Chapter 12
324 MIPS R4000 Microprocessor User's Manual
If the current cache line does not need to be written back, the coherency
attribute for the page that contains the requested cache line is update, and
potential updates are enabled, the processor issues a cluster consisting of
a read request, followed by a potential update request.
In an update protocol, the cache line requested by a processor coherent
read request can be returned in a shared state; the processor then has to
issue an update request before it can complete a store instruction. A
potential update issued with a read request in a cluster allows the external
agent to anticipate the read response on the system bus. If the read
response is in a shared state, the required update is quickly transmitted to
the rest of the system. This provides the processor with the acknowledge
and allows the processor to complete the store instruction as rapidly as
possible.
Without the potential update request, the response data must be returned
to the processor. If the line is returned in the shared or dirty shared state,
the processor issues an update request, which must then be forwarded to
the system bus before an acknowledge can be returned to the processor.
Note that potential updates behave as if they have not yet been issued by
the processor. Potential updates are not subject to cancellation, and do not
require an acknowledge. When a potential update is nullified, the
processor behaves as if no update request was ever issued; when a
potential update becomes compulsory, the processor behaves as if it had
issued an update request at that instant.
Compulsory Update: If the processor issues a cluster that contains a
potential update, and the response data for the read request is
returned with an indication that it must be placed in the cache in either
a shared or dirty shared state, the potential update then becomes
compulsory. Once a potential update becomes compulsory, it is
subject to cancellation, and the processor requires an acknowledge for
the update request. The external agent must forward the update to the
system, then signal the acknowledge to the processor when the update
is complete. The processor will not complete the store until it has
received an acknowledge for the update request.
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