PCI Express: Difference between revisions
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Older variations of PCI (e.g. "PCI Conventional") were limited to a maximum of 256 PCI bus segments. PCI Express extends this by introducing "PCI Segment Groups", where a system could (in theory) have up to 65536 PCI Segment Groups with 256 PCI bus segments per group, thereby allowing a single computer to have up to a maximum of 16777216 PCI bus segments.
PCI Segment Groups are numbered, and in most systems there is only one PCI Segment Group (PCI Segment Group number 0). Note that legacy PCI configuration space access mechanism #1 (which still exists for backward compatibility) has no "PCI Segment Group" field and therefore can only be used to access the PCI configuration space for PCI Segment Group number 0. The [[#Enhanced Configuration Mechanism]] must be used to access the PCI configuration space for any devices in other PCI Segment Groups.
The enhanced configuration mechanism makes use of memory mapped address space range/s to access PCI configuration space. Put simply, the memory address determines the segment group, bus, device, function and register being accessed. On x86 and x64 platforms, the address of each memory area is determined by the ACPI 'MCFG' table. The format of this ACPI table is:
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