在CISC结构下，一些功能复杂的指令运行时，被分解为一系列相对简单的指令来运行，这样一系列的简单指令就叫做微指令（microcode）。相关的概念最早在1947年开始出现。

微指令的作用是将机器指令与相关的电路实作分离，这样一来机器指令可以更自由的进行设计与修改，而不用考虑到实际的电路架构。与其他方式比较起来，使用微指令架构可以在降低电路复杂度的同时，建构出复杂的多步骤机器指令。撰写微指令一般称为微编程(microprogramming)，而特定架构下的处理器实做中微指令有时会称为微程序(microprogram)。

Microcode is "a technique that imposes an interpreter between the hardware and the architectural level of a computer." As such, the microcode is a layer of hardware-level instructions that implement higher-level machine code instructions or internal state machine sequencing in many digital processing elements. Microcode is used in general-purpose central processing units, in more specialized processors such as microcontrollers, digital signal processors, channel controllers, disk controllers, network interface controllers, network processors, graphics processing units, and in other hardware.