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SA-1100 Developer’s Manual 6-3
Caches, Write Buffer, and Read Buffer
memory-management page table. For this reason, in order to use the Dcaches, the MMU must be
enabled. The two functions may be enabled simultaneously with a single write to the control
register.
Note: The Dcaches operate with virtual addresses, so care must be taken to ensure that their contents
remain consistent with the virtual-to-physical mappings performed by the memory-management
unit. If the memory mappings are changed, the validity of the Dcaches must be ensured.
6.2.1 Cacheable Bit – C
The cacheable bit determines whether, on load misses, the data being read should be placed in one
of the two data caches. Cache hits are not affected by the cacheable bit; if a data access hits in the
cache, the data is assumed to be valid and the load or store is performed. Typically, main memory is
marked as cacheable to improve system performance and I/O space as noncacheable to stop the
data from being stored in SA-1100’s cache. For example, if the processor is polling a hardware flag
in I/O space, it is important that the processor is forced to read data from the external peripheral,
and not a copy of initial data held in the cache.
6.2.1.1 Cacheable Reads – C = 1
A linefetch of 8 words will be performed and it will be placed in a cache bank with a round-robin
replacement algorithm.
6.2.1.2 Noncacheable Reads – C = 0
An external memory access will be performed and the cache will not be written.
6.2.2 Bufferable Bit – B
The bufferable bit does not affect writes that hit the Dcaches. If a store hits in the Dcaches, the store
is assumed to be bufferable. Write-backs of dirty lines are treated as bufferable writes. See the
Section 6.3, “Write Buffer (WB)” on page 6-5 for more information on the B bit.
Table 6-1 summarizes the effects of the B and C bits on the Dcaches.
Table 6-1. Effects of the Cacheable and Bufferable Bits on the Data Caches
Load Store
BC
Cache Hit Cache Miss Cache Hit Cache Miss
0 0 Deliver cache data. Load from memory.
– No allocate.
Store to either cache.
– Mark line dirty.
Store to memory.
– No allocate.
0 1 Deliver cache data. Allocate to minicache. Store to either cache.
– Mark line dirty.
Store to memory.
– No allocate.
1 0 Deliver cache data. Load from memory.
– No allocate.
Store to either cache.
– Mark line dirty.
Store to memory.
– No allocate.
1 1 Deliver cache data. Allocate to main data cache. Store to either cache.
– Mark line dirty.
Store to memory.
– No allocate.