PCIE知识点-019：expansion rom地址可与bar0~5地址重叠

1、Expansion rom地址空间的特殊性

从PCIe 4.0协议的7.5.1.2.5章节可以得知：

Expansion ROM 的地址空间是允许与BAR0~5重叠的，无论是否支持Enhanced Allocation capability.

因此市面上的主流PCIe都支持此功能，当Expansion ROM 的地址空间与BAR0~5重叠的时，对重叠地址的访问能够正常响应，且被识别成对Expansion ROM的访问。

此处需要注意：BAR0~5不允许地址重叠

参考文档：PCI Express Base Specification, Rev. 4.0 Version 1.0

In order to minimize the number of address decoders needed, a Function may share a decoder  between the Expansion ROM Base Address register and other Base Address registers or entry in the Enhanced Allocation capability. When expansion ROM decode is enabled, the decoder is used for accesses to the expansion ROM and device independent software must not access the Function  through any other Base Address registers or entry in the Enhanced Allocation capability.