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Register Description
R
34 Intel
®
82845 MCH for SDR Datasheet
Primary PCI and Downstream Configuration Mechanism
If the Bus Number in the CONF_ADDR is non-zero, and is less than the value in the Host-AGP
device’s Secondary Bus Number register, or greater than the value in the Host-AGP device’s
Subordinate Bus Number register, the MCH will generate a Type 1 hub interface configuration
cycle. The ICH2 compares the non-zero Bus Number with the Secondary Bus Number and
Subordinate Bus Number registers of its P2P bridges to determine if the configuration cycle is
meant for Primary PCI, or a downstream PCI bus.
AGP Configuration Mechanism
From the chip-set configuration perspective, AGP is seen as a PCI bus interface residing on a
Secondary Bus side of the “virtual” PCI-PCI bridges referred to as the MCH Host-AGP bridge. On
the Primary Bus side, the “virtual” PCI-PCI bridge is attached to PCI Bus 0. Therefore, the
Primary Bus Number register is hardwired to 0. The “virtual” PCI-PCI bridge entity converts
Type 1 PCI bus configuration cycles on PCI Bus 0 into Type 0 or Type 1 configuration cycles on
the AGP interface. Type 1 configuration cycles on PCI Bus 0 that have a Bus Number that
matches the Secondary Bus Number of the MCH’s “virtual” Host-to-PCI_B/AGP bridge will be
translated into Type 0 configuration cycles on the AGP interface.
If the Bus Number is non-zero, greater than the value programmed into the Secondary Bus
Number Register, and less than or equal to the value programmed into the Subordinate Bus
Number Register, the MCH will generate a Type 1 PCI configuration cycle on AGP.
3.3 I/O Mapped Registers
The MCH contains two registers that reside in the processor I/O address space: the Configuration
Address (CONF_ADDR) register and the Configuration Data (CONF_DATA) register. The
Configuration Address register enables/disables the configuration space and determines what
portion of configuration space is visible through the configuration data window.
3.3.1 CONF_ADDR—Configuration Address Register
I/O Address: 0CF8h Accessed as a DWord
Default Value: 00000000h
Access: R/W
Size: 32 bits
CONF_ADDR is a 32 bit register that can be accessed only as a DWord. A Byte or Word
reference will "pass through" the Configuration Address register and the hub interface, onto the
PCI bus as an I/O cycle. The CONF_ADDR register contains the Bus Number, Device Number,
Function Number, and Register Number for which a subsequent configuration access is intended.