Compaq specifications 4 21264/EV68A-to-System Commands Descriptions

System Port

System designers can minimize pin count for systems with a small memory by config- uring both the bank interleave on cache block boundary mode and the page hit mode formats into a short bus format. The pin SysAddOut_L[1] and/or SysAddOut_L[0] are not used (selected by Cbox CSR SYS_BUS_SIZE[1:0]). Table 4–13lists the values for SYSBUS_FORMAT and SYS_BUS_SIZE[1:0] and shows the maximum physical memory size.

Table 4–13 Maximum Physical Address for Short Bus Format

SYSBUS_	SYSBUS_
FORMAT	SIZE[1:0]	Maximum PA	Comment
0	00	42	Bank interleave + full address
0	01	36	Bank interleave + SysAddOut_L[0] unused
0	10	Illegal	Illegal combination
0	11	34	Bank interleave + both SysAddOut_L[1:0] are used for I/O
1	00	38	Page hit mode + full address
1	01	36	Page hit mode + SysAddOut_L[0] unused
1	10	Illegal	Illegal combination
1	11	34	Page hit mode + both SysAddOut_L[1:0] are unused

Because addresses above the maximum PA are not visible to the external system, any memory transaction generated to addresses above the maximum PA are detected and converted to transactions to NXM (nonexistent memory) and processed internally by the 21264/EV68A.

4.7.4 21264/EV68A-to-System Commands Descriptions

Table 4–14describes the 21264/EV68A-to-system commands.

Table 4–14 21264/EV68A-to-System Commands Descriptions

	Command
Command	[4:0]	Function
NOP	00000	The 21264/EV68A drives this command on idle cycles during reset. After
		the clock forward reset period, the first NZNOP is generated and this
		command is no longer generated.
ProbeResponse	00001	Returns probe status and ID number of the VDB entry holding the
		requested cache block.
NZNOP	00010	This nonzero NOP helps to parse the command packet.
VDBFlushRequest	00011	VDB flush request. The 21264/EV68A sends this command to the system
		when an internally generated transaction Bcache index matches a Bcache
		victim or probe in the VDB. The system should flush VDB entries
		associated with all probe and WrVictimBlk transactions that occurred
		before this command.
MB1	00111	Indicates an MB was issued, optional when Cbox CSR
		SYSBUS_MB_ENA[0] is set.
ReadBlk	10000	Memory read.

21264/EV68A Hardware Reference Manual Cache and External Interfaces 4–21