82815 GMCH
R
40
Datasheet
3.3.
I/O Mapped Registers
The GMCH 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
I/O Address:
Default Value:
Access:
Size:
Configuration Address Register
0CF8h Accessed as a DWord
00000000h
Read/Write
32 bits
CONF_ADDR is a 32 bit register accessed only when referenced as a DWord. A Byte or Word reference
will “pass through” the Configuration Address Register onto the PCI0 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.
31
30
24
23
24
CFGE
Reserved (0)
Bus Number
15
11
10
8
7
2
1
0
Device Number
Function Number
Register Number
Reserved
Bit
Descriptions
31
Configuration Enable (CFGE).
This bit enables/disables accesses to PCI configuration space.
1 = Enabled.
0 = Disable.
30:24
Reserved.
These bits are read-only and have a value of 0.
23:16
Bus Number.
When the Bus Number is programmed to 00h the target of the Configuration Cycle is one
of the three devices in the GMCH or the PCI Bus (the hub interface is logically a PCI bus) that is directly
connected to the GMCH, depending on the Device Number field.
A type 0 Configuration Cycle is generated on the hub interface if the Bus Number is programmed to 00h
and the GMCH is not the target.
If the Bus Number is non-zero and matches the value programmed into the Secondary Bus Number
Register a Type 0 PCI configuration cycle will be generated on the AGP bridge.
If the Bus Number is non-zero, greater than the value in the Secondary Bus Number Register (Device 1)
and less than or equal to the value programmed into the Subordinate Bus Number Register (Device 1) a
Type 1 PCI configuration cycle will be generated on the AGP bridge.
If the Bus Number is non-zero, and is less than the value programmed into the Secondary Bus Number
or is greater than the value programmed into the Subordinate Bus Number Register a Type 1 hub
interface configuration cycle is generated.