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21264/EV68A Hardware Reference Manual
Cache and External Interfaces 4–29
System Port
Table 4–24 describes the SysDc[4:0] field.
The A bit in the first cycle indicates that the command is acknowledged. When A = 1, the
21264/EV68A decrements its command outstanding counter, but the A bit is not neces-
sarily related to the current SysDc command.
Probe commands can combine a SysDc command along with MBDone. In that event,
the probe is considered ahead of the SysDc command. If the SysDc command allows
the 21264/EV68A to retire an instruction before an MB, or allows the 21264/EV68A
itself to retire an MB (SysDc is MBDone), that MB will not complete until the probe is
executed.
The system can select the ending cache status for a cache fill operation by specifying
the status in one of the following SysDc commands:
ReadData (Clean) ReadDataShared (Clean/Shared)
ReadDataDirty (Dirty) ReadDataShared/Dirty (Shared/Dirty)
The system returns ReadDataShared or ReadData for ReadBlk commands, and ReadD-
ataDirty for a ReadMod command. However, other combinations are possible, but
should be used only after a careful study of the situation.
Table 4–24 SysDc[4:0] Field Description
SysDc[4:0] Command SysDc[4:0] Description
NOP 00000 NOP, SysData is ignored by the 21264/EV68A.
ReadDataError 00001 Data is returned for read commands. The system drives the SysData
bus, I/O, or memory NXM.
ChangeToDirtySuccess 00100 No data. SysData is ignored by the 21264/EV68A. This command is
also used for the InvalToDirty response.
ChangeToDirtyFail 00101 No data. SysData is ignored by the 21264/EV68A. This command is
also used for the Evict response.
MBDone 00110 Memory barrier operation completed.
ReleaseBuffer 00111 Command to alert the 21264/EV68A that the RVB, RPB, and ID
field are valid.
ReadData
(System Wrap)
100xx Data returned for read commands. The system drives SysData. The
system uses SysDc[1:0] to control the wrap order. See Section
4.7.8.6 for a description of the data wrapping scheme.
ReadDataDirty
(System Wrap)
101xx Data is returned for Rdx and RdModx commands. The ending tag
status is dirty. The system uses SysDc[1:0] to define the wrap order.
ReadDataShared
(System Wrap)
110xx Data is returned for read commands. The system drives the data. The
tag is marked shared. The system uses SysDc[1:0] to control the
wrap order.
ReadDataShared/Dirty
(System Wrap)
111xx Data is returned for the RdBlk command. The ending tag status is
Shared/Dirty. The system uses SysDc[1:0] to control the wrap order.
WriteData 010xx Data is sent for 21264/EV68A write commands or system probes.
The 21264/EV68A drives during the SysData cycles. The lower two
bits of the command specify the octaword address around which the
21264/EV68A wraps the data.