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4.1. Enhanced Configuration Access Method Base Address

On PC-compatible systems, the enhanced configuration access mechanism allows PCI configuration space to be accessed using memory primitives rather than I/O-based primitives (CF8/CFC mechanism).
For PCI Express and PCI-X Mode 2 hierarchies on these systems, the memory mapped configuration access mechanism is the only way to access the extended configuration space (offsets 256-4095).

This section defines an ACPI-based mechanisms to communicate the memory mapped configuration space base address(es) used for Enhanced Configuration Access Mechanism (defined in PCI-X and PCI Express Local Bus Specifications) to the operating system:

 MCFG: An ACPI table-based mechanism that is used to communicate the memory mapped configuration space base addresses corresponding to the (non-hot removable) PCI Segment Groups and/or base address ranges within a PCI Segment Group available to the system at boot.
The memory mapped configuration space address ranges described exclusively through the table mechanism are considered to be non-relocatable and non-hot removable for the current boot.

 _CBA: An ACPI method that is used to report the Enhanced Configuration Access base address for any PCI Segment Groups and/or base address ranges within a PCI Segment group.
This allows run-time update for the hot added PCI components.

The MCFG table is only used to communicate the base addresses corresponding to the non-hot removable PCI Segment Groups available to the system at boot.
The _CBA method enables the system to describe the base address of the memory mapped configuration space for hot plug capable PCI Segment Groups.

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