Mips Technologies R4000 - System Interface; Terminology; System Interface Description

Chapter 12
294 MIPS R4000 Microprocessor User's Manual
12.1 Terminology
The following terms are used in this chapter:
•Anexternal agent is any logic device connected to the processor,
over the System interface, that allows the processor to issue
requests.
•Asystem event is an event that occurs within the processor and
requires access to external system resources.
• Sequence refers to the precise series of requests that a processor
generates to service a system event.
• Protocol refers to the cycle-by-cycle signal transitions that occur
on the System interface pins to assert a processor or external
request.
• Syntax refers to the precise definition of bit patterns on
encoded buses, such as the command bus.
12.2 System Interface Description
The R4000 processor supports a 64-bit address/data interface that can
construct systems ranging from a simple uniprocessor with main memory
to a multiprocessor system with caches and complete cache coherency.
The System interface consists of:
• 64-bit address and data bus, SysAD
• 8-bit SysAD check bus, SysADC
• 9-bit command bus, SysCmd
• eight handshake signals:
- RdRdy*, WrRdy*
- ExtRqst*, Release*
- ValidIn*, ValidOut*
- IvdAck*, IvdErr*
The processor uses the System interface to access external resources such
as cache misses and uncached operations. In the case of R4000MC, the
System interface also supports multiprocessor cache coherency.