Single-chip CPU and microcomputer

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 2n 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.