The Problem
On any bus where an agent initiates the exchange of information (commands, data, status, etc.) with a target, a number of things can cause a delay (or even end) the normal completion of the intended transfer. The throttling of information delivery on a bus is referred to as flow control. PCI is a good example of a bus protocol which has reasonably high burst bandwidth, but is subject to performance hits caused by an unsophisticated flow control mechanism. Before looking at the HyperTransport approach to flow control, some of the general problems in bus flow control are described in the following section in terms of the PCI protocol. Refer to Figure 5-1 on page 100.
Figure 5-1. PCI Interface Handshake Signals
How PCI Handles Flow Control ...
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