Sysack And 21264/Ev67-To-System Commands Flow Control; Proberesponse Fields Descriptions - Compaq 21264 Hardware Reference Manual

Table 4–18 describes the ProbeResponse command fields.
Table 4–18 ProbeResponse Fields Descriptions
ProbeResponse Field Description
Command[4:0] The value 00001 identifies the command as a ProbeResponse.
DM Indicates that data movement should occur (copy of probe valid bit). See Section 4.4.
VS Write victim sent bit.
VDB[2:0] ID number of the VDB entry containing the requested cache block. This field is valid
when either the DM bit or the VS bit equals 1.
MS MAF address sent.
MAF[2:0] This field indicates the SharedToDirty, CleanToDirty, or
STCChangetoDirty MAF entry that matched the full probe address.
Status[1:0] Result of probe:
Status[1:0] Probe state
The system uses the SysDc signal lines to retrieve data for probes that requested a cache
block from the 21264/EV67. See Section 4.7.7.2 for more information about 2-cycle
data transfer commands. Probes that respond with M1, M2, or CH=1 will not be
reported to the system in a probe response command.

4.7.6 SysAck and 21264/EV67-to-System Commands Flow Control

Controlling the flow of 21264/EV67-to-system commands is a joint task of the 21264/
EV67 and the system. The flow is controlled using the A bit, which is asserted by the
system, and the Cbox CSR SYSBUS_ACK_LIMIT[4:0] counter. The counter has the
following properties:
•
The 21264/EV67 increments its command-outstanding counter when it sends a
command to the system. The 21264/EV67 decrements the counter by one each time
the A bit (SysAddIn_L[14]) is asserted in a system-to-21264/EV67 command. The
A bit is transmitted during cycle four of a probe mode command or during cycle
two of a SysDc command.
•
The 21264/EV67 stops sending new commands when the counter hits the maxi-
mum count specified by Cbox CSR SYSBUS_ACK_LIMIT[4:0]. When this
counter is programmed to zero, the CMD_ACK count is ignored (unlimited com-
mands are allowed in-flight).
•
Because RdBlkxVic and WrVictimBlk commands are atomic when the CSR
BC_RDVICTIM[0] is set, the 21264/EV67 does not send a RdBlkxVic command if
the SYSBUS_ACK_LIMIT[4:0] is equal to one less than the maximum outstanding
count. The limit cannot be programmed with a value of one when RdBlkxVic com-
mands are enabled unless the Cbox CSR RDVIC_ACK_INHIBIT command is also
asserted (see Table 5–24).
Alpha 21264/EV67 Hardware Reference Manual
00 HitClean
01 HitShared
10 HitDirty
11 HitSharedDirty
System Port
Cache and External Interfaces
4–25