Computer Architecture

Instruction Cycle

What is the instruction cycle:-

It is defined as a set of sub-cycles states of phases used to execute each and every instruction of a program. The instruction cycle in computer architecture consists of 4 sub-cycle such as





The instruction cycle in computer architecture is graphically shown as follows

instruction cycle

Fetch: –

It is defined as the sub-cycle of the instruction cycle, where the instructions are fetched from memory for execution. The PC will provide the address of the memory location to be accessed by the processor. The processor transfers the address through the address bus and selects the memory location to read the instruction. Now the PC is incremented by 1. i.e. PC ß PC+1.

Execute: –

The instruction sub-cycle is set to be executed if the instruction fetched by the fetched sub-cycle is executed. It starts with decoding the instruction in order to decide the type and nature of instruction. Depending upon the instruction time the necessary timing signal will be generated. Now the ALU is activated to perform the desired operation. After the operation is complete the result will be stored in a proper destination.

Indirect: –

It is the sub-cycle of instruction execution in which the operands are accessed indirectly. It uses either memory indirect of register indirect to need operand during execution. It deals with accessing memory or registers twice. Hence it takes more time to read operand as compare to direct addressing. It is the optional sub-cycle that’s it may be or may not be used during instruction execution.

Interrupt: –

The instruction cycle is said to be interrupt signal is used to perform a predefined task. This sub-cycle is generally used by the IO devices for the inputting of operands or outputting of the result. It deals with the creation of an interrupt and the execution same interrupt. It Is an optional sub-cycle of instruction execution. The sub-cycle is only used in case of interrupt handling of one or more devices. It is a time-consuming sub-cycle that further increases the execution time of instruction.

Conclusion: –

All the four sub-cycles i.e. fetch execute indirectly and interrupt are used to execute each and every instruction of the program. So, the execution forms a cycle known as instruction forms a cycle.

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