Chapter 12
306 MIPS R4000 Microprocessor User's Manual
Processor Read Request
When a processor issues a read request, the external agent must access the
specified resource and return the requested data. (Processor read requests
are described in this section; external read requests are described in
External Requests, later on in this chapter.)
A processor read request can be split from the external agent’s return of
the requested data; in other words, the external agent can initiate an
unrelated external request before it returns the response data for a
processor read. A processor read request is completed after the last word
of response data has been received from the external agent.
Note that the data identifier (see System Interface Commands and Data
Identifiers, in this chapter) associated with the response data can signal
that the returned data is erroneous, causing the processor to take a bus
error.
Processor read requests that have been issued, but for which data has not
yet been returned, are said to be pending. A read remains pending until the
requested read data is returned.
In secondary-cache mode, the external agent must be capable of accepting
a processor read request followed by a potential update request any time
all three of the following conditions are met:
• There is no processor read request pending.
• There is no unacknowledged processor update request.
• The signal RdRdy* has been asserted for two or more cycles.
In no-secondary-cache mode, the external agent must be capable of
accepting a processor read request any time the following two conditions
are met:
• There is no processor read request pending.
• The signal RdRdy* has been asserted for two or more cycles.
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