Microcode

In computer architecture, microcode is a layer of low-level control data or instructions used to implement a processor's instruction set architecture or internal control sequences. It consists of hardware-level operations that carry out higher-level machine code instructions or direct internal sequencing in many digital components. In many modern Intel and AMD general-purpose processors, common instructions are decoded directly into internal micro-operations, while microcode is used mainly for more complex instructions, special cases, and processor updates.

Housed in special high-speed memory, microcode translates machine instructions, state machine data, or other input into sequences of detailed circuit-level operations. It separates the machine instructions from the underlying electronics, thereby enabling greater flexibility in designing and altering instructions. Moreover, it facilitates the construction of complex multi-step instructions, while simultaneously reducing the complexity of computer circuits. The act of writing microcode is often referred to as microprogramming, and the microcode in a specific processor implementation is sometimes termed a microprogram.

Through extensive microprogramming, microarchitectures of smaller scale and simplicity can emulate more robust architectures with wider word lengths, additional execution units, and so forth. This approach provides a relatively straightforward method of ensuring software compatibility between different products within a processor family.