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L2 Description
4-16
The L2 uses a least recently used (LRU) replacement strategy to replace old
cached data with new data. To determine which cache lines to replace, the
address for the new data is used to calculate the set which that address maps
to. Each external address maps to one set in each cache way. Then, that set
in each way is interrogated to determine which way contains the least recently
used (LRU) data. The new data is stored at that location. If the cache location
to be replaced contains valid data, the previous data is evicted. An eviction
occurs as follows. The L2 first polls the L1D to determine if the evicted address
is also cached in the L1D. This is referred to as snooping the L1D. If data is
returned from the L1D, it is written out to the EDMA. Then, both the L1D and
L2 lines are invalidated. If the L1D does not cache the evicted address, the
data in the L2 to written out to the EDMA. In this case, only the L2 line is
invalidated. Finally, the requested data is stored in the L2 and sent to the re-
questor. This mechanism ensures that the CPU does not access stale data
and that no data is lost. Figure 4–14 diagrams the decision process used by
the L2 controller to service a data request from the CPU when the L2 is operat-
ing as a cache. The L2 performs evictions for read and write requests.
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