Introduction to the Packet-Based Protocol

The PCI Express protocol improves upon methods used by earlier busses (e.g. PCI) to exchange data and to signal system events. In addition to supporting basic memory, IO, and configuration read/write transactions, the links eliminate many sideband signals and replaces them with in-band messages.

With the exception of the logical idle indication and physical layer Ordered Sets, all information moves across an active PCI Express link in fundamental chunks called packets which are comprised of 10 bit control (K) and data (D) symbols. The two major classes of packets exchanged between two PCI Express devices are high level Transaction Layer Packets (TLPs), and low-level link maintenance packets called Data ...

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