MIPS R4000 Microprocessor User's Manual 375
System Interface
Data Identifier Bit Definitions
SysCmd(7) marks the last data element and SysCmd(6) indicates whether
or not the data is response data, for both processor and external coherent
and noncoherent data identifiers. Response data is data returned in
response to a read request or an intervention request.
SysCmd(5) indicates whether or not the data element is error free.
Erroneous data contains an uncorrectable error and is returned to the
processor, forcing a bus error. In the case of a block response, the entire
line must be delivered to the processor no matter how minimal the error.
The processor delivers data with the good data bit deasserted if a primary
parity error is detected for a transmitted data item. If the system is in ECC
mode, a secondary cache data ECC error is detected by comparing the
values transmitted on the SysAD and SysADC.
SysCmd(4) indicates to the processor whether to check the data and check
bits for this data element, for both coherent and noncoherent external data
identifiers.
SysCmd(3) is reserved for external data identifiers.
SysCmd(4:3) are reserved for both coherent and noncoherent processor
data identifiers.
SysCmd(2:0) indicate the data cache state to load the cache line, in
response to processor coherent read requests for coherent data identifiers.
SysCmd(2:0) also indicate the cache state for response data to an external
intervention request, or for the data cycle issued in response to an external
snoop request. SysCmd(2:0) are reserved for noncoherent data identifiers.
Table 12-25 lists the encodings of SysCmd(7:3) for processor data
identifiers. Table 12-26 lists the encodings of SysCmd(7:3) for external
data identifiers. Table 12-27 lists the encodings of SysCmd(2:0) for
coherent data identifiers.
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