PCIe replaced PCI, PCI-X, and AGP but the x1/x2/x4/x8/x16 and x32 are confusing as sometimes they are virtual
Posted by jpluimers on 2026/10/16
Each time I get confused about the various x in PCIe slots, this comes in useful: [Wayback/Archive] PCIe (Peripheral Component Interconnect Express) | On the Motherboard | Pearson IT Certification
TABLE 3.7 PCIe versions
PCIe version Speed (per lane per direction) 1.0 2.5GT/s (gigatransfers per second) or 250MB/s 2.0 5GT/s or 500MB/s 3.0 8GT/s or 1GB/s 4.0 16GT/s or 2GB/s
Note that
- PCIe x32 is almost never used.
- There are cards which look like x16 but fit in x8 and x4 slots too, for instance the [Wayback/Archive] Cablecc NGFF M-Key NVME AHCI SSD to PCI-E 3.0 16x x4 Adapter for XP941 SM951 PM951 A110 m6e 960 EVO SSD: Amazon.de: Computer & Accessories
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Some early PCIe motherboads mix in one or more older slot types:
Figure 3.35 Motherboard with PCIe and PCI slots
Bus bandwidth greatly improved from PCI, PCI-X and AGP to PCIe:
TABLE 3.8 Comparing bus bandwidth
Bus Maximum bandwidth PCI 133 or 266MB/s (depending on bus speed) PCI-X 266–4,266MB/s (depending on bus speed) AGP 2x 533MB/s PCIe x1 250MB/s (in each direction) PCIe x2 500MB/s (in each direction) PCIe x4 1,000MB/s (in each direction) PCIe x8 2,000MB/s (in each direction) PCIe x16 4,000MB/s (in each direction) PCIe x32 8,000MB/s (in each direction)
Images of slots and adapters:
A PCIe x1 adapter can fit in an x1 or higher slot. A larger card, such as a PCIe x16, cannot fit in a lower-numbered (x8, x4, x2, or x1) slot. Figure 3.33 shows this concept.
Figure 3.33 Correct slots for PCIe cards
–jeroen
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