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In processor design, microcode serves as an intermediary layer situated between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. It consists of a set of hardware-level instructions that implement higher-level machine code instructions or control internal finite-state machine sequencing in many digital processing components. While microcode is utilized in general-purpose CPUs in contemporary desktops, it also functions as a fallback path for scenarios that the faster hardwired control unit is unable to manage. 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 microprograming, 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. Some hardware vendors, notably IBM/Lenovo, use the term microcode interchangeably with firmware. In this context, all code within a device is termed microcode, whether it is microcode or machine code. For instance, updates to a hard disk drive's microcode often encompass updates to both its microcode and firmware. The lowest layer in a computer's software stack is traditionally raw machine code instructions for the processor. In microcoded processors, fetching and decoding those instructions, and executing them, may be done by microcode.

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