Chapter 8. Local Memory
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Cache Overview
Both the instruction and data cache implement line-fill buffers to optimize line-sized burst
accesses. The data cache supports operation of copyback, write-through, or cache-inhibited
modes. A four-entry, 32-bit buffer supports cache line-push operations, and can be
configured to defer write buffering in write-through or cache-inhibited modes. The cache
lock feature can be used to guarantee deterministic response for critical code or data areas.
A nonblocking cache services read hits or write hits from the processor while a fill (caused
by a cache allocation) is in progress.
The data and instruction caches are physically addressed. A cache hit occurs when an
address matches a cache entry. For a read, the cache supplies data to the processor. For a
write, which is permitted only to the data cache, the processor updates the cache. If an
access does not match a cache entry (misses the cache) or if a write access must be written
through to memory, the core system bus controller performs the necessary system bus
transactions.
Cache modules do not implement bus snooping; cache coherency with other possible bus
masters must be maintained in software.
8.7.1 Optimizing Cache Recommendation
The following is recommended for optimal cache performance.
Cache data and instruction space to improve system performance.
Do not cache the following:
— SIM space
— Memory-mapped I/O space
— DMA space
8.7.2 Cache Organization
A four-way set associative cache is organized as four ways (levels). Each line contains 16
bytes (4 longwords). Entire cache lines are loaded from memory by burst-mode accesses
that cache 4 longwords of data or instructions. All 4 longwords must be loaded for the cache
line to be valid.
Figure 8-11 shows the data cache organization, which differs from the instruction cache by
the inclusion of the modified bit, which indicates that the data cache block has been written
to without changing the corresponding location in system memory.
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