Compaq specifications System-to-21264/EV68A Commands, Probe Commands Four Cycles, Rvb Rpb

System Port

•There is no mechanism for the system to reject a 21264/EV68A-to-system com- mand. ProbeResponse, VDBFlushReq, NOP, NZNOP, and RdBlkxSpec (with a clear RV bit) commands do not require a response from the system. Systems must provide adequate resources for responses to all probes sent to the 21264/EV68A.

•Systems that program the Cbox CSR BC_RDVICTIM[0] to immediately follow victim write transactions with read transactions and allocate combined resources for the pair, may find it useful to increment the SYSBUS_ACK_LIMIT[4:0] counter only once for the pair. These systems may assert Cbox CSR RDVIC_ACK_INHIBIT, which does not increment the SYSBUS_ACK_LIMIT[4:0] count for RdBlkVic, RdBlkModVic, and RdBlkVicI commands.

•Systems that maintain victim data buffers may find it useful to limit the number of outstanding WrVictimBlk commands. This can be accomplished by using the Cbox CSR SYSBUS_VIC_LIMIT[2:0]. When the number of outstanding WrVictim commands or CleanVictim commands reaches this programmed limit, the Cbox stops generating victim commands on the system port. Because victim and read commands are atomic when BC_RDVICTIM[0] = 1, the RdBlkxVic commands are stalled when the victim limit is reached. Programming the SYSBUS_VIC_LIMIT[2:0] to zero disables this limit.

4.7.7System-to-21264/EV68A Commands

The system can send either probes (4-cycle) or data movement (2-cycle) commands to the 21264/EV68A. Signal pin SysAddIn_L[14] in the first command cycle indicates the type of command being sent (1 = probe, 0 = data transfer). Sections 4.7.7.1 and

4.7.7.2describe the formats of the two types of commands.

4.7.7.1Probe Commands (Four Cycles)

Probes are always 4-cycle commands that contain a field to indicate a valid SysDc com- mand. The format of the 4-cycle command is shown below.

Note: The SysAddIn_L[1:0] signal lines are optional and are used for memory designs greater than 32GB. The position of the address bits matches the selected format of the SysAddOut bus. The example below shows the bank interleave format.

Table 4–19shows the format of the system-to-21264/EV68A probe commands.

Table 4–19 System-to-21264/EV68A Probe Commands

	SysAddIn_L[14:2]							SysAddIn_L[1]	SysAddIn_L[0]
Cycle 1	1	Probe[4:0]			PA[34:28]			PA[36]	PA[38]
Cycle 2		PA[27:22], PA[12:6]						PA[35]	PA[37]
Cycle 3	0	SysDc[4:0]	RVB	RPB		A	ID[3:0]	PA[40]	PA[42]
Cycle 4	C	PA[21:13], PA[5:3]						PA[39]	PA[41]

4–26Cache and External Interfaces 21264/EV68A Hardware Reference Manual