What is Instruction Cycle?

Introduction to Instruction Cycle

The instruction cycle is related to the CPU (Central processing unit), a list of steps that follows when the computer system starts until the computer system is shut down. The other name of the instruction cycle is the fetch-decode-execute cycle. There are basically four phases in the instruction cycle: fetching an instruction from memory, decoding of the fetched instruction, reading of the address from memory, and the last phase include the instruction execution. The computer processor executes it.

What is the Instruction Cycle?

It is defined as the basic cycle carried out in a computer system in which the computer system fetched the instruction from memory, decodes the instruction and then executes the instruction. It is also known as Fetch-Execute-Cycle. In the computer system, all the instructions are executed in the RAM of the computer system. The CPU is responsible for executing the instruction. The CPU system first fetches the data and instruction from the main memory and store in the temporary memory, which is known as registers. This phase is known as the fetch cycle. After fetching an instruction from memory, the next step taken by the CPU is decoding of fetched instruction. This phase is known as the decode phase.

The CPU contains an instructions set which contain all the predefined list of instructions. In the last phase of the instruction cycle, data processing takes place. The instruction executes in this phase, and the result is stored in another register. After the fetch-decode-execute cycle, the CPU reset itself for another instruction cycle. The CPU system is considered the basic operation cycle, which is carried out in RAM of the central processing unit and executes the instruction. It is repeatedly continuously in the CPU when the computer system boots and then shut down. It is executed in a sequential manner means when one instruction cycle is completed, then only the next instruction cycle begins. But in the modern era central processing unit, the instructions can be executed in a parallel manner.

There are basically five stages of the instruction cycle which are described below:

1. Initiating Cycle

In this phase, the computer system boots up, and the operating system loads in the main memory (read-only memory) of the central processing unit. It immediately begins when the computer system starts.

2. Fetching of Instruction

The fetching of instruction is the first phase. The fetch instruction is common for each instruction executes in a central processing unit. In this phase, the central processing unit sends the PC to MAR and then sends the READ command into a control bus. After sending a read command on the data bus, the memory returns the instruction, which is stored at that particular address in the memory. Then, the CPU copies data from the data bus into MBR and then copies the data from MBR to registers. After all this, the pointer is incremented to the next memory location so that the next instruction can be fetched from memory.

3. Decoding of Instruction

The decoding of instruction is the second phase. In this phase, the CPU determines which instruction is fetched from the instruction and what action needs to be performed on the instruction. The opcode for the instruction is also fetched from memory and decodes the related operation which needs to be performed for the related instruction.

4. Read of an Effective Address

The reading of an effective address is the third phase. This phase deals with the decision of the operation. The operation can be of any type of memory type non-memory type operation. Memory instruction can be categorized into two categories direct memory instruction and indirect memory instruction.

5. Executing of Instruction

The executing of instruction is the last phase. In this stage, the instruction is finally executed. The instruction is executed, and the result of the instruction is stored in the register. After the execution of an instruction, the CPU prepares itself for the execution of the next instruction. For every instruction, the execution time is calculated, which is used to tell the processing speed of the processor.

Why we Need an Instruction Cycle?

• There is a need for an instruction cycle for the computer system so that proper understanding can be done to understand the flow of instructions and execution of an instruction in a computer processor.
• It deals with the complete flow of instruction when the computer system boots up until the computer system is shut down. By the instruction cycle, the internal flow of the central processing unit can be better understand so that if there is an issue, it can be easily resolved.
• It deals with basic operations of computer processor there is a need for proper understanding of various stages involved in it.
• The fetch-decode-execute cycle is common for all types of instructions for the computer processor system.

Importance of Instruction Cycle

• It is important for the processor system for the central processing unit as the instructions are the basic operations that are performed in the main memory of the central processing unit.
• It is a set of steps that help to understand the flow of instruction. By the instruction cycle, the end to end the flow of instructions can be visualized in the computer processor.
• It is common for all instruction set it needs to properly understand so that all the operations can be performed easily.
• By the instruction cycle, the processing time of the program can be easily calculated, which helps to determine the speed of the processor.
• As the speed of the processor tells how many instructions can be simultaneously executed in the central processing unit.

Conclusion

The instruction cycle is the basic operation cycle related to the computer system, which deals with the basic operation in the central processing unit. The other name of the instruction cycle is fetched decode execute cycle. It is continuously repeated until the system goes into the shutdown phase. It is common for all types of instructions defined for computer processor system.

This is a guide to What is Instruction Cycle? Here we discuss the introduction; why we need an instruction cycle? And importance. You may also have a look at the following articles to learn more –

1. Cache Memory Types
2. Types of Primary Memory
3. What is CPU Register
4. JSP Life Cycle