A miss in a write-back cache ( which requires a block to be replaced by another ) will often need two memory accesses : one to get the needed datum , and another to write replaced data from the cache to the store . For this reason , a read miss in a write-back cache ( which requires a block to be replaced by another ) will often require two memory accesses to service : one to retrieve the needed datum , and one to write replaced data from the cache to the store .