Control unit

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This article is about the component of a computer's CPU. For other uses, see Control unit (disambiguation).

The control unit is a component of a computer's central processing unit (CPU) which directs operation of the processor. It controls communication and co-ordination between input/output devices. It reads and interprets instructions and determines the sequence for processing the data.

It directs the operation of the other units by providing timing and control signals. All computer resources are managed by the CU (Control Unit). It directs the flow of data between the Central Processing Unit (CPU) and the other devices. The control unit was historically defined as one distinct part of the 1946 reference model of Von Neumann architecture. In modern computer designs, the control unit is typically an internal part of the CPU with its overall role and operation unchanged.

Functions of the control unit[edit]

The control unit implements the instruction set of the CPU. It performs the tasks of fetching, decoding, managing execution and, finally, storing results. The control unit may manage the translation of instructions (not data) to micro-instructions and manage scheduling the micro-instructions between the various execution units. On some processors, the control unit may be further broken down into other units, such as an instruction unit or scheduling unit to handle scheduling, or a retirement unit to deal with results coming from the instruction pipeline. The control unit handles the main functions of the CPU. N.B. The main functions are to carry out instructions in the software and to direct the flow of data through the computer

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 computer. The examples of devices that require a control unit are CPUs and graphics processing units (GPUs). The control unit receives external instructions or commands which it converts into a sequence of control signals that the control unit applies to the data path to implement a sequence of register-transfer level operations.[1]

Hardwired control unit[edit]

Hardwired control units are implemented through use of sequential logic units, featuring a finite number of gates that can generate specific results based on the instructions that were used to invoke those responses. Hardwired control units are generally faster than microprogrammed designs.

Their design uses a fixed architecture—it requires changes in the wiring if the instruction set is modified or changed. This architecture is preferred in reduced instruction set computers (RISC) as they use a simpler instruction set.

A controller that uses this approach can operate at high speed; however, it has little flexibility, and the complexity of the instruction set it can implement is limited.

The hardwired approach has become less popular as computers have evolved. Previously, control units for CPUs used ad-hoc logic, and they were difficult to design.

Microprogram control unit[edit]

Main article: Microcode

The idea of microprogramming was introduced by Maurice Wilkes in 1951 as an intermediate level to execute computer program instructions. 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 flowchart description.[2] 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.[3]

See also[edit]


  1. ^ Englander, Irv (2009). The Architecture of Computer Hardware, Systems Software, & Networking, 4th Ed. Hoboken, NJ: John Wiley & Sons, Inc. p. 200. ISBN 978-0471-71542-9. 
  2. ^ Barkalov, Alexander (2009). Logic synthesis for FSM based control units / Alexander Barkalov and Larysa Titarenko. Berlin: Springer. ISBN 978-3-642-04308-6. 
  3. ^ Wiśniewski, Remigiusz (2009). Synthesis of compositional microprogram control units for programmable devices. Zielona Góra: University of Zielona Góra. p. 153. ISBN 978-83-7481-293-1.