A cache memory system includes a cache memory including a plurality of cache memory lines and a dirty buffer including a plurality of dirty masks. A cache controller is configured to allocate one of the dirty masks to each of the cache memory lines when a write to the respective cache memory line is not a full write to that cache memory line. Each of the dirty masks indicates dirty states of data units in one of the cache memory lines. The cache controller may include a dirty buffer index which stores an identification (ID) information that associates the dirty masks with the cache memory lines to which the dirty masks are allocated. A cache line may include a fully dirty flag indicating when each byte in that cache line is dirty, so that a dirty mask does not need to be allocated for that cache line.