General

The chapter entitled “Overview of the Processor Role” on page 9 provides a good introduction to the processor's role in the system.

Refer to Figure 34-1 on page 825. In a system that incorporates PCI Express, the Root Complex plays the role that used to be played by the Memory Control Hub (MCH), or the North Bridge. It is a bridge between the processor(s) and the remainder of the system. It also incorporates the system memory controller. For a complete description of the PCI Express spec, refer to the MindShare book entitled PCI Express System Architecture (published by Addison-Wesley).

Figure 34-1. An Example System

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A system may incorporate one or more processors. The maximum number of processors currently supported on a FSB are four [in a system that utilizes Pentium® 4 Xeon MP (Multi Processor) processors]. A system using the Pentium® 4 Xeon DP (Dual Processor) processor supports up to two processors on the FSB.

The processors are tasked with fetching instructions from system memory (labelled DDR RAM in the illustration), decoding them and executing them. In a PC-compatible system, the processors are only permitted to cache information that is read from system memory. They are not permitted to cache from memory other than system memory.