Next time you plug in a Gen 5 SSD and it drops down to Gen 4 speeds, don't blame the hardware. Somewhere, the spec did its job. The link trained, the equalization failed, and the devices agreed on a slower, safer speed to keep your data intact.
: A hallmark of the PCIe spec is its interoperability. A newer PCIe 5.0 card can technically function in an older PCIe 3.0 slot, though it will be limited to the speeds of the older generation. Technical Evolution & Performance pcie spec
This is what your OS sees. It handles memory addressing, interrupts (MSI-X), and data packet routing. If a driver crashes, you're looking at a Transaction Layer issue. Next time you plug in a Gen 5
Here is the and Chapter 1: Overview text from the official PCI Express Base Specification (Revision 5.0) . This section defines the fundamental architecture and goals of the specification. : A hallmark of the PCIe spec is its interoperability