- Components
- Integrated unit
- Control signals
- Instruction decoder
- Microprogrammed unit
- Features
- Illustration
- References
The control unit is a digital circuit that governs operations and handles all control signals within a computer's processor. It allows the logic unit, memory, and input and output peripherals to know how to replicate commands received from a program. Thus, it routes all input and output flow, looks up code for program operations, and directs other units by sending timing and control signals.
A control unit acts by admitting input data, which it transforms into control signals and which are subsequently transmitted to the central processor. The processor orders the different devices what operations to perform.
Source: pixabay.com
This unit is the component of the processor that acts as your brain, generating the instructions for almost all operations and ensuring that they are executed correctly.
Examples of devices that use control units are the central processing unit (CPU) and the graphics processing unit (GPU).
Components
To execute an action, the CPU control unit must produce the necessary control signal in the proper sequence. There are two approaches that are used to generate these control signals in the correct sequence.
Integrated unit
The control unit is made up of logic gates, flip-flops, digital circuits, encoders and decoders, which are wired in an established way.
It can be thought of as a state apparatus, which varies from state to state in each clock cycle, depending on the content of the instruction register and external inputs.
The output of this state apparatus is the control signals. The sequence of operations performed by this machine is determined by the wiring of the logic elements. For this reason it is called "integrated."
The design is based on a specific architecture. When changes to the instruction set are required, changes should be made to the wiring and circuit.
This is the basis for the reduced instruction set computing (RISC) architecture, which has only a small number of instructions.
Control signals
Control signals are essential to direct the execution of operations. They are disseminated by expressly designed logic circuits, in which the signal generation method cannot be modified without a physical change in the circuit structure.
The control signals to perform an operation must be broadcast not at a single point in time, but over the entire period of time corresponding to the instruction execution cycle.
The opcode of an instruction includes the basic information to broadcast the control signal.
Instruction decoder
The opcode is decoded in the instruction decoder. This is made up of a set of decoders that decode the different fields of the operation code of the instruction.
The instruction decoder allows the control unit to enter the first state associated with the execution of the new operation, which lasts as long as the timing signals and other input signals remain unchanged.
A change in any of the above signals prompts the state of the control unit to change.
As a result, some lines coming out of the instruction decoder acquire values in the signal. These lines are coupled to the control signal generator device for the different units of the computer.
By repeating the organization of these phases, the appropriate ordering of internal states is structured in the control unit.
When a new instruction arrives at the control unit, the embedded units are in the initial state to retrieve the new instruction.
Microprogrammed unit
The essential difference of this approach with respect to the integrated unit is the existence of a control store, which is used to store the microprograms that include the coded control signals essential for the execution of an operation.
The operation code of the instruction is not going to be decoded to immediately create a control signal, but the address of a microprogram that is stored in the control store will be indicated.
That is, the control signals attached with the instructions are stored in special memory cells as replaceable firmware.
The microprogrammed control unit is slower than the built-in unit due to the time it takes to restore the operations found in the memory unit.
Features
It is the responsibility of the control unit to inform the input and output devices, the main memory and the logical-arithmetic unit of the computer how to replicate the operations that are issued to the processor.
Gets program operations into memory and takes them to the processor register. Depending on what this register includes, the control unit creates a signal that monitors the performance of these operations.
Depending on the type of operation you enter the control unit, you could change the number and order of the sequential steps generated to select and configure the parts of the CPU that will be used to achieve the objective of the operation, be it move, modify or store data.
This process of decoding operations is performed again as the counter is incremented to the next saved address of the program. From that address the new operation is entered into the control unit, and so on until the end of the program.
Illustration
The functions that a control unit performs will be dependent on the type of CPU, due to the existing disparities in the structure between the different brands. The following diagram illustrates how the instructions in a program are processed.
Source: By Kapooht - Own work, CC BY-SA 3.0, - Handles multiple tasks, such as search and retrieval of instructions, decoding, management of execution and storage of results.
- It receives the external instructions or commands which it converts into a sequence of control signals.
- Interpret the instructions.
- Controls the sequential execution of instructions.
- Coordinates the sequence of data movements in, out, and between the many processor sub-units.
- Regulates and controls the synchronization of the execution units, such as the arithmetic logical unit, and the data stores and registers contained in the processor.
- Sends and receives control signals to other computing devices.
- Directs and controls the flow of data through the different areas of the computer.
References
- Techopedia (2019). Control Unit (CU). Taken from: ceilingpedia.com.
- Computer Hope (2017). Control unit. Taken from: computerhope.com.
- Geeks for Geeks (2019). Computer Organization: Hardwired v / s Micro-programmed Control Unit. Taken from: geeksforgeeks.org.
- Geeks for Geeks (2019). Introduction of Control Unit and its Design. Taken from: geeksforgeeks.org.
- Wikipedia, the free encyclopedia (2019). Control unit. Taken from: en.wikipedia.org.