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21264/EV68A Hardware Reference Manual
Cache and External Interfaces 4–13
Cache Coherency
The 21264/EV68A sends a WrVictimBlk command to the system when it evicts a Dirty
or Dirty/Shared cache block. The 21264/EV68A may be configured to send a CleanVic-
timBlk to the system (by way of the Cbox CSR BC_CLEAN_VICTIM[0]) when evict-
ing a clean or shared block. Both commands allocate buffers in the VAF (victim address
file). This buffer is a coherent part of the 21264/EV68A cache system. Write data con-
trol and deallocation of the VAF can be directly controlled by using the SysDc Write-
Data and ReleaseBuffer commands.
4.5.5 Dcache States and Duplicate Tags
Each Dcache block contains an extra state bit (modified bit), beyond those required to
support the cache protocol. If set, this bit indicates that the associated block should be
written to the Bcache when it is evicted from the Dcache. The modified bit is set in two
cases:
1. When a block is filled into the Dcache from memory its modified bit is set, ensur-
ing that it also gets written back into the Bcache at some future time.
2. When the processor writes to a dirty Dcache block the modified bit is set, indicating
it should be written to the Bcache when evicted.
The contents of the modified bit are functionally invisible to the external cache environ-
ment, but knowledge of the bits function is useful to programmers optimizing the
scheduling of the Bcache data bus.
The Cbox contains a duplicate copy of the Dcache tag array. In contrast to the Dcache
tag array (DTAG), which is virtually indexed, the Cbox copy of the Dcache tag array
(CTAG) is physically-indexed. The Cbox uses the CTAG array entries in the following
situations.
InvalToDirty ChangeToDirtyFail Illegal. InvalToDirty instructions must provide a cache block.
Fetchx
Rdiox
ReadData
ReadDataShared
ReadDataShared/Dirty
ReadDataDirty
The 21264/EV68A delivers the data block, independent of its
status, to waiting load instructions and does not cache the block in the
21264/EV68A cache system.
Fetchx ReadDataError The cache block address was to an NXM address space. The 21264/
EV68A delivers the all-ones patterns to any dependent load instruc-
tions and does not cache the block in the 21264/EV68A cache system.
Rdiox ReadDataError The cache block access was to NXM address space. The 21264/
EV68A delivers an all-ones pattern to any load command and does
not cache the block in the 21264/EV68A cache system.
Evict ChangeToDirtyFail Retiring the MAF entry is the only legal response.
STCChangeTo
Dirty
ReadDataX
ChangeToDirtyFail
All fill and ChangeToDirtyFail responses will fail the STx_C require-
ments.
STCChangeTo
Dirty
ChangeToDirtySuccess The STx_C instruction succeeds.
MB MBDone Acknowledgment for MB.
Table 4–5 System Responses to 21264/EV68A Commands and Reactions (Continued)
21264/EV68A
CMD SysDc 21264/EV68A Action