microcode

This term describes the lowest level of software, acting as a bridge between the physical circuitry of a CPU and the machine language it executes. It is characterized by its invisibility to the end user and its role in defining the internal logic of hardware, often used to fix bugs or security flaws without requiring a physical chip replacement.

As a noun, it is treated as an uncountable mass noun. It refers to the collective set of instructions rather than individual lines of code, meaning it does not take a plural form in standard technical usage. To refer to a specific instance, one would use a partitive like a piece of microcode or a microcode update.