Microcode is a layer of hardware-level instructions or data structures involved in the implementation of higher level machine code instructions in many computers and other processors; it resides in special high-speed memory and translates machine instructions into sequences of detailed circuit-level operations. It helps separate the machine instructions from the underlying electronics so that instructions can be designed and altered more freely. It also makes it feasible to build complex multi-step instructions while still reducing the complexity of the electronic circuitry compared to other methods. Writing microcode is often called microprogramming and the microcode in a particular processor implementation is sometimes called a microprogram.
Modern microcode is normally written by an engineer during the processor design phase and stored in a ROM (read-only memory) or PLA (programmable logic array) structure, or a combination of both. However, machines also exist which have some (or all) microcode stored in SRAM or flash memory. This is traditionally denoted a "writeable control store" in the context of computers. Complex digital processors may also employ more than one (possibly microcode based) control unit in order to delegate sub-tasks which must be performed (more or less) asynchronously in parallel. Microcode is generally not visible or changeable by a normal programmer, not even by an assembly programmer. Unlike machine code which often retains some compatibility among different processors in a family, microcode only runs on the exact electronic circuitry for which it is designed, as it constitutes an inherent part of the particular processor design itself.
More extensive microcoding has also been used to allow small and simple microarchitectures to emulate more powerful architectures with wider word length, more execution units and so on; a relatively simple way to achieve software compatibility between different products in a processor family.
Some hardware vendors, especially IBM, use the term as a synonym for firmware, so that all code in a device, whether microcode or machine code, is termed microcode (such as in a hard drive for instance, which typically contains both).
Read more about Microcode: Overview, The Reason For Microprogramming, History, Examples of Microprogrammed Systems, Implementation, Writable Control Stores, Microcode Versus VLIW and RISC