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Register Description
R
Intel
®
82845 MCH for SDR Datasheet 35
Bit Descriptions
31 Configuration Enable (CFGE).
0 = Disable.
1 = Enable. Accesses to PCI configuration space are enabled.
30:24 Reserved. These bits are read only and have a value of 0.
23:16 Bus Number. When Bus Number is programmed to 00h, the target of the configuration cycle is
a hub interface agent (MCH, ICH2, etc.).
The configuration cycle is forwarded to the hub interface, if Bus Number is programmed to 00h
and the MCH is not the target (the device number is ≥2).
If Bus Number is non-zero and matches the value programmed into the Secondary Bus Number
Register of device 1, a Type 0 PCI configuration cycle will be generated on AGP.
If Bus Number is non-zero, greater than the value in the Secondary Bus Number register of
device 1 and less than or equal to the value programmed into the Subordinate Bus Number
register of device 1 a Type 1 PCI configuration cycle will be generated on AGP.
If Bus Number is non-zero, and does not fall within the ranges enumerated by device 1’s
Secondary Bus Number or Subordinate Bus Number register, then a hub interface Type 1
configuration cycle is generated.
15:11 Device Number. This field selects one agent on the PCI bus selected by the Bus Number field.
When the Bus Number field is 00, the MCH decodes the Device Number field. The MCH is
always Device Number 0 for the Host-Hub Interface bridge entity and Device Number 1 for the
Host-AGP entity. Therefore, when Bus Number =0 and Device Number=0–1, the internal MCH
devices are selected.
If the Bus Number is non-zero and matches the value programmed into the Secondary Bus
Number register a Type 0 PCI configuration cycle is generated on AGP. The MCH decodes the
Device Number field ([15:11]) and assert the appropriate GAD signal as an IDSEL. For PCI-to-
PCI Bridge translation, one of the 16 IDSELs is generated. When bit [15] = 0, bits [14:11] are
decoded to assert a signal AD[31:16] IDSEL. GAD16 is asserted to access Device 0, GAD17 for
Device 1, and so forth up to Device 15 which asserts AD31. All device numbers higher than 15
cause a type 0 configuration access with no IDSEL asserted, which results in a Master Abort
reported in the MCH’s “virtual” PCI-PCI bridge registers.
For Bus Numbers resulting in hub interface configuration cycles, the MCH propagates the device
number field as A[15:11]. For bus numbers resulting in AGP type 1 configuration cycles, the
device number is propagated as GAD[15:11].
10:8 Function Number. This field is mapped to GAD[10:8] during AGP configuration cycles and
A[10:8] during Hub Interface configuration cycles. This allows the configuration registers of a
particular function in a multi-function device to be accessed. The MCH ignores configuration
cycles to its internal devices if the function number is not equal to 0.
7:2 Register Number. This field selects one register within a particular bus, device, and function as
specified by the other fields in the Configuration Address register. This field is mapped to
GAD[7:2] during AGP configuration cycles and A[7:2] during hub interface configuration cycles.
1:0 Reserved.