Compaq specifications SysAck and 21264/EV68A-to-System Commands Flow Control

System Port

Table 4–18describes 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
	00	HitClean
	01	HitShared
	10	HitDirty
	11	HitSharedDirty

The system uses the SysDc signal lines to retrieve data for probes that requested a cache block from the 21264/EV68A. 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/EV68A-to-System Commands Flow Control

Controlling the flow of 21264/EV68A-to-system commands is a joint task of the 21264/EV68A 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/EV68A increments its command-outstanding counter when it sends a command to the system. The 21264/EV68A decrements the counter by one each time the A bit (SysAddIn_L[14]) is asserted in a system-to-21264/EV68A com- mand. The A bit is transmitted during cycle four of a probe mode command or dur- ing cycle two of a SysDc command.

•The 21264/EV68A 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/EV68A does not send a RdBlkxVic command if the SYSBUS_ACK_LIMIT[4:0] is equal to one less than the maximum outstand- ing count. The limit cannot be programmed with a value of one when RdBlkxVic commands are enabled unless the Cbox CSR RDVIC_ACK_INHIBIT command is also asserted (see Table 5–24).

21264/EV68A Hardware Reference Manual Cache and External Interfaces 4–25