The instruction cycle, also known as the fetch-decode-execute cycle, is the basic operation performed by a central processing unit (CPU) to execute an instruction. It is the cycle that the CPU follows from boot-up until the computer has shut down in order to process instructions. The instruction cycle consists of several steps, each of which performs a specific function in the execution of the instruction. The major steps in the instruction cycle are:
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Fetch: In the fetch cycle, the CPU retrieves the instruction from memory. The address instruction to be implemented is held at the program counter (PC). The processor fetches the instruction from the memory that is pointed by the PC. Next, the PC is incremented to display the address of the next instruction. This instruction is loaded onto the instruction register (IR) .
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Decode: During this stage, the Control Unit deciphers what the instruction stored in the IR means. As the instruction is decoded, they are turned into a series of control signals that are used to execute the instruction.
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Execute: In this stage, the CPU performs the operation specified by the instruction. This may involve reading or writing data from or to memory, performing arithmetic or logic operations on data, or manipulating the control flow of the program.
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Memory Access: If the instruction requires data from memory, the CPU accesses memory to retrieve it.
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Registry Write-Back: The result generated by the operation is stored in the main memory or sent to an output device. Based on the feedback from the arithmetic logic unit (ALU), the PC may be updated to a different address from which the next instruction will be fetched.
There are also some additional steps that may be performed during the instruction cycle, depending on the CPU architecture and instruction set. For example, in some CPUs, the operands needed for an instruction are fetched during a separate cycle before the execute cycle. By optimizing these cycles, CPU designers can improve the performance and efficiency of the CPU, allowing it to execute instructions faster and more efficiently.
