The microinstructions, especially of a microprocessor, that govern the details of operation. Microcoded functions can improve performance but add a layer of complexity. For example, microcode errors appear to software as being hardware failures.
Earlier CPUs were designed to execute instructions with circuitry directly decoding and executing program instructions. Microcode was a way of simplifying CPU design by allowing simpler hardware that executes simple microinstructions to interpret more complex machine instructions. It was first used commercially in the mid- and low-range IBM System/360. Implementations generally use either "horizontal" or "vertical" microcode, which differ primarily in number of bits. Microinstructions include a condition code and jump address ("jump" if a condition is true, "next instruction" if false), as well as the operation to be performed. In horizontal microcode, each operation bit triggers an individual control line (simple CPU controller but large microcode storage); in vertical microcode, the operation field is decoded to produce the control signals (smaller microcode but more complex controller). Some CPUs use a combination.
Translation layer between machine instructions and the elementary operations of a computer. Microcode is stored in ROM and allows the addition of new machine instructions without requiring that they be designed into electronic circuits when new instructions are needed.
hardware: Machine-language instructions that tell the CPU how to handle complex instructions such as those that take more than one clock cycle and/or use a lot of different parts of the chip. A high-overhead feature of CISC architecture, virtually eliminated by RISC chips.
A microprogram implements a CPU instruction set. Just as a single high level language statement is compiled to a series of machine instructions (load, store, shift, etc), in a CPU using microcode, each machine instruction is in turn implemented by a series of microinstructions, sometimes called a microprogram. Microprograms are often referred to as microcode.