3.3.2. Operation of the Bd and Cd bits

The Cd bit determines whether data being read must be placed in the DCache and used for subsequent reads. Typically, main memory is marked as cachable to reduce memory access time and therefore increase system performance. It is usual to mark input/output space as noncachable. For example, if a processor is polling a memory-mapped register in input/output space, it is important that the processor is forced to read data direct from the peripheral, and not a copy of initial data held in the DCache.

The Bd and Cd bits affect writes that both hit and miss in the DCache. If the Bd and Cd bits are both 1, the area of memory is marked as write back, and stores that hit in the DCache only update the cache, not external memory. If the Bd bit is 0 and the Cd bit is 1, the area of memory is marked as write through, and stores that hit in the DCache update both the cache and external memory.

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