In the case of uniprocessors, whenever a wordin the cache is modified, then the write-through policy ensures that the corresponding word in main memory is also update. In multiprocessors where each processor has its private cache, this policy cannot be easily enforced for, as we saw in previous section, the same data item may be present in several of the private cache memories. If a processor modifies this data item in its own cache, then to maintain coherence the same modification must be effected on that data item in all the other caches