Sun Microelectronics
95
7. UltraSPARC External Interfaces
7.6.1 State Transitions
Figure 7-20 on page 95 shows the cache coherency state diagram. Table 7-9 on
page 97 describes these transitions. It also shows the transactions that are initiat-
ed by either UltraSPARC or the SC, along with the expected acknowledgment fol-
lowing each transaction.
Figure 7-20
Cache Coherence Protocol State Diagram
Note:
These are not necessarily the transitions seen by a cache line at index [i];
rather, they are the transitions for a data block that is moving to/from a cache
line. The Invalid state in this context means that the block is not present in this
cache, but it may be present in another cache.
The following are invariants for the state transitions:
1.
Only one cache in the system can ever have the line in E or M state; while
a line is in E or M state, no other cache can have a copy of that line.
2.
Only one cache in the system can ever have the line in the O state; any
other cache having that line must have it in the S state.
3.
For ReadToOwn transactions, when data transfer is needed, the line should
be sourced from a cache that has the line in the M or O state. The line is
sourced from the addressed location in memory only if no cache has it.
4.
With a P_WRB_REQ transaction, a cache line is written to the destination
address only if its state is M or O. The Writeback is cancelled if its state is I.
5.
With a P_WRI_REQ transaction, data is written to memory regardless of its
state.
M
E
S
I
O
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