1. A data processing apparatus comprising: a first memory device for storing a plurality of instructions including at least first, second and third series of instructions into a plurality of memory locations which are ordered by location identifiers based on a predetermined rule and including at least first, second and third location identifiers; an execution unit connected to said first memory device for executing the first series of instruction consisting of one or more instructions stored in memory locations which are identified by the first location identifier in said first memory device; an instruction prefetching unit connected to said first memory device and to said execution unit for fetching the second series of instructions consisting of one or more instructions stored in memory locations which are identified by the second location identifier having an order which is subsequent to said memory locations which are identified by the first location identifier having an order which is subsequent to said memory locations which are identified by the first location identifier while said first series of instructions are being executed by said execution apparatus; and a second memory device connected to said first memory device and to said prefetching unit for storing the third series of instructions using the third location identifier as an index in the case where said third series of instructions are fetched when an amount of instruction data fetched in said instruction prefetching unit is below a constant value, said third series of instructions consisting of one or more instructions stored in memory locations which are identified by the third location identifier of said first memory device, wherein when a previously executed series of instructions are executed again, said third series of instructions are fetched into said instruction prefetching unit from said second memory device.