The Special Transaction
Under some circumstances, an IA32 processor generates a Special transaction to broadcast a message to the platform. In other words, this is not a point-to-point transaction that targets a particular agent. As indicated earlier, every transaction requires a Response Agent, and the Root Complex is typically the Response Agent (i.e., the central agent) for the Special transaction. This transaction has the following characteristics:
Although the content of the address bus in Packet A is “don't care,” it is factored into the address parity on AP[1:0]#.
In Packet A, the request type issued on REQ[4:0]# is 01000b (this is the logical, not electrical, value).
In Packet B, REQ[4:0]# is 00x01b, where x is “don't care.”