The Purpose of the Response Phase

The possible responses that the Response Agent may supply in the transaction's Response Phase are:

• The Response Agent may command the Request Agent to retry the transaction repeatedly until it succeeds (or fails). The Response Agent can't service the request now, but will be able to later.

• The Response Agent may inform the Request Agent that it will defer completion of the transaction until a later time. The Response Agent will service the request (read or write) off-line and will deliver the results to the Request Agent in a subsequent Deferred Reply transaction.

• The Response Agent may indicate a hard failure to the Request Agent. The Response Agent is broken and can't service the request at all.

• If the transaction is one that doesn't require the Response Agent to send data to the Request Agent (i.e., it is a write transaction, a Special transaction, a Memory Read, or Memory Read and Invalidate transaction for 0 bytes), the Response Agent indicates that, as requested, no data will be returned to the Request Agent.

• If the transaction is a memory read or write that results in a hit on a modified line in the Snoop Phase, the Response Agent indicates that the Snoop Agent will transfer the entire modified line to memory (referred to as an implicit writeback operation) in the Data Phase of the transaction (and, if it's a read transaction, to the Request Agent at the same time).

• If the transaction is any form of a read transaction (i.e., a Memory Read, a Memory Read and Invalidate for 64 bytes, an IO Read, or an Interrupt Acknowledge), the Response Agent indicates that it will return the requested data in the Data Phase (alternatively, it may choose to defer delivery of the read data until a later time). This is referred to as the normal data response.