Accelerated Graphics Port Interface Specification

AGP3.0 Interface Specification
Rev. 1.0
112
Since the Fan-out Bridge is transparent, it implies that the AGP Devices are logically on the primary
segment and are indistinguishable to software from multiple devices on a traditional multi-drop bus.
• The Fan-out Bridge cannot have an impact on the AGP flow control protocol. This implies that it
must supply adequate buffering to manage the flow control on each side of the bridge independently.
• No direct peer-to-peer capability should be assumed between devices behind a fan-out bridge. The
core-logic is not required to comprehend such traffic and no ordering rules for such traffic with
respect to the AGP traffic will be specified. Any peer-to-peer implementation scheme is outside the
scope of this specification. Indirect peer-to-peer between such devices using a shared area in
system memory is possible within the scope of this specification.
• AGP devices installed in the slots supported by the bridge are configured as Type 0 PCI devices.
Since the Fan-Out Bridge is transparent, it must direct the PCI configuration cycles to the
appropriate AGP3.0 device based on the IDSEL. At the AGP device level, IDSEL is always tied to
AD16. Therefore, the Fan-Out Bridge must implement a scheme to direct configuration cycles with
different IDSELs from the core-logic to the appropriate AGP device with IDSEL remapped to AD16.
Figure 4-5 illustrates a possible scheme.
Figure 4-5: Fan-out Bridge Scheme Example
Core-
logic
Fan-
Out
Bridg
Device 1
Device 2
PCI Config with
IDSEL
-
>AD16
IDSEL->AD16
IDSEL->AD16
Core-
logic
Fan-
Out
Bridg
Device 1
Device 2
PCI Config with
IDSEL
-
>AD17
IDSEL->AD16
IDSEL->AD16