A microcomputer MCU adopting the general purpose register method is enabled to have a small program capacity or a high program memory using efficiency and low system cost, while enjoying the advantage of simplification of the instruction decoding as in the RISC machine having a fixed length instruction format of the prior art, by adopting an instruction format of a fixed length of 2 n bits which is smaller than the length of the maximum data word fed to instruction execution means. The control of the coded division is executed by noting the code bits.