What is Control Unit (CU)- Definition | Types | Design

Controlling the instructions or operations is crucial for any system to work the way you want. Hence, a control unit(CU) drives the corresponding processing hardware by generating a set of signals. A CU's primary job is to get data from the main memory. Further, it completes the processes like identifying the devices and doing actions. Overall, it involves output control signals to carry out those tasks.

Read the blog to dive deeper into control unit definition including their types and design.

What Does a Control Unit in Computer Architecture Mean?

The CU is a part of the computer's CPU (central processing unit), which controls how the processor functions. Moreover, this unit’s role is to give instructions to the memory, input, and output devices. Even the arithmetic/logic unit and processor get information regarding how to process data.

Even if we talk about microprocessors, they have a microprocessor control unit. The CU and ALU (Arithmetic Logic Unit) for a microcomputer both reside within the microprocessor.  The CU retrieves internal program instructions from the main memory to the processor instruction register. Thus, it monitors the execution of these instructions. The establishment of a control signal takes place using the information in this register.

Types of Control Unit

Below are the two types of CUs

1. Hardwire based
2. Microprogrammed-based (single-level and two-level)

Hardwired CU

This type does not change the layout of the circuit; instead, a special hardware logic circuit generates the control signals. Thus, you cannot modify the resulting signal before the processor starts processing it.

Functioning

The hardwired CU operates as a sequential logic circuit or a finite state machine. Moreover, it produces a certain sequence of the control signal to carry out an instruction. The hardwired CU employs logic to read the instructions and produce control signals. Also, there are various considerations when developing this type of control unit, including:

• Total designing cost
• Number of hardware
• Speed of the operation (performance)

Advantages of Hardwired CU

• The use of combinational circuits to create signals makes the hardwired CU fast.
• The number of gates affects the potential amount of delay in the generation of control signals.
• You can adjust it to acquire the quickest mode of operation.
• Also, it gives greater speed than a microprogrammed control system.

Disadvantages of Hardwired CU

• The design becomes more complicated (requiring more encoders or decoders) with the need to generate more control signals.
• Control signal modifications are difficult since they call for rewiring the hardware circuit.
• Moreover, adding a new feature requires careful and slow development.
• Evaluation and correction of initial control unit design problems are challenging.
• It is expensive.

Microprogrammed Control Unit

This type uses the control store to store the control signals that are encoded during program execution. The address field remains in the control store by the microprogram. Hence, it delays the generation and decoding of the control signal. One level makes up the entire process.

Functioning

• The unit can produce the control signal for any instruction by reading the matching microprogram's control word. Thus, it is readable from memory in sequential order.
• A microprogram counter must exist to read the control word from the microprogram memory sequentially.
• The loading happens by the beginning address block. Hence, a new instruction loads into IR from the microprogram's starting location in the PC.
• The PC then reads the subsequent microinstruction from the memory after the clock boosts it.
• By doing so, each microinstruction supplies the control signal. Further, the microprogram counters ensure that it goes to the different CPU components in the proper order.

Pros of a Microprogrammed CU

• It permits a more methodical design for the CU.
• This kind of CU is simpler to alter and debug.
• Instead of controlling hardware-based functions, control software-based ones.
• It is more flexible.
• This CU is useful for performing difficult tasks with ease.
• It can preserve the core organization of the control function.
• Next, it can simplify the design of the CU. It is hence less expensive and less prone to mistakes or issues.
• It can create designs that are methodical and efficient.

Cons of a Microprogrammed CUs

• The cost of adaptability is higher.
• In comparison to a hardwired one, it is slower.

How does a CU work?

A CU converts the user's data into control signals, which then go to the central processor. Further, the corresponding hardware gets instructions from the computer's processor to carry out tasks. CPU architecture differs from manufacturer to manufacturer. Hence, the duties that a CU in a computer performs depend on the type of CPU. So, these units are essential for the following devices:

1. The Central Processing Units
2. Graphene Processing Units

Functions of a Control Unit in a Computer

The CU's functions are as follows:

• It supports the computer system's ability to execute the commands of stored programs.
• It carries out mathematical or logical operations.
• Both the main memory and the arithmetic logic unit are in communication with it.
• Moreover, it coordinates all operations involving the peripherals and other units.

Benefits of Thoughtful CUs

Assuming you are referring to Control Units, here are some potential benefits of having thoughtful and well-designed Control Units (CUs) in various systems:

Efficient execution of instructions

A comprehensive CU can execute instructions more quickly by streamlining the instruction pipeline. Moreover, it reduces the amount of clock cycles needed for each instruction.

Efficiency

An efficient CU can increase the CPU's scalability. Thus, it enables the handling of greater and more complex tasks.

More effective parallelism support

An effective CU can more effectively support parallelism. Therefore, it enables the CPU to carry out several instructions at once and enhance performance.

Lower power consumption

The thoughtful CUs can lower power consumption. Thus, it optimizes the usage of resources like registers and memory. It also cuts the number of clock cycles needed for each instruction. Moreover, An efficient CU can increase branch prediction accuracy. Thus, it lowers the number of incorrect predictions and enhances performance.

Conclusion

This blog well explains what is CU and its core components. The central processing unit (CPU) of a computer has a component, a control unit that regulates the use of the processor. Moreover, the CU is in charge of instructing the computer's memory, arithmetic/logic unit, input, and output devices. Further, it provides a control signal that monitors the execution of these instructions. It processes after retrieving the internal program instructions.

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