6.17 System Interface Request and Response Protocol
An external agent may only issue an external request to the processor when the System interface is in slave state. If the System interface is not already in slave state, the external agent must first negate SysGnt* and then wait for the processor to assert SysRel*. If the System interface is already in slave state, the external agent may issue an external request immediately. The total number of outstanding external requests, including interventions, allocate request numbers, and invalidates, cannot exceed eight.
External requests may be accepted by the processor in adjacent SysClk cycles. External intervention and invalidate requests are considered external coherency requests.
An external agent issues an external intervention request with a single address cycle; this address cycle consists of the following:
Figure 6-20 depicts three external intervention requests. Since the System interface is initially in master state, the external agent must first negate SysGnt* and then wait until the processor relinquishes mastership of the System interface by asserting SysRel*.
Figure 6-20 External Intervention Request Protocol
An external agent issues an external allocate request number request with a single address cycle; this address cycle consists of the following:
Figure 6-21 depicts three external allocate request number requests. Since the System interface is initially in master state, the external agent must first negate SysGnt* and then wait until the processor relinquishes mastership of the System interface by asserting SysRel*.
Figure 6-21 External Allocate Request Number Request Protocol
An external agent issues an external invalidate request with a single address cycle. This address cycle consists of the following:
Figure 6-22 depicts three external invalidate requests. Since the System interface is initially in master state, the external agent must first negate SysGnt* and then wait until the processor relinquishes mastership of the System interface by asserting SysRel*.
Figure 6-22 External Invalidate Request Protocol
An external agent issues an external interrupt request with a single address cycle. This address cycle consists of the following:
Figure 6-23 depicts three external interrupt requests. Since the System interface is initially in master state, the external agent must first negate SysGnt* and then wait until the processor relinquishes mastership of the System interface by asserting SysRel*.
Figure 6-23 External Interrupt Request Protocol
External Intervention Request Protocol
An external agent issues an external intervention request to obtain a Shared or Exclusive copy of a secondary cache block.
An external agent may only issue an external intervention request address cycle when the System interface is in slave state; typically a free request number is specified. An external agent may have as many as eight external intervention requests outstanding on the System interface at any given time.External Allocate Request Number Request Protocol
An external agent issues an external allocate request number request to reserve a request number for private use. Once allocated, the processor is prevented from using the request number until an external completion response for the request number is received.
An external agent may only issue an external allocate request number request address cycle when the System interface is in slave state and there is a free request number. The external agent may have as many as eight external allocate request number requests outstanding on the System interface at any given time.External Invalidate Request Protocol
An external agent issues an external invalidate request to invalidate a secondary cache block.
An external agent may only issue an external invalidate request address cycle when the System interface is in slave state; typically a free request number is specified. An external agent may have as many as eight external invalidate requests outstanding on the System interface at any given time.External Interrupt Request Protocol
An external agent issues an external interrupt request to interrupt the normal instruction flow of the processor.
An external agent may only issue an external interrupt request address cycle when the System interface is in slave state.
Copyright 1995, MIPS Technologies, Inc. -- 29 JAN 96
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