1. A method of operating a computer system having a CPU, local storage for said CPU, a shared memory, and at least one other processor accessing said shared memory, comprising the steps of: (a) initiating a memory operation by said CPU, including copying a data item that includes multiple words of selected non-contiguous locations in said shared memory to corresponding locations in said local storage, and setting an indication associated with said corresponding locations in said local storage to mark them as being inaccessible by said other processor; (b) performing an operation on said data item by said CPU to alter at least one or more parts of said data item in said corresponding location in said local storage; (c) monitoring said shared memory beginning at the time of said copying to detect any access to said data item in said shared memory by said other processor, and altering a status indicator if said other processor (1) reads a location in said shared memory corresponding to a part of said data item that has been altered, or (2) writes to a location in said shared memory corresponding to any part of said data item; (d) after said step of performing an operation, determining whether said status indicator has been altered, and, if not, changing said indication so that said corresponding locations in said local storage are accessible by said other processor, or, if so, discarding the content of said corresponding locations in said local storage and re-executing steps (a), (b), (c) and (d).