MIPS R10000 Microprocessor User's Manual - SGI TechPubs Library

130 Chapter 6.
External Completion Response Protocol
An external agent issues an external completion response to provide an
acknowledge, error, or negative acknowledge to an outstanding request, and to
free the associated request number.
An external agent issues an external completion response by driving the response
on SysResp[4:0] and asserting SysRespVal* for one SysClk cycle. SysResp[4:2]
contains the request number associated with the corresponding outstanding
request and SysResp[1:0] contains an acknowledge, error, or negative
acknowledge indication, as described below:
• The external agent issues an external ACK completion response for a
processor read or upgrade request to indicate that the request was
successful. An external ACK completion response may only be issued
for a processor read request if a corresponding external data response
is coincidentally or previously issued.
• The external agent issues an external ERR completion response for a
processor read or upgrade request to indicate that the request was
unsuccessful. Upon receiving an external ERR completion response, the
processor takes a Bus Error exception on the associated instruction. If
the processor read or upgrade request was caused by a PREFETCH
instruction, no exception is taken. Also, if the request was caused by a
speculative instruction, no exception is taken.
• The external agent issues an external NACK completion response for a
processor read or upgrade request to indicate that the request was not
accepted. Upon receiving an external NACK completion response, the
processor re-evaluates the associated instruction. Due to the
speculative nature of the R10000 processor, the re-evaluation may or
may not result in the reissue of a similar processor request.
An external ERR or NACK completion response issued in response to an external
intervention, allocate request number, or invalidate has no affect on the processor
except to free the request number.
Version 2.0 of January 29, 1997 MIPS R10000 Microprocessor User's Manual