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50
Intel
82865G/82865GV GMCH Datasheet
Register Description
3.3
Routing Configuration Accesses
The GMCH supports two bus interfaces: HI and AGP/PCI. PCI configuration cycles are selectively
routed to one of these interfaces. The GMCH is responsible for routing PCI configuration cycles to
the proper interface. PCI configuration cycles to ICH5 internal devices and Primary PCI (including
downstream devices) are routed to the ICH5 via HI. AGP/PCI_B configuration cycles are routed to
AGP. The AGP/PCI_B interface is treated as a separate PCI bus from a configuration point of view.
Routing of configuration AGP/PCI_B is controlled via the standard PCI-to-PCI bridge mechanism
using information contained within the Primary Bus Number, the Secondary Bus Number, and the
Subordinate Bus Number registers of the corresponding PCI-to-PCI bridge device.
A detailed description of the mechanism for translating processor I/O bus cycles to configuration
cycles on one of the buses is described in the following sub-sections.
3.3.1
Standard PCI Bus Configuration Mechanism
The PCI Bus defines a slot based “configuration space” that allows each device to contain up to
eight functions with each function containing up to 256, 8-bit configuration registers. The PCI
specification defines two bus cycles to access the PCI configuration space: Configuration Read and
Configuration Write. Memory and I/O spaces are supported directly by the processor.
Configuration space is supported by a mapping mechanism implemented within the GMCH. The
PCI 2.3 specification defines the configuration mechanism to access configuration space. The
configuration access mechanism uses the CONFIG_ADDRESS register (at I/O address 0CF8h
though 0CFBh) and CONFIG_DATA register (at I/O address 0CFCh though 0CFFh). To reference
a configuration register a DWord I/O write cycle is used to place a value into CONFIG_ADDRESS
that specifies the PCI bus, the device on that bus, the function within the device, and a specific
configuration register of the device function being accessed. CONFIG_ADDRESS[31] must be 1
to enable a configuration cycle. CONFIG_DATA then becomes a window into the four bytes of
configuration space specified by the contents of CONFIG_ADDRESS. Any read or write to
CONFIG_DATA will result in the GMCH translating the CONFIG_ADDRESS into the
appropriate configuration cycle.
The GMCH is responsible for translating and routing the processor’s I/O accesses to the
CONFIG_ADDRESS and CONFIG_DATA registers to internal GMCH configuration registers,
HI, or AGP/PCI_B.
3.3.2
PCI Bus #0 Configuration Mechanism
The GMCH decodes the Bus Number (bits 23:16) and the Device Number fields of the
CONFIG_ADDRESS register. If the Bus Number field of CONFIG_ADDRESS is 0, the
configuration cycle is targeting a PCI Bus 0 device. The Host-HI Bridge entity within the GMCH is
hardwired as Device 0 on PCI Bus 0. The Host-AGP/PCI_B Bridge entity within the GMCH is
hardwired as Device 1 on PCI Bus 0. Device 6 contains test configuration registers.
3.3.3
Primary PCI and Downstream Configuration Mechanism
If the Bus Number in the CONFIG_ADDRESS is non-zero, and is less than the value in the Host-
AGP/PCI_B device’s Secondary Bus Number register or greater than the value in the Host-AGP/
PCI_B device’s Subordinate Bus Number register, the GMCH generates a Type 1 HI Configuration