Coherency means that data is matched between cache and external memory.
For example, writing to external RAM is not performed if the cache is hit when the cache is used in write back mode.
Unmatching of data between the cache and external RAM occurs because only writing to relevant entry of cache is performed and writing to external RAM is not performed.
Unmatching data of external RAM is transferred to external I/O when performing DMA transfer between external RAM and external I/O in a state of unmatching. Be sure to match data by invalidating a specific entry or a specific address before DMA transfer.