PCI Express devices require setup after power-on, before devices in the system can begin application specific communication with each other. At least two devices connected through a PCI Express Link must have their Configuration spaces initialized and be enumerated to communicate.
Root Ports facilitate PCI Express enumeration and configuration by sending Configuration Read (CfgRd) and Write (CfgWr) TLPs to the downstream devices such as Endpoints and Switches to set up the configuration spaces of those devices. When this process is complete, higher-level interactions, such as Memory Reads (MemRd TLPs) and Writes (MemWr TLPs), can occur within the PCI Express System.
The Configurator example design described here performs the configuration transactions required to enumerate and configure the Configuration space of a single connected PCI Express Endpoint and allow application-specific interactions to occur.