PCIe 4.0 technology is becoming more common. But what are the differences between PCIe 5.0, 4.0, and 3.0? How does PCIe backward compatibility work? What are the benefits of CPU PCIe lanes compared to chipset PCIe lanes?
Let’s take a deep dive into how PCIe 4.0 works and why it’s an integral part of your PC experience.
What is PCIe 4.0?
If you've built a computer before, you'll recognize the PCIe slots arranged horizontally on the motherboard. PCIe (Peripheral Component Interconnect Express) is a high-bandwidth expansion bus commonly used to connect graphics cards and SSDs, as well as peripherals such as capture cards and wireless cards.
On a motherboard, PCIe lanes come in x1, x2, x4, x8, and x16 configurations. More lanes mean more bandwidth, and longer slots. Graphics cards are often installed in the top x16 slot, as it has the most bandwidth and traditionally has the most direct connection to the CPU. Modern PCIe m.2 SSDs use x4 lanes.
Each generation of PCIe is twice as fast as the previous one. While PCIe 3.0 has a data transfer rate of 8 Gigabits per second, PCIe 4.0 has a data transfer rate of 16 GT/s, and PCIe 5.0 has a data transfer rate of 32 GT/s. (Bitrates are measured in gigabytes to show theoretical maximum speeds before encoding -- actual speeds may be slower.)
On the surface, the new PCIe slots look identical to 3.0. They are also backwards and forwards compatible: not only can you connect a PCIe 3.0 SSD to a PCIe 4.0 slot, you can also connect a PCIe 4.0 SSD to a 3.0 slot.
What are PCIe lanes?
Not all PCIe lanes work the same - CPU PCIe lanes connect directly to the CPU, while chipset lanes (or "PCH lanes") connect to the CPU through the motherboard's chipset, which is connected to the CPU via a DMI (Direct Media Interface) link.
The PCH typically manages features on the motherboard, such as USB devices, Wi-Fi and Ethernet networking, and onboard sound. Since the link between the CPU and chipset is limited to x8 3.0 total bandwidth, it is possible to saturate the link if multiple storage devices are plugged in and using other resources. Connecting directly to the CPU bypasses this bottleneck.
In the past, users with 16 PCIe 3.0 lanes could only take advantage of this optimal path to the CPU by halving the bandwidth to the GPU, creating another potential bottleneck.
PCIe m.2 SSDs and NVMe SSDs using riser cards already enjoy a speed advantage over drives connected via SATA data cables. PCIe's higher throughput allows NVMe storage to quickly queue more data, and connecting directly to the motherboard reduces latency. Connecting to the CPU PCIe lanes further helps reduce latency by eliminating the distance data must travel through the chipset.
Why upgrade to PCIe 4.0?
As mentioned above, throughput doubles with each generation of PCIe. But the actual benefit of PCIe 5.0 is full backward compatibility and future proofing: you know new hardware won't be bottlenecked on your system.
Currently, PCIe 4.0 SSDs are designed to have higher maximum read/write speeds than PCIe 3.0 SSDs, but the actual benefits in areas like load times and large file transfers are currently small. However, as time goes on and new memory controllers are released, expect games and applications to take greater advantage of modern SSDs.
This may be achieved through upcoming technologies like DirectStorage, which are designed to improve the performance of SSDs in heavy I/O workloads. As SSDs become the norm in next-generation game development, this could lead to advances in load times, asset streaming, and level design.
The higher bandwidth of PCIe 4.0 and 5.0 may also benefit graphics cards, as the higher throughput helps transfer data to VRAM faster. But while PCIe 4.0 settings outperform 3.0 in synthetic benchmarks, the actual benefits in gaming are currently tiny.
Some testing has shown that even running games at 4K with current graphics cards won't saturate the bandwidth of a PCIe 3.0 x16 slot. Comparing the same GPU running in a PCIe 4.0 configuration versus 3.0, there may be a slight FPS advantage, but the difference is so small that it's barely noticeable.
What's needed to support PCIe 4.0 or 5.0?
CPU. PCIe 5.0 is supported from the ground up for gaming, with up to 16 CPU PCIe 5.0 lanes and up to four CPU PCIe 4.0 lanes. 4.0 is also supported from the ground up for gaming, with features like PCIe 4.0 and up to 20 CPU PCIe lanes.
Motherboard. You'll need a 600-series chipset with an LGA 1700 socket or a 500-series motherboard in the Z590 or B560 series depending on your CPU.
PCIe 4.0 and 5.0 devices. While you probably won't buy a PCIe 4.0 SSD or GPU on your initial build or purchase, it's easy to see why supporting them would be useful in the future. Maybe ports of new console games start to rely more on streaming assets, and a PCIe 4.0 SSD provides a smoother experience. Or maybe next-generation GPUs will benefit from the doubled throughput of PCIe 4.0 and 5.0 slots. (Note that due to backward compatibility, PCIe 3.0 devices will also work fine on PCIe 4.0 or 5.0 platforms.)
Why are PCIe 4.0 and 5.0 important?
PCIe 5.0 and 4.0 platforms give you more flexibility. You’ll have more options when you buy new peripherals and more options when you configure them. And with PCIe 5.0, you’ll have more time before future devices saturate the available bandwidth.
Many users will replace their storage drives or graphics cards at some point in their system’s life. If you’re one of them, PCIe 5.0 support ensures you can take full advantage of the capabilities of cutting-edge SSDs and GPUs released in the next few years.
Additionally, 20 CPU PCIe lanes provide a more direct CPU connection to two of your system’s most important peripherals – the GPU and SSD – with the potential for reduced latency.