Idle Response. RS[2:0]# are deasserted. This is the state of RS[2:0]# before and after the response has been delivered to the Request Agent. In other words, immediately upon entry into the Response Phase, RS[2:0]# are in this state and will remain in this state until a valid response is presented. When any of the valid (i.e., non-idle) responses are driven (for one clock), one or more of the RS[2:0]# signals are driven low. All of the valid response patterns have at least one of the RS signals asserted (remember, in this table, a 0 = deasserted = electrically high). After one clock, the response is removed. The RS signals are then returned to the deasserted state (in other words, back to the Idle state).

Retry Response. The Response Agent is commanding the Request Agent to retry the transaction repeatedly until the transaction succeeds (or fails). The Response Agent can't service the request now, but will be able to later. A classic case wherein a Response Agent would issue the Retry response would be as follows:

• A device (e.g., the Root Complex, North Bridge, or MCH) handles a memory write by posting it in a Posted Memory Write buffer.

• If the buffer is currently full, the device would return the Retry response.

Deferred Response. The Response Agent informs the Request Agent that it is deferring completion of the transaction until a later time. In other words, it will service the request off-line and will deliver the results to the Request Agent in a subsequent Deferred Reply transaction.

Hard Failure Response. The Response Agent is indicating a hard failure to the Request Agent. The Response Agent is broken and can't service the request at all.

No Data Response. This response indicates that no data was requested by the Request Agent and therefore no data will be delivered.

• This is the proper response to a write (although data is written to the device, none is requested from it).

• It is also the proper response to a transaction that doesn't require any data to be transferred—the Special transaction, the Memory Read and Invalidate for 0 bytes, the Memory Code or Data Read for 0 bytes, or the IO Read for 0 bytes.

Implicit Writeback Response. This response is issued by the Response Agent if a memory transaction resulted in a hit on a modified line (i.e., HITM# was asserted in the Snoop Phase). The Snoop Agent that has the modified line will supply the modified line to the Response Agent (i.e., the memory controller) as well as to the Request Agent (if it's a read transaction). The author thinks of this as the “don't be startled” response. A non-processor Request Agent (e.g., a bridge on behalf of a device adapter) may be attempting to read less than a line of information and, if the Snoop Agent has a hit on a modified line, it always supplies the full line. The implicit writeback response tells the Request Agent that eight qwords (64 bytes) will be transferred, rather than the smaller data packet actually requested. The eight qwords are transferred in toggle mode order, critical qword first. This means that the first qword sent back by the Snoop Agent will be the first one requested by the Request Agent and, if a second qword was also requested (assume, for example, that this is a 16 byte read request), the second qword sent back is the first qword's toggle mode partner. The Request Agent should just accept the qword(s) requested and ignore the rest. The memory controller (i.e., the Response Agent), on the other hand, will accept the full line.

Normal Data Response. This is the proper response to any read request (that doesn't hit on a modified line and is not deferred)—a Memory Code or Data Read, a Memory Read and Invalidate for 64 bytes, an IO Read, an Interrupt Acknowledge, or a Deferred Reply that is returning previously-requested read data.