PCIe Transaction Protocol

Transaction Layer Packet (TLP)
requester and completer

Transaction types:

• Memory: MRd, MRdLk, MWr
  
• IO: IORd, IOWr
  
• Configuration:CfgRd0, CfgRd1, CfgWr0, CfgWr1
  
• Message: Msg, MsgD

Transaction categories:

• Non-posted transaction
  

  - completer will returns a TLP completion packet back to the requester

  • MRd, IORd, CfgRd0, CfgRd1
  • MRdLk
    
    • requester can only be a root complex
    • on behalf of CPU
  • IOWr, CfgWr0, CfgWr1
• Posted transaction
  

  - completer will NOT returns a TLP completion packet back to the requester, optimized for best performance, may or may not contain data in the request TLP

  • MWr
  • Msg, MsgD
    
    • message request support eliminates the need for side-band signals in a PCIe system
    • used for
      • PCI style legacy interrupt signaling
      • power management protocol
      • error signaling
      • unlocking a path in the PCIe fabric
      • slot power support
      • hot plug protocol
      • vender defined purposes

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Refrence
PCI Express System Architecture, Addison Wesley