A technique for starting up a data processing system in which a "basic" or "kernel" instruction set is stored in an auxiliary store and transferred to the processor's control store in start-up. The kernel instruction set in used to execute an initialization program which identifies an I/O device in which a microcode local program and a desired microcode file are stored. No microcode load program is transferred to the processor's main memory, using the kernel instructions and the desired microcode file is then transferred in selected blocks from the I/O device to the main memory and there to the microcode control store. The microinstructions of the desired microcode file then can be used to load appropriate operating system software so that the overall processor is then ready for use in whatever applications are desirable.