Difference Between Arithmetic Logical Unit (ALU) And Control Unit (CU)


The Central Processing Unit (CPU) stands as the heart and brain of modern computing devices. Within the CPU, two indispensable components work tirelessly in tandem to execute instructions and process data efficiently: the Arithmetic Logical Unit (ALU) and the Control Unit (CU).

While both units are critical to the CPU’s functionality, they actually do different roles. The ALU performs arithmetic and logical operations on data, while the CU manages the overall operation of the CPU, fetching and decoding instructions, and coordinating data flow between different units. Let us learn more about these two components:

What is Arithmetic Logic Unit (ALU)?

An arithmetic logic unit (ALU) is the main part of the central processing unit (CPU) of a computer system. It performs all processes related to arithmetic and logic operations that need to be done on instruction words. In some processors, the arithmetic logical unit is divided into two units, an arithmetic unit (AU) and a logic unit (LU).

The (ALU) is designed in such a way that it has a direct input and output access to the processor main memory –the random access memory (RAM). Now, the input and outputs flow along an electronic path that is known as a bus. The input consists of an instruction word that contains an operation code or sometimes a format code. The operation code informs the ALU what operation to perform and the operands are used in the operation. Examples of operations that are regularly carried out by the ALU include:

  • Arithmetic Operations: This is basically addition and subtraction. Addition can be substituted for multiplication and subtraction for division.
  • Logical Operations:  This include NOR, NOT, OR, XOR etc.
  • Bit shifting Operations:  This entails shifting the position of bits by a certain number of places to either the right or left, which is considered a multiplication operation.

In other words, the ALU has storage places for input operands, operands that are being added, the accumulated results and shifted results. The flow of data and the operations performed on them in subunits of the ALU is controlled by gated circuits. Division and multiplication are done by a series of adding or subtracting and shifting operations. 

Also Read: Difference Between Microprocessor And Microcontroller

What is Control Unit (CU)?

The control unit (CU) is a component of the central processing unit of the computer system that controls the operations of the processor. It informs the arithmetic and logic unit, the computer’s main memory and the output and input devices how to respond to the command that have been sent to the processor.  The control unit is taken to be the processor brain because it issues orders to everything and ensure that the best results are produced.

Generally, the control unit has been designed with digital circuits, encoders, decoders, logic gates and flip-flops that are arranged in a certain fixed way. There is also a special control memory that stores microprograms based on flowcharts.

The functions of the Control Unit (CU) include:

  • Interprets instructions, regulate and control processor timing.
  • Directs data flow through different components of the CPU.
  • Handle tasks such as decoding, fletching, executing the command and storing results.
  • Sends and receives control signals from other computer gadgets.
  • Directs sequential data flow.
  • Interprets commands and instructions.

Also Read: Difference Between Logical And Physical Address

Control Unit Vs Arithmetic Logical Unit In Tabular Form

FunctionThe Control Unit is responsible for managing and coordinating the operations of various components within the CPU. It fetches instructions from memory and directs the data flow between different units.The Arithmetic Logical Unit is responsible for performing arithmetic operations (such as addition, subtraction, multiplication, and division) and logical operations (like AND, OR, NOT) on data.
OperationsControls the sequence of operations, fetches instructions from memory, and manages the flow of data between the CPU and memory.Handles arithmetic and logical operations directly on the data provided by the registers.
Data ProcessingFocuses on controlling the overall operation of the CPU and coordinating the activities of various units.Focuses on processing data and executing operations on that data.
ComponentsConsists of control circuits, decoders, and sequencers to manage the control flow.Consists of a combination of logic gates, multiplexers, and other components to perform calculations.
SpeedOperates at a slower speed as it manages the sequencing of instructions and coordinates data movement.Operates at a higher speed as it performs the actual arithmetic and logic operations.
Instruction InterpretationDecodes and interprets the instructions fetched from memory, determining the appropriate actions to be taken.Does not interpret instructions; it performs calculations based on the instructions provided by the CU.
Data FlowControls the flow of data between CPU registers, memory, and other components.Receives data from registers, processes it according to the instructions, and sends the result back to registers or memory.
Size and ComplexityIt is more extensive and more complex due to its role in managing the CPU’s operation and instruction execution.Generally smaller and less complex compared to the CU, as it mainly focuses on performing calculations.

Key Takeaway

  • ALU is a circuit component of the CPU that deals with mathematics calculations, data processing and deducting all the logical conclusions and outputs. Arithmetic operations include basic calculations such as addition, subtraction, multiplication and division.
  • CU is one of the two main components of the computer processor; it deals with the coordination between the hardware devices attached with each other and works as an interactive bridge between software and hardware of the computer system. Parts of the internal components that the control unit directs include the arithmetic logical unit and buses.
  • ALU also deals with manipulation of the data and works according to the instructions of the control unit, whereas Control Unit provides instructions to the arithmetic logical unit and coordinates according to the nature of the data provided and nature of work to be performed.
  • Arithmetic logical unit engages in comparison operations whereby one data item is compare with the other data item to determine which item is greater then, equal to or less than the other item. Control unit interprets instructions provided by a program and the initiates the appropriate action to carry out the instruction.
  • ALU depend entirely on the Control unit for its function. The control unit does not in any way depend on the arithmetic logic unit.
  • The Arithmetic logical unit helps in understanding the information entered and produces the results in a better way. The control unit acts as a communicator between the memory and the arithmetic logical unit.

Also Read: Difference Between System Unit And Central Processing Unit