Compaq EV68A specifications 24describes the SysDc40 field, SysDc40 Field Description

Models: EV68A

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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/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

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/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.

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.

21264/EV68A Hardware Reference Manual

Cache and External Interfaces 4–29

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Compaq EV68A specifications 24describes the SysDc40 field, SysDc40 Field Description