MIPS R4000 Microprocessor User's Manual 315
System Interface
Mastership of the System interface always returns to the processor after an
external request is issued. The processor does not accept a subsequent
external request until it has completed the current request. The processor
accepts external requests between the issue of a processor read request, or
a processor read request followed by a potential update request and the
issue of a processor write request within a cluster.
If there are no processor requests pending, the processor decides, based on
its internal state, whether to accept the external request, or to issue a new
processor request. The processor can issue a new processor request even if
the external agent is requesting access to the System interface.
The external agent asserts ExtRqst* indicating that it wishes to begin an
external request. The external agent then waits for the processor to signal
that it is ready to accept this request by asserting Release*. The processor
signals that it is ready to accept an external request based on the criteria
listed below.
• The processor completes any processor request or processor
request cluster that is in progress.
• While waiting for the assertion of RdRdy* to issue a processor
read request, the processor can accept an external request if the
request is delivered to the processor one or more cycles before
RdRdy* is asserted.
• While waiting for the assertion of WrRdy* to issue a processor
write request, the processor can accept an external request
provided the request is delivered to the processor one or more
cycles before WrRdy* is asserted.
• If waiting for the response to a read request after the processor
has made an uncompelled change to a slave state, the external
agent can issue an external request before providing the read
response data.
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