What Is PCIe? A Basic Definition

PCIe Slot Credit: MMXeon/ShutterstockPCIe Slot Credit: MMXeon/Shutterstock

PCIe (peripheral component interconnect express) is an interface standard for connecting high-speed components. Every desktop PC motherboard has a number of PCIe slots you can use to add GPUs (aka video cards aka graphics cards), RAID cards, Wi-Fi cards or SSD (solid-state drive) add-on cards. The types of PCIe slots available in your PC will depend on your motherboard.

PCIe slots come in different physical configurations: x1, x4, x8, x16, x32. The number after the x tells you how many lanes (how data travels to and from the PCIe card) that PCIe slot has. A PCIe x1 slot has one lane and can move data at one bit per cycle. A PCIe x2 slot has two lanes and can move data at two bits per cycle (and so on).

Credit: Erwin Mulialim/Wikimedia CommonsCredit: Erwin Mulialim/Wikimedia CommonsYou can insert a PCIe x1 card into a PCIe x16 slot, but that card will receive less bandwidth. Similarly, you can insert a PCIe x8 card into a PCIe x4 slot, but it’ll only work with half the bandwidth compared to if it was in a PCIe x8 slot. Most GPUs require a PCIe x16 slot to operate at their full potential.

PCIe Generations: PCIe 4.0 and PCIe 5.0

PCIe standards currently come in three different generations: PCIe 1.0, PCIe 2.0, PCIe 3.0 and PCIe 4.0. Bandwidth doubles with each generation. How do you know what performance you’ll get with a PCIe expansion card? Your PCIe card will run at the lowest generation present. So if you put a PCIe 2.0 card in a PCIe 3.0 slot, you’ll get PCIe 2.0 performance.

Credit: PCI-SIGCredit: PCI-SIGPCIe 4.0

The PCIe 4.0 standard debuted in 2017 and offers 64 GBps of throughput. It’s available for enterprise-grade servers, but only just started becoming usable with SSDs this year. The upcoming AMD Ryzen 3000-series CPUs, debuting this year, will be the first desktop CPUs to support PCIe 4.0 x16 out of the box. For full support, users will need new motherboards running the upcoming 500-series chipsets

To learn more about PCIe 4.0, check out our article What We Know About PCIe 4.0 So Far

PCIe 5.0

PCIe 5.0 is also in the works and will bring 128 GBps of throughput. It will be backwards compatible with previous PCIe generations but also include new features, including electrical changes to improve signal integrity and backward-compatible CEM connectors for add-in cards. The first PCIe 5.0 devices are expected to debut in 2019 with broader availability in 2020. PCI-SIG, which defines PCIe standards, expects PCIe 4.0 and PCIe 5.0 to co-exist for a while, with PCIe 5.0 used for high-performance needs craving the most throughput, like GPUs for AI workloads and networking applications. So, PCIe 5.0 will mainly be used be in data center, networking and high-performance computing (HPC) environments, while less-intense applications, like those used by desktop PCs, will be fine with PCIe 4.0.

This article is part of the Tom's Hardware Glossary.

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