Sun Microelectronics
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UltraSPARC User’s Manual
7.11.1 Clean Victim Handling
When the victimized line is clean (E, S, or I state), the read request for the new
line is issued with DVP=0, and the following rules apply:
1. UltraSPARC inhibits reading and writing the victimized line by blocking
any activity to the same E-Cache index, except for loads and stores of the
first level caches. Since the D-Cache is writethrough, stores are not
considered to be in the coherence domain until they complete to the
E-Cache.
2. UltraSPARC keeps the victimized block in the coherence domain for
copyback-invalidate requests from SC until it receives the S_REPLY for the
missed line; that is, until the read completes.
7.11.2 Dirty Victim Handling
When the victimized line is dirty (M or O state), the read request for the new line
is issued with DVP=1, and the following rules apply:
1. Reads and writes by UltraSPARC to the same E-Cache index are blocked,
just like for clean victims.
2. UltraSPARC keeps the dirty victimized block in the coherence domain for
copyback-invalidate requests from SC until it receives the S_REPLYs for
both the read and Writeback transactions; that is, until both the read and the
Writeback complete.
3. Each UltraSPARC models supports a limited number of outstanding
coherent reads with DVP=1. Table 7-16 and the paragraphs that follow it
discuss these limits.
4. The dirty victimized block transitions to I State only if the associated read
fails; that is, is completed with either S_RTO or S_ERR. When the read
completes normally, the new data overwrites the dirty victimized block.
7.11.3 Writeback Cancellation Requirement
A classic problem in designing cache-coherent interfaces is handling coherency
requests to a line that has a pending Writeback. In this case, UltraSPARC correctly
returns the writeback data, even if the read miss that caused the Writeback has al-
ready completed. However, UltraSPARC does not flush the Writeback if a coher-
ency request took ownership of the line; that is, if SC sent an invalidate
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