Intel
®
81341 and 81342 I/O Processors
December 2007
Developer’s Manual
Order Number: 315037-002US
407
Messaging Unit—Intel
®
81341 and 81342
4.2.1
Transaction Ordering
From a PCI standpoint, the Messaging Unit is a piece of the ATU and therefore must
maintain ordering requirements against ATU transactions. Transaction ordering is
achieved for the Index Registers, the Doorbell Register, and the Message Registers
since these transactions are routed through the standard set of ATU read/write queues.
The Circular Queues (Inbound/Outbound Queue Port) are separate queue structures
and therefore require ordering. The Inbound Post Queue (contains PCI writes) must be
ordered against the inbound write queue of the ATU to allow the data that is
represented by the Inbound Post interrupt to be written to local memory before the
interrupt is delivered. See
for a summary of Messaging Unit transaction
ordering.
Table 259. Circular Queue Ordering Requirements
Messaging Unit Feature
Transaction Ordering Mechanism
Message Registers
Doorbell Registers
Index Registers
Through ATU Queues
Through ATU Queues
Through ATU Queues
Circular Queues
Inbound Post
Ordered Against ATU Inbound Write Queue (PMW Can
Not Pass Another PMW)
Inbound Free
No Specific Hardware Ordering
Outbound Post
No Specific Hardware Ordering
Outbound Free
No Specific Hardware Ordering