Compaq specifications 21264/EV68A Command NXM Address System/21264/EV68A Response

System Port

Table 4–32shows each 21264/EV68A command, with NXM addresses, and the appro- priate system response.

Table 4–32 21264/EV68A Commands with NXM Addresses and System Response

21264/EV68A
Command NXM
Address	System/21264/EV68A Response
ProbeResponse	Probe responses for addresses to NXM space are of UNPREDICTABLE status. Although
	the final status of a ReadDataError is Invalid, the 21264/EV68A fills the block Valid/
	Clean and uses an atomic Evict command to invalidate the block. Systems that send
	probes to NXM space to the 21264/EV68A must disregard the probe result.
RdBlk	Load references to NXM space can be speculative. In this case, systems should respond
RdBlkSpec	with a SysDc ReadDataError fill that the 21264/EV68A uses to service the original load/
RdBlkVic	Istream command. If the original load command was speculative, the 21264/EV68A will
	remove the load instruction that generated the NXM command, and start processing
	instructions down the correctly predicted path. If the command was not speculative, there
	must be an error in the operating system mapping of a virtual address to an illegal physi-
	cal address, and the 21264/EV68A provides an all ones pattern as a signature for this bug.
	The NXM block is not cached in the Dcache or Bcache.
RdBlkI	Istream references to NXM space can be speculative. In this case, systems should respond
RdBlkSpecI	with a SysDc ReadDataError fill, which the 21264/EV68A will use to service and exe-
RdBlkVicI	cute the original Istream reference. If the original Istream reference was speculative, the
	21264/EV68A will remove the instructions started after the mispredicted instruction that
	generated the NXM reference, and start instruction processing down the correctly pre-
	dicted path. If the reference was not speculative, there must be an error in the operating
	system mapping of a virtual address to an illegal physical address, and the 21264/EV68A
	provides an all ones pattern as a signature for this bug. The NXM block is not cached in
	the Bcache, but can be cached in the Icache.
RdBlkMod	Store instructions to NXM space initiate RdBlkMod commands. Again, speculative store
RdBlkModSpec	instructions are removed. Nonspeculative store instructions are forced to fail, much like
RdBlkModVic	STx_C instructions that fail. The NXM block is not cached in the Dcache or Bcache.
WrVictimBlk	Dirty Victims to NXM space are illegal. Systems should perform a machine check, with
	the 21264/EV68A indicating a severe error.
CleanVictimBlk	The 21264/EV68A can generate CleanVictimBlk commands to NXM space if the Cbox
	CSR BC_CLEAN_VICTIM[0] bit is asserted and a SysDc ReadDataError has been gen-
	erated. Systems that use clean victims must faithfully deallocate the CleanVictim VAF
	entry.
Evict	If the Cbox CSR ENABLE_EVICT is asserted, the 21264/EV68A will generate Evict
	commands to NXM space. Systems may use this command to invalidate their duplicate
	tags. Systems must respond with SysDc ChangeToDirtyFail to retire the NXM MAF
	entry.
RdBytes	Load instructions to I/O space are not speculative, so an I/O reference to NXM space is
RdLWs	an error. Systems must respond with ReadDataError and should generate a machine
RdQWs	check to indicate an operating system error.
WrBytes	Store instructions to I/O space are not speculative, so an I/O reference to NXM space is
WrLWs	an error. Systems must respond by deallocating the appropriate IOWB entries, and should
WrQWs	generate a machine check to indicate an operating system error.
FetchBlk	Loads to noncached memory in NXM space may be speculative. Systems must respond
FetchBlkSpec	with a SysDc ReadDataError to retire the MAF entry.

21264/EV68A Hardware Reference Manual Cache and External Interfaces 4–39