A CPU (Central Processing Unit) is the main processor in a computer that runs instructions from software and the operating system. It follows a simple loop called fetch, decode and execute, using internal parts such as registers, an arithmetic and logic unit, control logic and fast memory called cache. Modern CPUs are multi-core, which means several processing cores sit on one chip, and many support simultaneous multithreading so each core can work on more than one stream of instructions at once. Performance depends on clock speed (measured in GHz), how much work the CPU does per clock, the size and speed of its caches and memory, and the efficiency of its design. Common instruction set families include x86-64 and ARM. Desktop CPUs usually plug into a socket on the motherboard, while laptops and phones often use a system-on-chip that combines the CPU with graphics, memory controllers and other functions. A CPU is general purpose and good at many different tasks, which is why it differs from a GPU that is built to process large numbers of similar operations in parallel.
