Compaq 21264 - Manual (Page 113)

Alpha 21264/EV67 Hardware Reference Manual Cache and External Interfaces 4–25
System Port
Table 4–18 describes the ProbeResponse command fields.
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).
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
00 HitClean
01 HitShared
10 HitDirty
11 HitSharedDirty