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82801AA and 82801AB Datasheet
5-1
Functional Description
Functional Description
5
5.1
Hub Interface to PCI Bridge (D30:F0)
The hub interface to PCI Bridge resides in PCI Device 30, Function 0 on bus #0. This portion of the
ICH implements the buffering and control logic between PCI and the Hub Interface. The
arbitration for the PCI bus is handled by this PCI device. The PCI decoder in this device must
decode the ranges for the Hub Interface. All register contents are lost when core well power is
removed.
5.1.1
PCI Bus Interface
The ICH PCI interface provides a 33 MHz, Rev. 2.2 compliant implementation. All PCI signals are
5V tolerant, except PME#. The ICH integrates a PCI arbiter that supports up to four
(ICH0: 82801AB) or six (ICH: 82801AA) external PCI bus masters in addition to the internal ICH
requests.
Note:
Most transactions targeted to the ICH first appear on the external PCI bus before being claimed
back by the ICH. The exceptions are I/O cycles involving USB, IDE, and AC’97. These
transactions complete over the Hub Interface without appearing on the external PCI bus.
Configuration cycles
targeting USB, IDE or AC’97 appears on the PCI bus.
Note:
If the ICH is programmed for positive decode, the ICH grabs the cycles appearing on the external
PCI bus in medium decode time. If the ICH is programmed for subtractive decode, the ICH grabs
these cycles in subtractive time. If the ICH is programmed for subtractive decode, these cycles can
be grabbed by another positive decode agent out on PCI.
Note:
This architecture enables the ability to boot off of a PCI card that positively decodes the boot
cycles. To boot off a PCI card it is necessary to keep the ICH in subtractive decode mode. When
booting off a PCI card, the BOOT_STS bit (bit 2, TCO2 Status Register) is set.
Note:
PCI configuration write cycles, initiated by the processor, with the following characteristics are
converted to a special cycle with the Shutdown message type.
— Device Number (AD[15:11]) = 11111
— Function Number (AD[10:8]) = 111
— Register Number (AD[7:2]) = 000000
— Data = 00h
— Bus number matches secondary bus number
5.1.2
PCI-to-PCI Bridge Model
From a software perspective, the ICH contains a PCI-to-PCI bridge. This bridge connects the hub
interface to the PCI bus. By using the PCI-to-PCI bridge software model, the ICH can have its
decode ranges programmed by existing plug-and-play software such that PCI ranges do not
conflict with graphics ranges in the host controller.