PCI Express Base Specification v1.0 - 2002.pdf

PCI EXPRESS BASE SPECIFICATION, REV. 1.0FramingSequenceNumberHeader Data CRC FramingTransaction LayerData Link LayerPhysical LayerFigure 1-5: Packet Flow Through the LayersOM13754Note that a simpler form of packet communication is supported between two Data LinkLayers (connected to the same Link) for the purpose of Link management.1.5.1. Transaction LayerThe upper layer of the architecture is the Transaction Layer. The Transaction Layer’sprimary responsibility is the assembly and disassembly of Transaction Layer Packets (TLP).TLP are used to communicate transactions, such as read and write, as well as certain types ofevents. The Transaction Layer is also responsible for managing credit-based flow controlfor TLP.Every request packet requiring a response packet is implemented as a split transaction. Eachpacket has a unique identifier that enables response packets to be directed to the correctoriginator. The packet format supports different forms (memory, I/O, configuration, andmessage) of addressing depending on the type of the transaction. The Packets may also haveattributes such as “no-snoop,” and “relaxed-ordering” which may be used to optimally routethese packets through the system.The transaction layer supports four address spaces: it includes the three PCI address spaces(memory, I/O, and configuration) and adds a Message Space. This specification uses theMessage Signaled Interrupt concept as a primary method for interrupt processing and usesMessage Space to support all prior side-band signals, such as interrupts, power-managementrequests, and so on, as in-band Message transactions. You could think of PCI ExpressMessage transactions as “virtual wires” since their effect is to eliminate the wide array ofsideband signals currently used in a platform implementation.1.5.2. Data Link LayerThe middle layer in the stack, the Data Link Layer, serves as an intermediate stage betweenthe Transaction Layer and the Physical Layer. Responsibilities of Data Link Layer includeLink management, error detection, and error correction.The transmission side of the Data Link Layer accepts TLP assembled by the TransactionLayer, calculates and applies data protection code and TLP sequence number, and submitsthem to Physical Layer for transmission across the Link. The receiving Data Link Layer isresponsible for checking the integrity of received TLP and for submitting them to theTransaction Layer for further processing. On detection of TLP error(s), this layer is36