microcode

Microcode is a low-level layer of instructions that implements a computer processor’s machine instruction set. It sits between the instruction decoding logic and the hardware datapath, translating each architectural instruction into a sequence of simpler operations that the control unit can execute. By encoding this behavior in a control store, microcode provides a level of software-like flexibility that lets designers modify how instructions are carried out without altering the physical circuitry.

In operation, the processor fetches an architectural instruction, uses a microcode address derived from that instruction to access the microcode store, and then executes a sequence of microinstructions. Each microinstruction steers the processor’s control signals, routing data through registers, ALUs, and buses, and then advances to the next microinstruction. The microcode can be stored in read-only memory, programmable ROM, or writable RAM, enabling updates or patches.

Microcode can be implemented in different architectural styles. Horizontal microcode uses many control bits per microinstruction, granting fine-grained control over the datapath but increasing code size. Vertical microcode uses compact encoding with higher-level operations that expand into several micro-operations. In modern processors, many complex instructions are ultimately translated into micro-operations; a microcode layer remains in use for implementing legacy instructions, handling rare cases, or applying bug fixes and feature updates.

History and status: microcode originated in early computers to simplify hardware design, with notable use in IBM System/360 and later CPUs. Today, vendors frequently distribute microcode updates via firmware to fix errata, add instructions, or improve performance and compatibility, without requiring new silicon. Microcode thus remains a foundational but evolving mechanism for implementing and maintaining a processor’s instruction set.