PCI Express System Architecture

Enumerating a System With a Single Root Complex

Figure 21-2 on page 748 illustrates an example system before the buses and devices have been enumerated, while Figure 21-3 on page 749 shows the same system after the buses and devices have been enumerated. The discussion that follows assumes that the configuration software uses either of the two configuration mechanisms defined in the previous chapter. At startup time, the configuration software executing on the processor performs bus/device/function enumeration in the following manner:

1. Starting with device 0 (bridge A), the enumeration software attempts to read the Vendor ID from function 0 in each of the 32 possible devices on bus 0.

   • If a valid (not FFFFh) Vendor ID is returned from bus 0, device ...