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4–28 Cache and External Interfaces
21264/EV68A Hardware Reference Manual
System Port
The 21264/EV68A holds pending probe commands in a 8-entry deep probe queue. The
system must count the number of probes that have been sent and ensure that the probes
do not overrun the 21264/EV68A queue. The 21264/EV68A removes probes from the
internal probe queue when the probe response is sent.
The 21264/EV68A expects to hit in cache on a probe response, so it always fetches a
cache block from the Bcache on system probes. This can become a performance prob-
lem for systems that do not monitor the Bcache tags, so the 21264/EV68A provides
Cbox CSR PRB_TAG_ONLY[0], which only accesses Bcache tags for system probes.
For a Bcache hit, the 21264/EV68A retries the probe reference to get the associated
data. In this mode, the 21264/EV68A has a cache-hit counter that maintains some his-
tory of past cache hits in order to fetch the data with the tag in the cases where streamed
transactions are being performed to the host processor.
4.7.7.2 Data Transfer Commands (Two Cycles)
Data transfer commands use a 2-cycle format on SysAddIn_L[14:0]. The SysDc[4:0]
field indicates success or failure for ChangeToDirty and MB commands, and error con-
ditions as shown in Table 4–24.
The pattern of data is controlled by the SysDataInValid_L and SysDataOutValid_L
signals. These signals are valid each cycle of data transfer, indicating any gaps in the
data cycle pattern. The SysDataInValid_L and SysDataOutValid_L signals are
described in Section 4.7.8.4. Table 4–23 shows the format of the data transfer com-
mand.
101 Invalid
110 Transition1
2
:Clean Clean/Shared
Dirty
Dirty/Shared
111 Reserved
1
Transition3 is useful in nonduplicate tag systems that want to give writable status to the reader and do
not know if the block is clean or dirty.
2
Transition1 is useful in nonduplicate tag systems that do not update memory on ReadBlk hits to a
dirty block in another processor.
Table 4–23 Data Transfer Command Format
SysAddIn_L[14:2] SysAddIn_L[1] SysAddIn_L[0]
Cycle 1 0 SysDc[4:0] RVB RPB A ID[3:0] X X
Cycle 2 C X X X
Table 4–22 Next Cache Block State Selection by Probe[2:0] (Continued)
Probe[2:0] Next Tag State