PCI Express uses a packet based protocol to exchange information between the Transaction Layers
of the two components communicating with each other over the Link. PCI Express supports the
following basic transaction types: Memory, I/O, Configuration, and Messages. Two addressing
formats for Memory Requests are supported: 32 bit and 64 bit.Transactions are carried using Requests and Completions. Completions are used only where
required, for example, to return read data, or to acknowledge Completion of I/O and Configuration
Write Transactions. Completions are associated with their corresponding Requests by the value in
the Transaction ID field of the Packet header.
All TLP fields marked Reserved (sometimes abbreviated as R) must be filled with all 0’s when a TLP is formed. Values in such fields must be ignored by Receivers and forwarded unmodified by
Switches. Note that for certain fields there are both specified and reserved values – the handling of
reserved values in these cases is specified separately for each case.
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