
Chapter 3. Memory Access Protocol
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A processor with a snoop hit that requires a push uses the window to request the address
bus. After it gains the address bus, it uses the address and data tenures only to perform a
push. In some cases, a processor may have a queued snoop push and receive a snoop hit that
requires another push. The processor can use the window to perform the queued push and
not queue the second push. If the processor is parked in the cycle after AACK, a processor
with a snoop does not generate a fast push; instead, it acts as if it were not parked.
The SHD signal can also be asserted either coincident with ARTRY or alone to indicate that
another bus device has a copy of the requested data and that the requesting device should
mark its corresponding cache block as shared (S).
3.3 Data Bus Tenure
This section describes the data bus arbitration, transfer, and termination phases, which are
nearly identical to address tenure phases.
3.3.1 Data Bus Arbitration
Data bus arbitration uses the data arbitration signal group—DBG, DBWO, and DBB.
Additionally, the combination of TS and TT[0–4] provides information about the data bus
request to external logic. Asserting TS is an implied data bus request; the arbiter must
qualify TS with the transfer type (TT[0–4]) encodings to determine if the current address
transfer is an address-only operation (see Table 2-1). If the data bus is needed, the arbiter
grants data bus mastership by asserting DBG to the processor. As with the address bus
arbitration phase, the processor must qualify DBG before assuming bus mastership, as
described in Section 2.6.1, “Data Bus Grant (DBG)—Input.” As shown in Figure 3-7, the
processor asserts DBB on the bus clock cycle after recognition of a qualified data bus grant.
Figure 3-7. Data Bus Arbitration
0
1
2
3
TS
dbg
dbb
drtry
qual DBG
DBB