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Hardware

PCIe

PCI Express, the high speed link between the CPU and graphics cards, SSDs and other add-in cards. A link has 1, 4, 8 or 16 lanes, and each new generation doubles the speed per lane, so the same SSD or GPU runs at the speed of the slower end.

Lanes times generation

A PCIe link is a bundle of lanes. Each lane carries data both ways at once, and the link's width is how many lanes it has: x1, x2, x4, x8 or x16. The generation sets the speed of every lane, and since 3.0 each one has doubled the transfer rate of the one before. Usable data per lane in each direction, after encoding overhead:

  • PCIe 3.0: 8 GT/s, about 0.985 GB/s
  • PCIe 4.0: 16 GT/s, about 1.969 GB/s
  • PCIe 5.0: 32 GT/s, about 3.938 GB/s
  • PCIe 6.0: 64 GT/s, about 7.563 GB/s
  • PCIe 7.0: 128 GT/s, about 15.125 GB/s

The card and the slot negotiate the best generation and width they both support, so PCIe is backward compatible. A Gen 5 SSD in a Gen 4 slot runs at Gen 4 speed, and a x1 card in a x16 slot uses one lane.

What uses the slot

Graphics cards use x16. NVIDIA lists the RTX 4090 as a PCIe Gen 4 card and the RTX 5090 as Gen 5. NVMe SSDs typically use x4. Taking a quarter of the published x16 rates of 31.508 GB/s for Gen 4 and 63.015 GB/s for Gen 5, a Gen 4 x4 link carries about 7.9 GB/s each way and a Gen 5 x4 link about 15.8 GB/s.

The Throughput Visualizer models a Gen 4 drive reading at 7,000 MB/s and a Gen 5 drive at 14,000 MB/s. A drive can't read past what its link allows, which is why a Gen 5 SSD needs a Gen 5 slot to show its speed. A GPU also reads its own VRAM much faster than it can pull data across this link, which is one reason a model needs to fit in VRAM.

PCIe 6.0 and 7.0

PCIe 6.0 doubled the rate again to 64 GT/s and moved to PAM4 signaling with FLIT based encoding, and products began to appear in 2025. PCI-SIG released the PCIe 7.0 specification to its members in June 2025 at 128 GT/s. It says a x16 link reaches up to 512 GB/s in both directions combined, or 256 GB/s each way, and it pitches the generation at the bandwidth demands of AI.

For a PC you're choosing today, the number that counts is the generation of the slot and of the device in front of you. The spec sheet lists both.

Checking a real slot

  • Read the slot, not its size. A slot can be x16 in size and wired for fewer lanes, such as x4. The motherboard manual gives each slot's real lane count and generation.
  • Check both ends. The link runs at the lower generation and the narrower width of the two, so a fast drive in a slower slot is held to the slot's speed.
  • Compare real workloads before paying for a newer generation. The Gen 4 vs Gen 5 NVMe page under Read next shows where the extra bandwidth is noticeable.
Work it out yourselfOpen the Memory and Storage Throughput Visualizer

Read next

Gen 4 vs Gen 5 NVMe SSDs on real work NVMe vs SATA SSD Writing 100 GB to a Gen 4 NVMe SSD

Related terms

NVMe vs SATATwo SSD interfaces. NVMe connects over PCIe and reaches 3,000-14,000 MB/s for sequential reads. SATA is the legacy 6 Gbps interface shared with HDDs and tops out around 550 MB/s. NVMe also has lower random-read latency and handles many more requests at once, which is what makes an NVMe SSD feel faster than a SATA SSD in real workloads.USB4A USB standard built on the Thunderbolt protocol that shares one USB-C link between data, video and PCIe. Version 1.0 runs at 20 or 40 Gbps and version 2.0 adds 80 Gbps, so check the speed on the port or cable label.ThunderboltIntel's connection standard that uses the USB-C connector and carries data, video and power over one cable. Thunderbolt 4 guarantees 40 Gbps, and Thunderbolt 5 raises the minimum to 80 Gbps, which is more than USB4 requires.VRAMThe memory on a graphics card, next to the GPU. To run a large language model on a GPU, its weights, its KV cache and some runtime overhead all have to fit in VRAM, so VRAM decides which models and context lengths a card can handle at full speed.

Sources

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