PCI Express System Architecture

Flow Control Concept

The ports at each end of every PCI Express link must implement Flow Control. Before a transaction packet can be sent across a link to the receiving port, the transmitting port must verify that the receiving port has sufficient buffer space to accept the transaction to be sent. In many other architectures including PCI and PCI-X, transactions are delivered to a target device without knowing if it can accept the transaction. If the transaction is rejected due to insufficient buffer space, the transaction is resent (retried) until the transaction completes. This procedure can severely reduce the efficiency of a bus, by wasting bus bandwidth when other transactions are ready to be sent.

Because PCI Express is a point-to-point ...