Page "Microcode" Paragraph 17

Microcode was originally developed as a simpler method of developing the control logic for a computer.

Initially CPU instruction sets were " hardwired ".

Each step needed to fetch, decode and execute the machine instructions ( including any operand address calculations, reads and writes ) was controlled directly by combinatorial logic and rather minimal sequential state machine circuitry.

While very efficient, the need for powerful instruction sets with multi-step addressing and complex operations ( see below ) made such " hard-wired " processors difficult to design and debug ; highly encoded and varied-length instructions can contribute to this as well, especially when very irregular encodings are used.