Sun Microelectronics
102
UltraSPARC User’s Manual
7.7 Cache Coherent Transactions
This section specifies the cache coherent transactions (that is, transactions issued
to access cacheable main memory address space), and the final Etag cache state of
the requesting interconnect master after the transaction completes.
7.7.1 ReadToShare (P_RDS_REQ)
Coherent Read to share. Generated by UltraSPARC due to a load miss.
The system provides the data to the UltraSPARC with S_RBS (Read Block
S
hared) reply if another cache also shares it, and S_RBU (Read Block Unshared)
reply if no other cache has it.
If this read transaction displaces a dirty victim block in the cache (Etag state is M
or O), UltraSPARC sets the Dirty Victim Pending (DVP) bit in the request packet.
If no other cache has this datum (that is, if this is the first read of the datum), then
Etag transitions to E. This gives exclusive access to the requesting UltraSPARC to
later write this datum without generating another interconnect transaction.
If SC determines that another cache also has this datum, Etag transitions to S.
Table 7-10 shows the number of outstanding ReadToShare transactions that each
UltraSPARC model supports.
7.7.1.1 Error Handling
The system can reply with S_RTO (time-out, typically if the address is for unim-
plemented memory), or S_ERR (bus error, typically if the access is illegal). These
in turn generate data access or instruction access error exceptions as described in
Chapter 11, “Error Handling.”
7.7.2 ReadToShareAlways (P_RDSA_REQ)
Coherent Read to share always. Generated by a UltraSPARC for an I-Cache miss.
Table 7-10
Supported Number of Outstanding ReadToShare Transactions
UltraSPARC-I
UltraSPARC-II
Number
1
3
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