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How CPUs Actually Work: Fetch Execute Cycle Explained

The instruction cycle (also known as the fetch–decode–execute cycle, or simply the fetch–execute cycle) is the cycle that the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instructions. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage. This is a simple diagram illustrating the individual stages of the fetch-decode-execute cycle. Abbreviations: PC – Program counter MAR – Memory address register MDR – Memory data register CIR – Current instruction register CU – Control unit ALU – Arithmetic logic unit In simpler CPUs, the instruction cycle is executed sequentially, with each instruction being processed before the next one starts. In most modern CPUs, the instruction cycles are instead executed concurrently, and often in parallel, through an instruction pipeline: the next instruction starts being processed before the previous instruction has finished, which is possible because the cycle is broken up into separate steps. IGCSE REQUIREMENTS: Understand the purpose of the components in a CPU, in a computer that has a Von Neumann architecture Describe the process of the fetch–decode–execute (FDE) cycle including the role of each component in the process