Compaq EV67 User Manual

Cache and External Interfaces

The 21264/EV67 sends a WrVictimBlk command to the system when it evicts a Dirty 
or Dirty/Shared cache block. The 21264/EV67 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/EV67 cache system.  Write data control 
and deallocation of the VAF can be directly controlled by using the SysDc WriteData 
and ReleaseBuffer commands.

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/EV67 delivers the data block, independent of its
status, to waiting load instructions and does not cache the block in the 
21264/EV67 cache system.

Fetchx

ReadDataError

The cache block address was to an NXM address space. The 21264/
EV67 delivers the all-ones patterns to any dependent load instructions 
and does not cache the block in the 21264/EV67 cache system.

Rdiox

ReadDataError

The cache block access was to NXM address space. The 21264/EV67 
delivers an all-ones pattern to any load command and does not cache 
the block in the 21264/EV67 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.