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4–40 Cache and External Interfaces
21264/EV68A Hardware Reference Manual
System Port
4.7.10 Ordering of System Port Transactions
This section describes ordering of system port transactions. The two classes of transac-
tions are listed here:
• 21264/EV68A commands and system probes
• System probes and SysDc transfers
4.7.10.1 21264/EV68A Commands and System Probes
This section describes the interaction of 21264/EV68A-generated commands and sys-
tem-generated probes that reference the same cache block. Some definitions are pre-
sented here:
• ProbeResponses generated by the 21264/EV68A respond to all system-generated
probe commands. System-generated data transfer commands respond to all 21264/
EV68A-generated data transfer commands.
• The victim address file (VAF) and victim data buffer (VDB) entries each have inde-
pendent valid bits for both a victim and a probe.
• Probe results indicate a hit on a VAF/VDB and when a WrVictim command has
been sent to the system. Systems can decide whether to move the buffer once or
twice.
• ProbeResponses are issued in the order that the system-generated probes were
received; however, there is no requirement for the system to retain order when issu-
ing release buffer commands.
• Probe processing can stall inside the 21264/EV68A when the probe entry index
matches PA[19:6] of a previous probe entry in the VAF.
• The 21264/EV68A reserves one VAF entry for probe processing, so that VAF-full
conditions cannot stall the processing of probes at the head of the queue.
Table 4–33 lists all interactions between pending internal 21264/EV68A commands
and the Probe[2:0] command field, Next Cache Block State, described in Table 4–22.
Table 4–33 shows the 21264/EV68A response to system probe and in-flight command
interaction. In the table, note the following:
• ReadBlkVic and ReadBlkModVic commands do not appear in Table 4–33. If there
is interaction between the probe and the victim, it is the same as a WrVictimBlk
command.
CleanToDirty
SharedToDirty
STCChangeToDirty
ChangeToDirty commands to NXM space are impossible in the 21264/EV68A because
all NXM references to memory space are atomically filled with an Invalid cache status.
InvalToDirty
InvalToDirtyVic
InvalToDirty commands are not speculative, so InvalToDirty commands to NXM space
indicate an operating system error. Systems should respond with a SysDc ReadDataError,
and should generate a machine check to indicate error.
Table 4–32 21264/EV68A Commands with NXM Addresses and System Response (Continued)
21264/EV68A
Command NXM
Address System/21264/EV68A Response