Sun Microelectronics
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UltraSPARC User’s Manual
2. If UltraSPARC receives the S_REQ for the dirty cache block in the
Writeback Buffer after the S_WAB/S_WBCAN reply for the Writeback
transaction and before the S_RBU/S_RBS reply for the read transaction, the
S_REQ completes atomically and can either result in P_SACK or
P_SNACK. Both P_REPLYs are correct, since the former ends up sourcing
the same data that was just written to memory.
If an S_REQ receives a P_SNACK, SC can send an S_CRAB, but UltraSPARC re-
turns undefined data. There is no reason for SC to send an S_CRAB in this case.
7.6 Cache Coherence Protocol
This section describes the protocol used to maintain coherency between an
UltraSPARC’s internal caches, the E-Cache, and the system. “System” refers to
any other location within the same coherency domain as UltraSPARC; for exam-
ple, it includes caches of other processors connected to the interconnect. The
cache coherence protocol operates on Physically Indexed, Physically Tagged
(PIPT) writeback caches.
The E-Cache maintains inclusion for both the I-Cache and the D-Cache; that is, all
lines in the internal caches are also in the E-Cache. The system is responsible only
for maintaining E-Cache coherency; UltraSPARC ensures that the internal caches
are coherent.
The cache coherence protocol is point-to-point write-invalidate; that is, SC must
issue separate S_INV requests to each cache containing a copy of the line it needs
to invalidate. There are no “broadcast” transmissions on the interconnect.
The protocol is based on the MOESI states maintained in the E-Cache tags of each
master port. Note that subsets of the states, such as MSI, or MOSI, could be used.
Bits within each E-Cache tag define the cache line state of each line:
Table 7-7 E-Cache Coherency State Definition
State Bit
Line State Valid Modified Exclusive
Invalid (I) 0 X X
Shared Clean (S) 1 0 0
Exclusive Clean (E) 1 0 1
Shared Modified (O) 1 1 0
Exclusive Modified (M) 1 1 1
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