture’s user manual.
An example of how an electrical PCIe lane can be different than its physical PCIe slot can be found in riser cards. Riser cards help increase the number of physical slots available and distributes the bandwidth evenly across the open slots. In the image blow, the riser card features two PCIe x16 slot and splits the lanes into two even x8 bandwidth lanes.
Top Uses of PCIe Slots
PCIe slots have a variety of uses for a number of different types of peripherals. Here are just a few of the top uses for each type of PCIe slot that are commercially available.
PCIe x1 Slots:
- Sound Cards
- Network Cards
- USB Expansion Cards
PCIe x4 Slots:
- M.2 NVMe SSD Expansion Cards
- SATA 3 Expansion Cards
- Network Adaptors
- Video Capture Cards
PCIe x8 & PCIe x16 Slots:
- Graphics Cards
PCIe Generations
Currently, PCIe lanes have gone through 4 iterations, or generations. Currently, PCIe Gen 4 is the most used and can be found in many computer motherboards today. However, in 2019, PCI-SIG (Peripheral Component Interconnect Special Interest Group), the special interest group that is responsible for specifying the PCIe buses as a market standard, officially announced the 5th generation PCIe. And again, just this year (2022) the organization officially announced the final specifications for the 6th Generation PCIe. Despite the announcement, products containing 6th generation PCIe aren’t expected to be seen available for quite some time due to real-work demands and application needs
The computing world is, however, seeing the first CPUs and peripherals emerge on the market that support PCIe Gen 5. This now means that customers may obtain bandwidth and data transfer speeds even faster than the current PCIe Gen 4. Both AMD and Intel have released CPUs (Alder Lake and Ryzen 7000 respectively) that support PCIe 5, and we should be seeing an increase in peripherals that can utilize Gen 5 speeds soon after.
So. just how much faster is PCIe Gen 5 in comparison to the current generation?
PCIe 5.0 Speeds, Technical Specs, and More
PCIe speeds are very quick to pick up. With each generation, the data transfer speed increases linearly, meaning that with iteration the transfer rate and bandwidth double. When we compare the speeds from the very first iteration of a PCIe, we can see just how much improvement is made over each generation through this doubling.
PCIe 5 brings huge boosts in performance speed and bandwidth. With Gen 1, we saw bandwidth speeds of 250MB/s and data transfer rates of 2.5 GT/s (gigatransfers) per lane. Now with Gen 5, with the PCIe next-gen rule, speeds will reach up to 32 GT/s of data transfer and 4GB/s of bandwidth per lane.
The additional bandwidth of PCIe 5.0 means that devices may be able to achieve the same throughput while using fewer lanes, thus freeing up the number of lanes available. For example, a graphics card that used to require x16 bandwidth to run may now run at the same speed with x8 lanes, thus freeing up x8 lanes for use. PCIe Gen 5 effectively allows these lanes to become more available for further additions via PCIe slots.
Backwards and Forwards Compatibility of PCIe Slots and Cards
The great thing about PCIe slots and their different generations is that each of these iteration’s actual physical designs of a PCIe slot do not change, allowing for backwards and forwards compatibility. A PCIe 4 is backwards compatible with Gen 1, Gen 2, and Gen 3 while Gen 1, 2, or 3 can slot into a Gen 4. And with the release of PCIe Gen 5 compatible products, this will also ring true (PCIe Gen 1-4 will be compatible with PCIe Gen 5 and vice versa). However, there is a small caveat. A PCIe Gen 4 device will be limited to the 4th generation specifications when inserted into a Gen 5 slot, and a Gen 5 support component will be limited to the speeds of a Gen 4 when inserted into Gen 4 slot.
Said simply, PCIe 5 cards will work on motherboards with PCIe 4 slots, and PCIe 4 cards will work on motherboards with PCIe 5 slots. That said, although PCIe 4 cards will work on PCIe 5 motherboards, their speed will be limited according to PCIe 4 standards.
As confusing as it sounds, this makes it very practical for those looking to upgrade their peripherals to something newer. And with Gen 5 support, it opens up options for the latest and most up to date peripherals and frees up the available bandwidth as previously mentioned. Customers can look to buy CPUs supporting PCIe 5.0, use PCIe 4.0 endpoints for the time being, and upgrade when PCIe 5.0 endpoints are readily available.
CPUs with PCIe 5.0 Support
As of this year, both Intel and AMD have released their latest generation of processors, Alder Lake and Ryzen 7000 series, that support PCIe Gen 5. As of the writing of this blog, here are the available PCIe Gen 5 supported CPUs
| Processor | Model |
| AMD Ryzen 9 | Ryzen 9 7950X, 7900X |
| AMD Ryzen 7 | 7700X |
| AMD Ryzen 5 | 7600X |
| Intel Core i9 | 13900K, 13900KF, 12900K, 12900KF, 12900, 12900T |
| Intel Core i7 | 13700K, 13700KF, 12700K, 12700KF, 12700F, 12700, 12700T |
| Intel Core i5 | 13600K, 13600KF, 12600K, 12600KF, 12600, 12600T, 12500, 12500T, 12400F, 12400, 12400T |
| Intel Core i3 | 12300, 12300T, 12100, 12100F, 12100T |
| Intel Pentium Gold | G7400, G7400T |
| Intel Celeron | G69000, G6900T |
PCIe 5 and The Rugged Edge
Now the question begs, when will we see components that are compatible to run PCI Gen 5 when in industrial applications? The answer is, very soon. With Intel and AMD introducing their new generation of processors, we can be seeing a quick evolution of peripherals adopting PCIe Gen 5 soon after.
Although industrial systems are developed with little to no maintenance and upgrading in mind, the ever-evolving world of artificial intelligence and machine learning is proving to many industries that speed and real time data processing is critical. This is a huge plus in many edge applications that require real time decision making, such as autonomous vehicles, medical inferencing, and more. Many current rugged edge applications sit comfortably at Gen 3 or Gen 4 and don’t necessarily have the immediate urge to upgrade. However, within the complex, ever-evolving world of high-performance computing, this might not be the case in the not so far future. The high data transfer rates, lower power consumption, and increased bandwidth provided by PCIe 5.0 may prove to be critical in enhancing data processing in artificial intelligence and machine learning applications to deliver real-time actionable insights. As AI, machine learning, and the number of IoT devices continue grow at the edge, the demand for more power and the amount of data generated will only continue to grow as well. Essentially, the faster applications can look adopt PCIe Gen 5, the quicker they are able to alleviate potential bottlenecks that may appear in these applications.
This is a huge plus in many edge applications that require real time decision making, such as autonomous vehicles, medical inferencing, and more. With Gen 4 currently, many applications are taking advantage of components such as M.2 NVMe SSDs to bring out the full potential of data transfer. Although many systems benefit enough with PCIe Gen 4, new AI and ML applications are constantly pushing the threshold of innovation in order to push real time inferencing at the edge to new heights. A great case is the advent of M.2 hardware accelerators (Domain Specific Architectures or DSAs) that help run very specific AI workloads. With Gen 5, DSAs can further push the envelope in deep learning training and inferencing. These significant improvements can greatly improve how these AI and ML workloads compute large amounts of data in real time.
What About PCIe Gen 6 and Gen 7?
As Gen 5 rolls out, PCI-SIG makes no stops to continue pushing along to advance the technology behind the interface. As of today, both Gen 6 and 7 have been announced, although in different stages. Gen 6’s final specifications were officially announced in January of 2022, while the 7th generation’s development was announced in June. Despite making the next generations known to the public, there is a large gap between technological innovation versus the market. As mentioned before, it will be a while before we see any development of possible compatible components that can meet the specifications that are provided in both Gen 6 and 7. For now, the speed offered by PCIe Gen 5 should be plenty enough to help satisfy the demand for the volume of data that is needed to be processed at the edge.
FAQ:
Is PCIe Gen 5 worth the upgrade?
- With Gen 5 slowly being adopted, it is a very viable option to consider upgrading to PCI Gen 5 for consumers who are looking to prioritize speed. However, it is only just recently that Intel and AMD released PCIe Gen 5 compatible processors. Customers can look to buy CPUs/chipsets supporting PCIe 5.0, use PCIe 4.0 endpoints for the time being, and upgrade when PCIe 5.0 endpoints are readily available.
- As mentioned before, with the release of Intel and AMD CPUs that support Gen 5, it will be a matter of time that Gen 5 becomes the standard just like how Gen 4 is now.
- Physically, there is no difference between a PCIe Gen 4 and PCIe Gen 5. The biggest main difference in generations is the bandwidth and speed that they allow data to pass through. With Gen 5, add-ons like a graphics card can effectively utilize less PCIe lanes and make more lanes available for use.
- Yes, Intel’s 12th Generation (code named: Alder Lake), is the first processor that can support PCIe Gen 5.
- Yes, with AMD’s Ryzen 7000 release in September 2022, they also now house processors that can support PCI Gen 5.
- PCIe 5, and like its previous iterations, are used as slots to connect peripheral components (such as graphics cards and storage drives) to the motherboard (or a host) to allow for the data transfer and communication between the peripheral and the host.