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62 21555 Non-Transparent PCI-to-PCI Bridge User Manual
PCI Bus Transactions
• A target retry in response to a posted write is allowed, but only due to temporary conditions,
such as a buffer-full condition.
The ordering rules apply to transactions crossing the bridge in the same direction.
— A posted write.
— A delayed write and read request.
— A delayed write and read completion.
Delayed completions cross the bridge in the opposite direction of its respective delayed request. Table 16 lists the
21555 transaction ordering rules.
The only ordering restriction the 21555 enforces is ordering with respect to posted writes. No other transaction
other than a delayed write completion can pass a posted write. Posted writes are delivered in the order in which they
are accepted.
Delayed transactions may be initiated by the 21555 in any order, and are not necessarily initiated in the order in
which they are received. When the 21555 initiates a delayed transaction, the 21555 can do the following:
• When the Delayed Transaction Order Control configuration bit is not set, the 21555 uses a rotating fairness
algorithm to select which delayed transaction it initiates next, regardless of the type of target termination is
returned (retry, TRDY#, etc.).
• When the Delayed Transaction Order Control configuration bit is set, the 21555 continues to initiate the same
transaction until a response other than target retry is received.
Table 16. 21555 Transaction Ordering Rules
↓ Pass→
Posted
Write
Delayed Read
Request
Delayed Write
Request
Delayed Read
Completion
Delayed Write
Completion
Posted Write No Yes Yes Yes Yes
Delayed Read
Request
No Yes/No
†
Yes/No
†
Yes Yes
Delayed Write
Request
No Yes/No
†
Yes/No
†
Yes Yes
Delayed Read
Completion
No Yes Yes Yes Yes
Delayed Write
Completion
Yes Yes Yes Yes Yes
† Dependent on the state of the Delayed Transaction Ordering bit.