MIPS R4000 Microprocessor User's Manual 341
System Interface
External Request Protocols
External requests can only be issued with the System interface in slave
state. An external agent asserts ExtRqst* to arbitrate (see External
Arbitration Protocol, below) for the System interface, then waits for the
processor to release the System interface to slave state by asserting
Release* before the external agent issues an external request. If the System
interface is already in slave state—that is, the processor has previously
performed an uncompelled change to slave state—the external agent can
begin an external request immediately.
After issuing an external request, the external agent must return the
System interface to master state. If the external agent does not have any
additional external requests to perform, ExtRqst* must be deasserted two
cycles after the cycle in which Release* was asserted. For a string of
external requests, the ExtRqst* signal is asserted until the last request
cycle, whereupon it is deasserted two cycles after the cycle in which
Release* was asserted.
The processor continues to handle external requests as long as ExtRqst* is
asserted; however, the processor cannot release the System interface to
slave state for a subsequent external request until it has completed the
current request. As long as ExtRqst* is asserted, the string of external
requests is not interrupted by a processor request.
This section describes the following external request protocols:
• read
• null
• write
• invalidate and update
• intervention
• snoop
• read response
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