How the CPU Runs Programs with the Fetch-Execute Cycle

Every program you run — a game, a browser, a chat app — is ultimately a list of simple instructions stored in memory. The CPU's entire job is to work through that list, one instruction at a time, using a loop called the fetch-decode-execute cycle. Here is how each step works: FETCH: The CPU contains a special register called the Program Counter (PC). The PC holds the memory address of the next instruction to run. The CPU reads (fetches) that instruction from memory and loads it into the Instruction Register (IR). Then the PC advances to point at the next instruction so the cycle can continue. DECODE: The Control Unit examines the bits in the IR and figures out what kind of operation the instruction is requesting. Is it adding two numbers? Loading a value from memory? Jumping to a different part of the program? Decoding translates the raw bit pattern into signals the rest of the CPU understands. EXECUTE: The appropriate hardware carries out the operation. Arithmetic or logic work goes to the Arithmetic Logic Unit (ALU). Memory reads or writes go to the memory interface. Branch instructions may update the PC to a new address instead of the usual increment — this is how loops and if-statements work. How fast does this happen? A 3 GHz processor executes 3 billion clock pulses per second. Each clock pulse is not a full fetch-decode-execute cycle — a classic CPU may need several clock cycles per instruction, while a pipelined CPU achieves roughly one instruction completed per clock cycle on average. The program itself does not run 'all at once' — it runs one instruction at a time. Modern CPUs use pipelining to overlap the fetch, decode, and execute stages across different instructions so that while one instruction is being executed, the next is already being decoded and the one after that is already being fetched. Each individual instruction still passes through all three stages in order; speed comes from billions of fast clock cycles and that pipeline overlap, not from running all instructions simultaneously. Key insight: the same cycle runs every program on every CPU. Your CPU does not 'know' whether it is playing music or rendering a web page — it just fetches the next instruction, decodes it, and executes it, over and over, until the program ends.