Microcoded

Microcoded refers to implementing a computer's control unit with microcode, a small program of microinstructions stored in a control store. Each microinstruction encodes signals that drive the datapath, enabling sequencing, conditional branches, and basic operations needed to execute machine instructions.

Microprogramming originated in the 1950s and gained prominence with IBM's System/360, which used a large microcode store to implement its instruction set. The approach allows complex instructions to be defined or modified without changing hardware. Microcode is typically stored in ROM, PROM, or RAM.

A microcoded control unit comprises a microcode memory, a microsequencer that selects the next microinstruction, and a set of microoperations that produce the required control signals. Microinstructions may specify the operation, next address, and conditions, enabling structured decoding and sequencing.

In modern processors, microcode remains common for implementing complex or infrequently used instructions and for applying fixes without hardware changes. Some architectures translate complex x86 instructions into micro-ops, while others rely on dedicated microcode for fidelity or performance. Vendors issue microcode updates via firmware to address errata or security issues.

Benefits include flexibility, easier ISA evolution, and bug fixes without hardware redesign. Drawbacks include potential performance overhead and added complexity, which can reduce speed compared with fully hardwired control.