microcode

[′mī·krō‚kōd] (computer science) A code that employs microinstructions; not ordinarily used in programming.

microcode

(programming)A technique for implementing the instruction set of a processor as a sequence of microcode instructions("microinstructions"), each of which typically consists of a(large) number of bit fields and the address of the nextmicroinstruction to execute. Each bit field controls somespecific part of the processor's operation, such as a gatewhich allows some functional unit to drive a value onto thebus or the operation to be performed by the ALU. Severalmicroinstructions will usually be required to fetch, decodeand execute each machine code instruction("macroinstruction"). The microcode may also be responsiblefor polling for hardware interrupts between eachmacroinstruction. Writing microcode is known as"microprogramming".

Microcode may be classified as "horizontally encoded" or"vertically encoded". Horizontal microcode is as describedabove where there is a fairly direct correspondence betweenthe bit fields in a microinstruction and the control signalssent to the various parts of the CPU. Not all combinations ofbits will be valid (e.g. two units driving the bus at once).Vertical microcode is closer to machine code because a bitfield value may pass through some intermediate combinatorylogic which generates the actual control signals. This allowsa few bits of a microinstruction to determine several controlsignals and ensure that only valid combinations of thosesignals are generated (e.g. a field may be decoded to determinewhich unit drives the bus). The disadvantage with verticalencoding is that the encoding is usually fixed and takes extratime compared with horizontal encoding which allows anycombination of signals to be generated and takes no time todecode.

The alternative to a microcoded processor is a hard-wiredone where the control signals are generated directly from thebits of the machine code instruction. This is more commonin modern RISC architectures because it is faster.

Microcode is usually stored in ROM chips though someprocessors (e.g. the Orion) use fast RAM, making themdynamically microprogrammable.

microcode

A set of elementary instructions in a complex instruction set computer (CISC). The microcode resides in a separate high-speed memory and functions as a translation layer between the machine instructions and the circuit level of the computer. Microcode enables the computer designer to create machine instructions without having to design electronic circuits. Writing microcode is called "microprogramming," and the microcode for a given computer is called a "microprogram."

RISC computers do not use microcode, which is the reason why RISC compilers generate more instructions than CISC compilers.

Source Code to Machine Code to Microcode
When software is written, the source code is converted into machine instructions by assemblers and compilers. At execution time, the machine instructions are converted into microinstructions, and the microinstructions cause transistors to open and close in the circuits. See microinstruction, CISC and RISC.

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Synonyms for microcode

noun (computer science) coded instructions that are stored permanently in read-only memory

Synonyms

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