Operation of the Control Unit of CPU

The control unit performs the computer’s traffic control. It coordinates and controls operations of the central processing unit (CPU). Let’s know detail about operation of the control unit of CPU.

It does this like the human brain coordinates and controls the activities of the human body. The control unit does not input, output, process, or store data; rather, it initiates and controls these operations.

The control unit also communicates with input devices to begin the transfer of data or instructions into memory and with output devices to begin the transfer of results from memory.

Operation of the control unit

Data transfer involves the moving of data or instructions from one location to another. When an item of data is stored in a given location, it replaces the previous contents of that location.

But when an item of data is moved from one location to another the data item is not physically removed from its initial location; actually the data item is copied to the new location.

When the computer executes a program contained in primary memory, the control unit obtains the instructions in the sequence in which they are executed, interprets the instructions, and issues in the sequence in which they are executed, interprets the instructions, and issues signals to execute them.

To accomplish this, the control unit must communicate with ALU and primary storage.

Operation of the Control Unit

Operation of the Control Unit

The control unit generally performs all or most of the following functions for execution of instructions:

  • Determines the instruction to be executed
  • Determines the operation to be performed
  • Determines what data are needed and where they are stored
  • Determines where results, if any, are to be stored
  • Determines where the next instruction is located
  • Causes the instruction to be executed
  • Transfers control to the next instruction

Machine Cycle

The activities of the control unit are composed of thousands of individual steps, each of which takes place in a fixed interval of time. These intervals are controlled by an internal electronic generally measured in megahertz (MHz) or million pulses per second.

In general, operations within the CPU of the computer take place in terms of a fixed number of clock pulses. This number determines the machine cycle for the computer. During a machine cycle, the computer can perform one machine operation.

The number of machine operations required to execute a single instruction vary from instruction to instruction. Execution of instruction takes place under the direct supervision of the control unit.

Instruction Format

Program instructions stored in memory must be in a machine-readable form. In general, these instructions consist of two distinct parts,

  • An operation code, and
  • One (or more ) operand

The operation code (Opcode) tells the machine what task is to be performed; the operands specify what are to be used to perform the task. Examples of operands are:

  • Address of a data item or an instruction in primary memory.
  • Address of a data item or a program outside the CPU on a secondary memory.
  • Address of an input or an output unit.
  • Address of a register or special-purpose temporary memory area.

For example, the instruction

MOV          8000           9000

Contains the operation code MOV and the operands 8000 and 9000. This could mean that the machine is to move the contents of storage location 8000 to storage location 9000.

In addition to the address of an item of data in primary storage, an operand can be used to indicate the address of a register, the address of data stored in secondary storage, or even the address of an input or output device.

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