Compaq 21264, EV67 specifications 24describes the SysDc40 field, SysDc40 Field Description

System Port

Table 4–24describes the SysDc[4:0] field.

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/EV67.
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/EV67. This command is
		also used for the InvalToDirty response.
ChangeToDirtyFail	00101	No data. SysData is ignored by the 21264/EV67. This command is
		also used for the Evict response.
MBDone	00110	Memory barrier operation completed.
ReleaseBuffer	00111	Command to alert the 21264/EV67 that the RVB, RPB, and ID field
		are valid.
ReadData	100xx	Data returned for read commands. The system drives SysData. The
(System Wrap)		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	101xx	Data is returned for Rdx and RdModx commands. The ending tag
(System Wrap)		status is dirty. The system uses SysDc[1:0] to define the wrap order.
ReadDataShared	110xx	Data is returned for read commands. The system drives the data. The
(System Wrap)		tag is marked shared. The system uses SysDc[1:0] to control the
		wrap order.
ReadDataShared/Dirty	111xx	Data is returned for the RdBlk command. The ending tag status is
(System Wrap)		Shared/Dirty. The system uses SysDc[1:0] to control the wrap order.
WriteData	010xx	Data is sent for 21264/EV67 write commands or system probes. The
		21264/EV67 drives during the SysData cycles. The lower two bits of

the command specify the octaword address around which the 21264/ EV67 wraps the data.

The A bit in the first cycle indicates that the command is acknowledged. When A = 1, the 21264/EV67 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/EV67 to retire an instruction before an MB, or allows the 21264/EV67 itself to retire an MB (SysDc is MBDone), that MB will not complete until the probe is exe- cuted.

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.

Alpha 21264/EV67 Hardware Reference Manual Cache and External Interfaces 4–29