Control Unit (CU)

A control unit in general is a central (or sometimes distributed but clearly distinguishable) part of the machinery that controls its operation, provided that a piece of machinery is complex and organized enough to contain any such unit. One domain in which the term is specifically used is the area of computer design. In the automotive industry, the control unit helps maintain various functions of the motor vehicle.
The rest of this article describes control unit in terms of computer design. There is no further article on other uses under this lemma as yet. (Disambiguation and/or integration of this article in Computer with respective linkage—and retention/creation of a more broad-sense article—may be appropriate.)

Microprogram Control Unit

The idea of microprogramming was introduced by M. V. Wilkes in 1951 as an intermediate level to execute computer program instructions (see also: microcode). Microprograms were organized as a sequence of microinstructions and stored in special control memory. The algorithm for the microprogram control unit is usually specified by flow-chart description.^[1] The main advantage of the microprogram control unit is the simplicity of its structure. Outputs of the controller are organized in microinstructions and they can be easily replaced.

Functions of the Control Unit

The functions performed by the control unit vary greatly by the internal architecture of the CPU, since the control unit really implements this architecture. On a regular processor that executes x86 instructions natively the control unit performs the tasks of fetching, decoding, managing execution and then storing results. On a x86 processor with a RISC core, the control unit has significantly more work to do.It manages the translation of x86 instructions to RISC micro-instructions, manages scheduling the micro-instructions between the various execution units, and juggles the output from these units to make sure they end up where they are supposed to go. On one of these processors the control unit may be broken into other units (such as a scheduling unit to handle scheduling and a retirement unit to deal with results coming from the pipeline) due to the complexity of the job it must perform.