1. A system, comprising: a memory having a plurality of memory blocks; a memory directory having a read/write bit associated with each of the plurality of memory blocks and logic configured to set the read/write bit when the memory block associated with the read/write bit has been read by more than one processor and written by at least one other processor and presence bits to indicate processors have or have had a copy of an associated memory block; and at least two processors, each having a cache to store a plurality of memory blocks from the memory as cache lines and logic configured to update presence bits in the memory directory when replacing an unmodified cache line corresponding to a memory block only when the associated read/write bit is set.