Chapter 12
312 MIPS R4000 Microprocessor User's Manual
A processor potential update request remains potential until the read
response to the pending processor read request which began the cluster is
received by the external agent.
• If the read response data is returned in one of the shared
states—shared or dirty shared—the potential update becomes
compulsory and is no longer potential. A compulsory update
must receive an acknowledge either by the signal IvdAck* or
IvdErr*.
• If the read response data is returned in one of the exclusive
states—clean exclusive or dirty exclusive—the potential update is
nullified and the processor neither expects nor requires an
acknowledge.
Write Request as Part of a Cluster
A write request that is part of a cluster obeys the WrRdy* timing rules for
issuing, as shown earlier in Figure 12-3.
Null Write Request as Part of a Cluster
Since the processor accepts external requests between the issue of a read-
with-write-forthcoming request that begins a cluster and the issue of the
write request that completes a cluster, it is possible for an external request
to eliminate the need for the write request in the cluster. For example, if
the external agent issued an external invalidate request that targeted the
cache line the processor was attempting to write back, the state of the
cache line would be changed to invalid and the write back for the cache
line would no longer be needed. In this event, the processor issues a
processor null write request after completing the external request to
complete the cluster.
Processor null write requests do not obey the WrRdy* flow control rules
for issuance, rather they issue with a single address cycle regardless of the
state of WrRdy*. Any external request that changes the state of a cache
line from dirty exclusive or dirty shared to clean exclusive, shared, or
invalid obviates the need for a write back of that cache line.
To Next Page
Loading...