The Request Agent starts driving a transaction request on the rising-edge of BCLK0. In Figure 49-1 on page 1205, a request is issued at the start of BCLK cycle 1 and another at the start of BCLK cycle 3. The signals that comprise the Request Phase signal group are divided into subgroups on the system board:
The Address Strobe 0 signal trace is routed with the A[16:3]# and REQ[4:0]# signal traces.
The Address Strobe 1 signal trace is routed with the A[35:17]# signal traces.
In the first half of the BCLK cycle, the Request Agent:
- drives out address bits [35:3] on A[35:3]#,
- drives out the transaction type on REQ[4:0]#,
- drives both of the Address Strobe signals (ADSTB[1:0]#) low.
The information on A[35:3]# and REQ[4:0]# comprises request Packet A.
All of the FSB agents use the falling-edge of the two Address Strobes to latch Packet A into their input receivers. At this point, processors on the FSB have all of the information necessary to determine whether or not it is a memory transaction (i.e., the memory address and the transaction type) and, if it is, they initiate a snoop in their internal caches. The snoop result will be delivered when the transaction has entered its Snoop Phase.
In the second half of the BCLK cycle, the Request Agent:
- drives out additional transaction information on A[35:3]#,
- drives out additional transaction information on REQ[4:0]#,
- drives both of the Address Strobe signals (ADSTB[1:0]#) high.
The information on A[35:3]# and REQ[4:0]# comprises request Packet B.
All of the FSB agents use the rising-edge of the two Address Strobes to latch Packet B into their input receivers.
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