If one is posed the question, "what is the CPU for?", the answer is pretty obvious: the CPU is the heart of the machine – it reads in, decodes, and executes machine instructions, working on memory and peripherals. It does this by incorporating various stages.

Very simplistically, in the Instruction Fetch stage, it reads in machine instructions (which we represent in various human-readable ways – in hexadecimal, assembly, and high-level languages) from memory (RAM) or CPU cache. Then, in the Instruction Decode phase, it proceeds to decipher the instruction. Along the way, it makes use of the control unit, its register set, ALU, and memory/peripheral interfaces.