Abstract: The disclosure describes means for allowing the sequencing of operand fetches to deviate from the conceptual sequence specified in the program. Allowing fetch sequencing deviations may improve system performance, while not causing any deviation in program execution results. Out-of-sequence (OOS) fetching may be caused by issuing each fetch without regard to the following conditions: 1. a delay in issuance to storage (such as a delay in generating the address for a fetch request); 2. a speed up in operand data return (such as due to fetching the operand data from a store buffer in the execution unit without going to storage for the data); 3. a delay in the return of operand data (such as when a fetch request has a cache miss and its data must be obtained from the storage hierarchy); or 4. an overlap in the return of fetch data for a serializer instruction with execution of instructions preceding the serializer or data and instruction fetching for instructions following the serializer before its completion.