is only possible if one is sure that the writeback will complete enabling a snoop or querying request to be stalled in this manner means that problems that might arise were for example the read unique to retrieve the data from the cache while the cache was performing a writeback for that item and then itself perform a writeback of that data item to memory are avoided in addition to not transmitting the writeback to coherency control circuitry there are further rules that the interconnect follows to be sure that the writeback is guaranteed to complete looking at memory 30 one sees that although there are separate read and write paths going to this memory inside memory controller 33 they may combine to form a single path thus writes and reads will be performed in an order defined by the memory and will need to wait for each other to complete thus coherency control circuitry 55 is configured to only issue read transactions to memory 30 where it knows that they are sure to complete similarly it only issues write transactions where it can also be sure that they will complete this ensures that a writeback transaction does not get held up indefinitely behind.