Historically, microcode was invented around 1951 and was heavily used in most of the IBM System/360 computers (1964). Once your instruction set is even moderate

Historically, microcode was invented around 1951 and was heavily used in most of the IBM System/360 computers (1964). Once your instruction set is even moderately complicated, using microcode is a lot easier than hard-wired control circuitry. You can view microcode as one of the many technologies that started in mainframes, moved into minicomputers, and then moved into microprocessors. In a sense, the idea behind RISC was the recognition that this natural progression was not necessarily the best way to build microprocessors.