Explained
How a cpu works.
How a CPU works
A CPU, or central processing unit, runs the instructions that make up every program. It fetches an instruction from memory, decodes what it means, carries it out and stores the result, billions of times a second. Modern processors have several cores, each able to run its own stream of instructions, plus shared cache memory that keeps recent data close at hand.
What you buy as a CPU is a package. The actual processor is the silicon die in the middle. Everything around it exists to connect the die to the motherboard, power it, and get its heat out. This model is a generic desktop processor without any brand markings.
From the lid down
On top is the integrated heat spreader, a nickel-plated copper lid. The die makes a lot of heat in a small area, and the lid spreads it out so the cooler on top can take it away. Between the die and the lid is a thermal interface material, either paste or a thin layer of solder, which fills tiny gaps that would otherwise hold insulating air.
The die sits on a substrate, a small circuit board with many layers of fine copper wiring. The die has thousands of microscopic connection points, far too close together for a motherboard, so the substrate fans them out to the gold contact pads on the underside. Many of those pads only carry power and ground, because a fast processor can draw large currents at low voltage. The small capacitors on the substrate smooth the supply when the demand suddenly changes.
Down to the transistor
Inside the die, everything is built from transistors, tiny switches that let current flow or block it under the control of a voltage on their gate. Combine a few and you get logic gates; combine millions and you get adders, caches and whole cores. The Intel 4004 of 1971 had about 2,300 transistors. In 1965 Gordon Moore predicted that the number on a chip would keep doubling at a steady rate, an observation now known as Moore's law, and today's processors have billions.
Modern transistors are FinFETs or newer designs, where the gate wraps around a thin fin of silicon to control the current more tightly. Open Play and use Zoom to go from the package to the die, to a single core, and finally to one transistor.
Parts glossary
- Integrated heat spreader
- The nickel-plated copper lid protects the die and spreads its heat across a wider area for the cooler.
- Thermal interface
- Thermal paste or solder fills the microscopic gaps between the die and the lid, because air conducts heat very badly.
- Sealant
- A bead of adhesive glues the lid to the substrate.
- Silicon die
- This is the actual processor: cores, cache and controllers made from billions of transistors on a piece of silicon a few centimetres square or smaller. Fun fact: The Intel 4004 of 1971, one of the first commercial microprocessors, had about 2,300 transistors.
- Capacitors
- Tiny capacitors smooth out the power supply as the chip's demand jumps from moment to moment.
- Substrate
- A small multi-layer circuit board fans the die's thousands of tiny connections out to contacts the socket can reach.
- Contact pads
- Gold-plated pads press against spring pins in the motherboard socket. Many carry power and ground, not data.
