PCI-E Slots Explained

PCI-E Slots were introduced for expanding stuff that you could plug into Motherboards. They vary in physical size and generation, both of which impact transfer speed between the Motherboard and the Peripheral (usually Graphics Cards, Sound Cards, Storage Cards, Expansion Cards, etc)

Slot Size (Physical)

The physical slot size determines whether a card will physically fit into a slot or not. Smaller cards are backwards compatible on Larger slots (usually). The Size is expressed as x16, x8, x4, x1, etc. where a larger number means a larger slot. Slot Size doesn’t impact performance directly.

Slot Size (No.of Pins/Mode)

But there is a catch. Motherboard manufacturers will sometimes make a large sized PCI-E slot but decrease the number of pins in them (let’s call them as mode). So, an x16 sized physical slot might in reality be in x8 mode. This means that a peripheral that utilises x16 lanes can only use x8 lanes causing a drop in performance even though it is physically compatible and works just fine. Always look out for this while buying motherboards.

PCI-E Generation/Version

Newer specifications enable more and more bandwidth to PCI-E.

On an x1 lane PCI-E bus,

• Gen5 32 Gbps
• Gen4 16 Gbps
• Gen3 8 Gbps
• Gen2 4 Gbps
• Gen1 2 Gbps

Similiarly for an x16 lane, the speeds get 16 times bigger,

• Gen5 512 Gbps
• Gen4 256 Gbps
• Gen3 128 Gbps
• Gen2 64 Gbps
• Gen1 32 Gbps

Peripheral-Motherboard-CPU Bottlenecking

The transfer speed between a Peripheral and the CPU depends on three things. The maximum theoretical speed will depend on the minimum compatible generation of the 3 and also on the minimum compatible no.of lanes.

Example : Ryzen 5600G (PCI-E 3.0), Asrock B550M Pro4 (PCI-E 4.0 x16), AMD RX470 (PCI-E 3.0 x16)

This implies that the maximum speed will be PCI-E 3.0 x16, which corresponds to 128 Gbps