6. System Interface Operations

6.16 System Interface Arbitration

The processor supports a simple System interface arbitration protocol, which relies on an external arbiter. This protocol is used in uniprocessor systems, multiprocessor systems using dedicated external agents, and multiprocessor systems using the cluster bus. System interface arbitration is handled by the SysReq*, SysGnt*, and SysRel* signals (request, grant, and release).

As described earlier in this chapter, the System interface resides in either master or slave state; the processor enters slave state during all of the reset sequences.

When mastership of the System interface changes, there is always one dead SysClk cycle during which the bidirectional signals are not driven; the processor ignores all bidirectional signals during this dead SysClk cycle.

The protocol supports overlapped arbitration which allows arbitration to occur in parallel with requests and responses. This results in fewer wasted cycles when mastership of the System interface changes.

Grant parking is also supported, allowing a device to retain mastership of the System interface as long as no other device requests the System interface.

In multiprocessor systems using the cluster bus, the external arbiter typically implements a round-robin priority scheme.

Copyright 1995, MIPS Technologies, Inc. -- 29 JAN 96

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