Write policy

When data is written to the cache, it must at some point be written to main memory as well. The timing of this write is controlled by what is known as the write policy. In a write-through cache, every write to the cache causes a write to main memory. Alternatively, in a write-back or copy-back cache, writes are not immediately mirrored to memory. Instead, the cache tracks which locations have been written over (these locations are marked dirty). The data in these locations is written back to main memory when that data is evicted from the cache. For this reason, a miss in a write-back cache will often require two memory accesses to service: one to first write the dirty location to memory and then another to read the new location from memory.