CPU

In simple terms

The CPU (central processing unit) is the part of a computer that does the work. It reads instructions out of memory one after another and carries them out: add these numbers, copy this byte, jump to that address.

More detail

A CPU is built from a few cooperating pieces:

• Registers — a tiny pile of very fast storage cells, the CPU’s “hands”
• ALU (Arithmetic Logic Unit) — does the maths and the boolean operations
• Control unit — decodes instructions and decides what happens next
• Caches — small, fast memory close to the CPU to hide the slowness of main memory
• Clock — a metronome that ticks billions of times per second; one tick (or a few) per instruction

The cycle every CPU runs is the fetch–decode–execute loop:

1. Fetch the next instruction from memory.
2. Decode what it means.
3. Execute it.
4. Repeat forever.

Modern CPUs have several cores (independent CPUs on one chip), pipelines (overlapping the steps above), and out-of-order execution (doing instructions in a smarter order while preserving the result).

Why it matters

The CPU is the heart of a computer. Everything a program does — every pixel drawn, every packet sent, every neural-network multiply — eventually becomes a stream of CPU instructions.

Real-world examples

• An Apple M-series chip contains a CPU with several “performance” and “efficiency” cores.
• A Raspberry Pi has a single ARM CPU running at around 1.5 GHz.
• A high-end server CPU may have 64 or more cores.

Common misconceptions

• “GHz tells you how fast a CPU is.” Clock speed matters, but so does what each tick can do — modern CPUs do far more per tick than CPUs from twenty years ago.
• “A CPU and a GPU are interchangeable.” A CPU is a generalist; a GPU is specialised for doing the same operation on many pieces of data in parallel.

Learn next

The CPU is almost useless without memory to read from and write to, and without an operating system to share it.