Chapter 5. Flow Control

The Previous Chapter

The previous chapter described the use of HyperTransport control and data packets to construct HyperTransport link transactions. Control packet types include Information, Request, and Response variants; data packets contain a payload of 0-64 valid bytes. The transmission, structure, and use of each packet type is presented.

This Chapter

This chapter describes HyperTransport flow control , used to throttle the movement of packets across each link interface. On a high-performance connection such as HyperTransport, efficient management of transaction flow is nearly as important as the raw bandwidth made possible by clock speed and data bus width. Topics covered here include background information on bus flow control and the initialization and use of the HyperTransport virtual channel flow control buffer mechanism defined for each transmitter-receiver pair.

The Next Chapter

The next chapter describes the rules governing acceptance, forwarding, and rejection of packets seen by HyperTransport devices. Several factors come into play in routing, including the packet type, the direction it is moving, and the device type which sees it. A related topic also covered in this chapter is the fairness algorithm used by a tunnel device as it inserts its own packets into the traffic it forwards upstream on behalf of devices below it. The HyperTransport specification provides a fairness algorithm and a hardware method for tunnel management packet insertion.