A control unit in general is a central (or sometimes distributed but clearly distinguishable ) part of whatsoever 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.

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.)

Definition

The Control Unit can be thought of as the brain of the CPU itself. It controls based on the instructions it decodes, how other parts of the CPU and in turn, rest of the computer systems should work in order that the instruction gets executed in a correct manner. There are two types of control units, the first type is called hardwired control unit. Hardwired control units are constructed using digital circuits and once formed cannot be changed. The other type of control unit is microprogrammed control unit. A microprogrammed control unit itself decodes and execute instructions by means of executing microprograms.

Application in Computer Design

In computers, the control unit was historically

General operation

The outputs of the control unit controls the activity of the rest of the device. A control unit can be thought of as a finite state machine. -->

The control unit is the circuitry that controls the flow of data through the processor, and coordinates the activities of the other units within it. In a way, it is the "brain within the brain", as it controls what happens inside the processor, which in turn controls the rest of the PC.Vague: date=November 2008

A few examples of devices that require a control unit are CPUs and GPUs. The modern information age would not be possible without complex control unit designs.Vague: date=November 2008

Hardwired Control

At one time, control units for CPUs were ad-hoc logic, and they were difficult to designs . These can be identified as the main part of the computer and the main device that helps the computer to function in an appropriate manner.

Microprogram Control

main: microcode

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.