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21264/EV68A Hardware Reference Manual
Cache and External Interfaces 4–9
Cache Coherency
Figure 4–3 Cache Subset Hierarchy
The following tasks must be performed to maintain cache coherency:
• Istream data from memory spaces may be cached in the Icache and Bcache. Icache
coherence is not maintained by hardware—it must be maintained by software using
the CALL_PAL IMB instruction.
• The 21264/EV68A maintains the Dcache as a subset of the Bcache. The Dcache is
set-associative but is kept a subset of the larger externally implemented direct-
mapped Bcache.
• System logic must help the 21264/EV68A to keep the Bcache coherent with main
memory and other caches in the system.
• The 21264/EV68A requires the system to allow only one change to a block at a
time. This means that if the 21264/EV68A gains the bus to read or write a block, no
other node on the bus should be allowed to access that block until the data has been
moved.
• The 21264/EV68A provides hardware mechanisms to support several cache coher-
ency protocols. The protocols can be separated into two classes: write invalidate
cache coherency protocol and flush cache coherency protocol.
4.5.2 Cache Block States
Table 4–2 lists the cache block states supported by the 21264/EV68A.
Table 4–2 21264/EV68A-Supported Cache Block States
State Name Description
Invalid The 21264/EV68A does not have a copy of the block.
Clean This 21264/EV68A holds a read-only copy of the block, and no other agent in the system
holds a copy. Upon eviction, the block is not written to memory.
System
Main Memory
Icache
Bcache
Dcache
FM-05824.AI4