MIPS R4000 Microprocessor User's Manual 313
System Interface
External Requests
External requests include read, write, invalidate, update, snoop,
intervention, and null requests, as shown in Figure 12-11. External
invalidate, update, snoop and intervention requests, as a group, are
referred to as external coherence requests. This section also includes a
description of read response, a special case of an external request.
Figure 12-11 External Requests
Read request asks for a word of data from the processor’s internal resource.
Write request provides a word of data to be written to the processor’s
internal resource.
Invalidate request specifies a cache line, in the primary and secondary
caches of the processor, that must be marked invalid.
Update request provides a doubleword, partial doubleword, word, or
partial word of data to be written to the processor’s primary and
secondary caches.
Snoop request checks the processor’s secondary cache to see if a valid copy
of a particular cache line exists. If a valid copy exists, the processor returns
the state of the cache line at the specified physical address in the secondary
cache, and can modify the state of the cache line.
Intervention request requires the processor to return the state of the
secondary cache line at the specified physical address. Under certain
conditions related to the state of the cache line and the nature of the
intervention request, the contents of the primary and secondary cache line
can be returned. The state of the line can also be modified by this request.
R4000
External Agent
External Requests
• Read
• Write
• Null
• Invalidate
• Update
• Snoop
• Intervention
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