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Switches implement both port arbitration and VC arbitration to determine the prioritywith which to forward packets from ingress ports to egress ports. Enumerating the SystemStandard PCI Plug and Play enumeration software will still be able to enumerate a PCIExpress system. The Links are numbered as they are in PCI, using a depth-first searchalgorithm. An example of the bus numbering is shown in Figure 3 on page 11. Each PCIExpress Link is equivalent to a logical PCI bus and is assigned a bus number by the busenumeration software. A PCI Express endpoint is device zero on a PCI Express Link of
An Introduction to PCI Express14a given bus number, since only one device (device zero) exists per PCI Express Link.The internal bus within a switch that connects all the virtual bridges together is alsonumbered. The first Link used by the root complex, for example, is bus number onebecause bus zero is an internal bus. Buses downstream of a PCI Express-to-PCI(-X)bridge are enumerated as they are in a PCI(-X) system.As in conventional PCI, endpoints may implement up to 8 functions per device and asystem could theoretically include up to 256 PCI Express Links or PCI(-X) buses.PCI Express System Block DiagramLow Cost PCI Express ChipsetFigure 4 on page 15 is a block diagram of a hypothetical low-cost PCI Express-basedsystem based on existing chipset architectures. In this solution, the AGP connectionbetween the MCH and a graphics controller in earlier designs is replaced with a PCIExpress Link, and so is the Hub Link that connects MCH to ICH. The ICH chip itselfsupports 4 PCI Express Links which can connect directly to devices on the motherboardor be routed to connectors where peripheral cards are installed.The CPU can communicate with PCI Express devices via the ICH as well as through thePCI Express graphics controller. PCI Express devices can communicate with systemmemory or the graphics controller through the MCH, and PCI devices may also commu-nicate with PCI Express devices and vice versa. In other words, the chipset supportspacket routing between PCI Express endpoints, PCI devices, memory and graphics. It isyet to be determined if the first PCI Express chipsets will actually support peer-to-peerpacket routing between PCI Express endpoints. The specification does not require theroot complex to support peer-to-peer between the multiple Links of the root complex.The design shown in Figure 4 on page 15 does not show any switches and, if the numberof PCI Express devices to be connected does not exceed the number of Links available,none would be required for this design.
An Introduction to PCI Express16Another Low Cost PCI Express ChipsetFigure 5 on page 16 is a block diagram of another low cost PCI Express system. In thisdesign, a Hub Link connects the root complex to an ICH device, so the ICH can be anexisting design which has no PCI Express Links. Instead, all PCI Express Links areassociated with the root complex. One of these Links connects to a graphics controller,