CPU

The CPU (Central Processing Unit) is the part of a computer that executes program instructions. It reads commands stored in memory and then performs arithmetic operations, comparisons, and data transfers.

Modern CPUs are made from a single silicon chip called a microprocessor. They are among the essential components of personal computers, servers, modern mobile phones, and embedded devices.

Internal components

A CPU typically has three main components. The ALU (Arithmetic Logic Unit) handles arithmetic and logical operations. The Control Unit directs the flow of instructions. Registers are small, fast storage locations that hold data being processed. There is also cache, which is fast memory that reduces the wait time for accessing the larger memory (RAM).

Execution cycle

The basic task is divided into a repeating cycle: fetching an instruction from memory, decoding its meaning, and executing it. This cycle is controlled by a clock, measured in hertz; clock speed is one measure of performance, but not the only one.

Evolution of design

Earlier designs executed one instruction at a time. Later, pipelining was introduced, which allows different stages to run simultaneously, along with out-of-order execution. Today, the most common way to increase performance is to put multiple cores (processing units) on a single chip that run in parallel, since further increases in clock speed are limited by heat and power consumption.

Instruction architecture

Each CPU supports a set of instructions called an instruction set architecture (ISA). Well-known examples include x86 (commonly found in computers and servers) and ARM (common in mobile devices). A program written for one ISA will not run on another without recompilation or emulation.