1. An information processing system comprising: a main memory; a processing unit which executes a prefetch instruction included as one of a plurality of instructions of a program in said main memory; two hierarchical level data caches connected to said processing unit and said main memory and arranged so that a primary cache close to said processing unit is at a first level of a cache hierarchy and a secondary cache close to said main memory is at a second level in said cache hierarchy, wherein said prefetch instruction, when executed, causes said processing unit to perform a prefetch operation by transferring operand data to be used in a subsequent operation from said main memory to said secondary cache only, not to said primary cache, prior to executing said subsequent operation.