Intel
®
81341 and 81342—Messaging Unit
Intel
®
81341 and 81342 I/O Processors
Developer’s Manual
December 2007
410
Order Number: 315037-002US
4.5
Circular Queues
The MU implements four circular queues. There are 2 inbound queues and 2 outbound
queues. In this case, inbound and outbound refer to the direction of the flow of posted
messages.
Inbound messages are either:
• posted messages by other processors for the Intel XScale
®
processor to process or
• free (or empty) messages that can be reused by other processors.
Outbound messages are either:
• posted messages by the Intel XScale
®
processor for other processors to process or
• free (or empty) messages that can be reused by the Intel XScale
®
processor.
Therefore, free inbound messages flow away from the 81341 and 81342 and free
outbound messages flow toward the 81341 and 81342.
The four Circular Queues are used to pass messages in the following manner. The two
inbound queues are used to handle inbound messages and the two outbound queues
are used to handle outbound messages. One of the inbound queues is designated the
Free queue and it contains inbound free messages. The other inbound queue is
designated the Post queue and it contains inbound posted messages. Similarly, one of
the outbound queues is designated the Free queue and the other outbound queue is
contains a summary of the queues.
The two outbound queues allow the Intel XScale
®
processor to post outbound
messages in one queue and to receive free messages returning from the host
processor. The Intel XScale
®
processor posts outbound messages, the host processor
receives the posted message and when it is finished with the message, places it back
on the outbound free queue for reuse by the Intel XScale
®
processor.
The two inbound queues allow the host processor to post inbound messages for the
81341 and 81342 in one queue and to receive free messages returning from the 81341
and 81342. The host processor posts inbound messages, the Intel XScale
®
processor
receives the posted message and when it is finished with the message, places it back
on the inbound free queue for reuse by the host processor.
Warning:
External PCI write transactions to the Inbound Post Queue or the Outbound Free Queue
with NULL byte enables (all byte enables deasserted) causes the Head Pointer to
increment.
Table 260. Circular Queue Summary
Queue Name
Purpose
Action on PCI
Interface
Inbound Post
Queue
Queue for inbound messages from other processors waiting to be
processed by the 81341 and 81342
Written
Inbound Free
Queue
Queue for empty inbound messages from the 81341 and 81342 available
for use by other processors
Read
Outbound Post
Queue
Queue for outbound messages from the 81341 and 81342 that are being
posted to the other processors
Read
Outbound Free
Queue
Queue for empty outbound messages from other processors available for
use by the 81341 and 81342
Written