How a CPU Thinks — The Foundry

The loop behind everything

A CPU does not really "think" like you. It repeats three small jobs, over and over.

FetchGet the next instruction

DecodeWork out what it means

ExecuteDo it

A CPU repeats fetch, decode, execute billions of times every second. That loop is the whole show. Everything a computer does is this tiny cycle, run faster than you can blink.

Here is the secret: the instructions and the data are just numbers. A program is a list of numbers, and the answers are numbers too. The CPU only ever moves and adds numbers around.

Read the program

This toy CPU knows only three commands. Here is a tiny program written in them.

The program3 instructions

0SET a, 2

1ADD a, 3

2SHOW a

SET a, 2 puts the number 2 into a box called a. ADD a, 3 adds 3 to whatever is in a. SHOW a reads it out loud. Those boxes are called , and each one just holds a number.

Down inside the chip, every one of these steps is built from and , the same switches you wired earlier. Stack enough of them and switches turn into a machine that runs programs.

Run it, one step at a time

Press Step. Watch the CPU fetch the line, decode it, then execute it. The register fills as it goes.

Programnext line: 0

0SET a, 2

1ADD a, 3

2SHOW a

FetchGet the line

DecodeWhat does it mean?

ExecuteDo it

Ready. Press Step to start the cycle.

Registers

a –

PC 0

PC is the "program counter": it remembers which line is next.

Tip: each Step moves one phase forward.

You just ran a program the slow way. A real CPU does this exact loop, but billions of times a second, and never gets tired. That speed is the only magic. The steps are simple.