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7. UltraSPARC External Interfaces
This is the same as the ReadToShare transaction, except that the Etag of the re-
questing UltraSPARC always transitions to S, and the system provides the data
with S_RBS reply. ReadToShareAlways avoids the overhead of taking read only
lines from E to S state when sharing eventually occurs.
If this 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.
UltraSPARC supports only one outstanding ReadToShareAlways transaction.
7.7.2.1 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.”
7.7.3 ReadToOwn (P_RDO_REQ)
Coherent Read to Own. Generated by UltraSPARC for a store miss or atomic
miss, or for a store hit or atomic hit on a shared line.
Etag transitions to M.
For a store miss or atomic miss, SC gets data from memory or another processor
and provides it to UltraSPARC with the S_RBU reply, after SC receives P_SACK
or P_SACKD reply from all other interconnect ports sharing this block.
If UltraSPARC already has the block in the S or O state and wants exclusive own-
ership in order to write the block (store hit or atomic hit), no data is transferred
and SC replies with S_OAK (Exclusive Ownership Ack) after receiving P_SACK
or P_SACKD from all other interconnect ports sharing this block. It is legal to
transfer data to the processor even in this case. In systems without Dtags, this
must be done.
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.
Table 7-11 shows the number of outstanding ReadToOwn transactions that each
UltraSPARC model supports.
Table 7-11
Supported Number of Outstanding ReadToOwn Transactions
UltraSPARC-I
UltraSPARC-II
Number
1
3
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